Co-reporter:Dimitrios Maganas, Serena DeBeer, and Frank Neese
Inorganic Chemistry October 2, 2017 Volume 56(Issue 19) pp:11819-11819
Publication Date(Web):September 18, 2017
DOI:10.1021/acs.inorgchem.7b01810
In this work, a new protocol for the calculation of valence-to-core resonant X-ray emission (VtC RXES) spectra is introduced. The approach is based on the previously developed restricted open configuration interaction with singles (ROCIS) method and its parametrized version, based on a ground-state Kohn–Sham determinant (DFT/ROCIS) method. The ROCIS approach has the following features: (1) In the first step approximation, many-particle eigenstates are calculated in which the total spin is retained as a good quantum number. (2) The ground state with total spin S and excited states with spin S′ = S, S ± 1, are obtained. (3) These states have a qualitatively correct multiplet structure. (4) Quasi-degenerate perturbation theory is used to treat the spin–orbit coupling operator variationally at the many-particle level. (5) Transition moments are obtained between the relativistic many-particle states. The method has shown great potential in the field of X-ray spectroscopy, in particular in the field of transition-metal L-edge, which cannot be described correctly with particle–hole theories. In this work, the method is extended to the calculation of resonant VtC RXES [alternatively referred to as 1s-VtC resonant inelastic X-ray scattering (RIXS)] spectra. The complete Kramers–Dirac–Heisenerg equation is taken into account. Thus, state interference effects are treated naturally within this protocol. As a first application of this protocol, a computational study on the previously reported VtC RXES plane on a molecular managanese(V) complex is performed. Starting from conventional X-ray absorption spectra (XAS), we present a systematic study that involves calculations and electronic structure analysis of both the XAS and non-resonant and resonant VtC XES spectra. The very good agreement between theory and experiment, observed in all cases, allows us to unravel the complicated intensity mechanism of these spectroscopic techniques as a synergic function of state polarization and interference effects. In general, intense features in the RIXS spectra originate from absorption and emission processes that involve nonorthogonal transition moments. We also present a graphical method to determine the sign of the interference contributions.
Co-reporter:David Schweinfurth, J. Krzystek, Mihail Atanasov, Johannes Klein, Stephan Hohloch, Joshua Telser, Serhiy Demeshko, Franc Meyer, Frank Neese, and Biprajit Sarkar
Inorganic Chemistry May 1, 2017 Volume 56(Issue 9) pp:5253-5253
Publication Date(Web):April 12, 2017
DOI:10.1021/acs.inorgchem.7b00371
Understanding the origin of magnetic anisotropy and having the ability to tune it are essential needs of the rapidly developing field of molecular magnetism. Such attempts at determining the origin of magnetic anisotropy and its tuning are still relatively infrequent. One candidate for such attempts are mononuclear Co(II) complexes, some of which have recently been shown to possess slow relaxation of their magnetization. In this contribution we present four different five-coordinated Co(II) complexes, 1–4, that contain two different “click” derived tetradentate tripodal ligands and either Cl– or NCS– as an additional, axial ligand. The geometric structures of all four complexes are very similar. Despite this, major differences are observed in their electronic structures and hence in their magnetic properties as well. A combination of temperature dependent susceptibility measurements and high-frequency and -field EPR (HFEPR) spectroscopy was used to accurately determine the magnetic properties of these complexes, expressed through the spin Hamiltonian parameters: g-values and zero-field splitting (ZFS) parameters D and E. A combination of optical d-d absorption spectra together with ligand field theory was used to determine the B and Dq values of the complexes. Additionally, state of the art quantum chemical calculations were applied to obtain bonding parameters and to determine the origin of magnetic anisotropy in 1–4. This combined approach showed that the D values in these complexes are in the range from −9 to +9 cm–1. Correlations have been drawn between the bonding nature of the ligands and the magnitude and sign of D. These results will thus have consequences for generating novel Co(II) complexes with tunable magnetic anisotropy and hence contribute to the field of molecular magnetism.
Co-reporter:Ajay Sharma, Michael Roemelt, Michael Reithofer, Richard R. Schrock, Brian M. Hoffman, and Frank Neese
Inorganic Chemistry June 19, 2017 Volume 56(Issue 12) pp:6906-6906
Publication Date(Web):June 1, 2017
DOI:10.1021/acs.inorgchem.7b00364
The molybdenum trisamidoamine (TAA) complex [Mo] {[3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2N]Mo} carries out catalytic reduction of N2 to ammonia (NH3) by protons and electrons at room temperature. A key intermediate in the proposed [Mo] nitrogen reduction cycle is nitridomolybdenum(VI), [Mo(VI)]N. The addition of [e–/H+] to [Mo(VI)]N to generate [Mo(V)]NH might, in principle, follow one of three possible pathways: direct proton-coupled electron transfer; H+ first and then e–; e– and then H+. In this study, the paramagnetic Mo(V) intermediate {[Mo]N}− and the [Mo]NH transfer product were generated by irradiating the diamagnetic [Mo]N and {[Mo]NH}+ Mo(VI) complexes, respectively, with γ-rays at 77 K, and their electronic and geometric structures were characterized by electron paramagnetic resonance and electron nuclear double resonance spectroscopies, combined with quantum-chemical computations. In combination with previous X-ray studies, this creates the rare situation in which each one of the four possible states of [e–/H+] delivery has been characterized. Because of the degeneracy of the electronic ground states of both {[Mo(V)]N}− and [Mo(V)]NH, only multireference-based methods such as the complete active-space self-consistent field (CASSCF) and related methods provide a qualitatively correct description of the electronic ground state and vibronic coupling. The molecular g values of {[Mo]N}− and [Mo]NH exhibit large deviations from the free-electron value ge. Their actual values reflect the relative strengths of vibronic and spin–orbit coupling. In the course of the computational treatment, the utility and limitations of a formal two-state model that describes this competition between couplings are illustrated, and the implications of our results for the chemical reactivity of these states are discussed.
Co-reporter:Gerard Sabenya, Laura Lázaro, Ilaria Gamba, Vlad Martin-Diaconescu, Erik Andris, Thomas Weyhermüller, Frank Neese, Jana Roithova, Eckhard Bill, Julio Lloret-Fillol, and Miquel Costas
Journal of the American Chemical Society July 12, 2017 Volume 139(Issue 27) pp:9168-9168
Publication Date(Web):June 9, 2017
DOI:10.1021/jacs.7b00429
Iron complex [FeIII(N3)(MePy2tacn)](PF6)2 (1), containing a neutral triazacyclononane-based pentadentate ligand, and a terminally bound azide ligand has been prepared and spectroscopically and structurally characterized. Structural details, magnetic susceptibility data, and Mössbauer spectra demonstrate that 1 has a low-spin (S = 1/2) ferric center. X-ray diffraction analysis of 1 reveals remarkably short Fe–N (1.859 Å) and long FeN–N2 (1.246 Å) distances, while the FT-IR spectra show an unusually low N–N stretching frequency (2019 cm–1), suggesting that the FeN–N2 bond is particularly weak. Photolysis of 1 at 470 or 530 nm caused N2 elimination and generation of a nitrido species that on the basis of Mössbauer, magnetic susceptibility, EPR, and X-ray absorption in conjunction with density functional theory computational analyses is formulated as [FeV(N)(MePy2tacn)]2+ (2). Results indicate that 2 is a low-spin (S = 1/2) iron(V) species, which exhibits a short Fe–N distance (1.64 Å), as deduced from extended X-ray absorption fine structure analysis. Compound 2 is only stable at cryogenic (liquid N2) temperatures, and frozen solutions as well as solid samples decompose rapidly upon warming, producing N2. However, the high-valent compound could be generated in the gas phase, and its reactivity against olefins, sulfides, and substrates with weak C–H bonds studied. Compound 2 proved to be a powerful two-electron oxidant that can add the nitrido ligand to olefin and sulfide sites as well as oxidize cyclohexadiene substrates to benzene in a formal H2-transfer process. In summary, compound 2 constitutes the first case of an octahedral FeV(N) species prepared within a neutral ligand framework and adds to the few examples of FeV species that could be spectroscopically and chemically characterized.
Co-reporter:Giovanni Bistoni, Christoph Riplinger, Yury Minenkov, Luigi Cavallo, Alexander A. Auer, and Frank Neese
Journal of Chemical Theory and Computation July 11, 2017 Volume 13(Issue 7) pp:3220-3220
Publication Date(Web):June 12, 2017
DOI:10.1021/acs.jctc.7b00352
The validity of the main approximations used in canonical and domain based pair natural orbital coupled cluster methods (CCSD(T) and DLPNO-CCSD(T), respectively) in standard chemical applications is discussed. In particular, we investigate the dependence of the results on the number of electrons included in the correlation treatment in frozen-core (FC) calculations and on the main threshold governing the accuracy of DLPNO all-electron (AE) calculations. Initially, scalar relativistic orbital energies for the ground state of the atoms from Li to Rn in the periodic table are calculated. An energy criterion is used for determining the orbitals that can be excluded from the correlation treatment in FC coupled cluster calculations without significant loss of accuracy. The heterolytic dissociation energy (HDE) of a series of metal compounds (LiF, NaF, AlF3, CaF2, CuF, GaF3, YF3, AgF, InF3, HfF4, and AuF) is calculated at the canonical CCSD(T) level, and the dependence of the results on the number of correlated electrons is investigated. Although for many of the studied reactions subvalence correlation effects contribute significantly to the HDE, the use of an energy criterion permits a conservative definition of the size of the core, allowing FC calculations to be performed in a black-box fashion while retaining chemical accuracy. A comparison of the CCSD and the DLPNO-CCSD methods in describing the core–core, core–valence, and valence–valence components of the correlation energy is given. It is found that more conservative thresholds must be used for electron pairs containing at least one core electron in order to achieve high accuracy in AE DLPNO-CCSD calculations relative to FC calculations. With the new settings, the DLPNO-CCSD method reproduces canonical CCSD results in both AE and FC calculations with the same accuracy.
Co-reporter:Manuel Sparta, Marius Retegan, Peter Pinski, Christoph Riplinger, Ute Becker, and Frank Neese
Journal of Chemical Theory and Computation July 11, 2017 Volume 13(Issue 7) pp:3198-3198
Publication Date(Web):June 7, 2017
DOI:10.1021/acs.jctc.7b00260
The linear-scaling local coupled cluster method DLPNO-CCSD(T) allows calculations on systems containing hundreds of atoms to be performed while reproducing canonical CCSD(T) energies typically with chemical accuracy (<1 kcal/mol). The accuracy of the method is determined by two main truncation thresholds that control the number of electron pairs included in the CCSD iterations and the size of the pair natural orbital virtual space for each electron pair, respectively. While the results of DLPNO-CCSD(T) calculations converge smoothly toward their canonical counterparts as the thresholds are tightened, the improved accuracy is accompanied by a fairly steep increase of the computational cost. Many applications study events that are confined to a relatively small region of the system of interest. Hence, it is viable to develop methods that allow the user to treat different parts of a large system at various levels of accuracy. In this work we present an extension to the native DLPNO method that fragments the system into preselected molecular parts and uses different thresholds or even different levels of theory for the interaction between individual fragments. Thereby chemical intuition can be used to focus computational resources on a more accurate evaluation of the properties at the center of interest, while permitting a less demanding description of the surrounding moieties. The strategy was implemented within the DLPNO-CCSD(T) framework. We tested the scheme for a series of realistic quantum chemical applications such as the calculation of the dimerization energies, potential energy surfaces, enantiomeric excess in organometallic catalysis, and the binding energy of the anticancer drug ellipticine to DNA. This work demonstrates the power of the approach and offers guidance to its setup.
Co-reporter:Christina Römelt, Jinshuai Song, Maxime Tarrago, Julian A. Rees, Maurice van Gastel, Thomas Weyhermüller, Serena DeBeer, Eckhard Bill, Frank Neese, and Shengfa Ye
Inorganic Chemistry April 17, 2017 Volume 56(Issue 8) pp:4745-4745
Publication Date(Web):April 5, 2017
DOI:10.1021/acs.inorgchem.7b00401
Iron porphyrins can act as potent electrocatalysts for CO2 functionalization. The catalytically active species has been proposed to be a formal Fe(0) porphyrin complex, [Fe(TPP)]2– (TPP = tetraphenylporphyrin), generated by two-electron reduction of [FeII(TPP)]. Our combined spectroscopic and computational investigations reveal that the reduction is ligand-centered and that [Fe(TPP)]2– is best formulated as an intermediate-spin Fe(II) center that is antiferromagnetically coupled to a porphyrin diradical anion, yielding an overall singlet ground state. As such, [Fe(TPP)]2– contains two readily accessible electrons, setting the stage for CO2 reduction.
Co-reporter:Saurabh Kumar Singh, Julien Eng, Mihail Atanasov, Frank Neese
Coordination Chemistry Reviews 2017 Volume 344(Volume 344) pp:
Publication Date(Web):1 August 2017
DOI:10.1016/j.ccr.2017.03.018
•Ab-initio ligand field theory –a new perspective in ligand field paradigm.•Bridging quantum chemistry and ligand-field theory.•Experimental trends in ligand field parameters are well reproduced.•Spin-allowed d-d transitions are reproduced with an off-set of 2000–3000 cm−1.•Ab initio ligand field theory –a powerful predictive tool in molecular magnetism.•Ab initio ligand field theory –now freely available in ORCA 4.0 package.In this work, a general, user-friendly method – ab initio ligand field theory (AILFT), is described and illustrated. AILFT allows one to unambiguously extract all ligand field parameters (the ligand field one-electron matrix VLFT, the Racah parameters B and C, and the spin-orbit coupling parameter ζ) from relatively straightforward multi-reference ab initio calculations. The method applies to mononuclear complexes in dn or fn configurations. The method is illustrated using complete active space self-consistent field (CASSCF) and N-electron valence perturbation theory (NEVPT2) calculations on a series of well documented octahedral complexes of CrIII with simple ligands such as F−, Cl−, Br−, I−, NH3 and CN−. It is shown that all well-known trends for the value of 10Dq (the spectrochemical series) are faithfully reproduced by AILFT. By comparison of B and ζ for CrIII in these complexes with the parameters calculated for the free ion Cr3+, the covalency of the Cr-ligand bond can be assessed quantitatively (the non-relativistic and relativistic nephelauxetic effects). The variation of ligand field parameters for complexes of 3d, 4d and 5d elements is studied using MCl63− (M = CrIII, MoIII, WIII) as model examples. As reflected in variations of 10Dq, B and ζ across this series, metal-ligand covalency increases from CrCl63− to MoCl63− to WCl63−. Using the angular overlap model, the one-electron parameters of the ligand field matrix are decomposed into increments for σ- and π- metal-ligand interactions. This allows for the quantification of variations in σ- and π-ligand donor properties of these ligands. Using these results, the well documented two-dimensional spectroscopic series for complexes of CrIII is quantitatively reproduced. Comparison of the results obtained using CASSCF and NEVPT2 reveals the importance of dynamic electron correlation. Finally, the limitations of the AILFT method for complexes with increasing metal-ligand covalency are analyzed and discussed.Download high-res image (141KB)Download full-size image
Co-reporter:Elizaveta A. SuturinaJoscha Nehrkorn, Joseph M. Zadrozny, Junjie Liu, Mihail Atanasov, Thomas Weyhermüller, Dimitrios MaganasStephen Hill, Alexander Schnegg, Eckhard Bill, Jeffrey R. Long, Frank Neese
Inorganic Chemistry 2017 Volume 56(Issue 5) pp:
Publication Date(Web):February 22, 2017
DOI:10.1021/acs.inorgchem.7b00097
The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)2[Co(SPh)4] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)4]2– upon structural variations, depending on the counterion and crystallization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)4]2–: (PPh4)2[Co(SPh)4] (1) and (NEt4)2[Co(SPh)4] (2). The characterization methods employed include multifield SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic circular dichroism (VTVH-MCD), and frequency domain Fourier transform THz-EPR (FD-FT THz-EPR). Notably, the paramagnetic Co(II) complex [Co(SPh)4]2– shows strong axial magnetic anisotropy in 1, with D = −55(1) cm–1 and E/D = 0.00(3), but rhombic anisotropy is seen for 2, with D = +11(1) cm–1 and E/D = 0.18(3). Multireference ab initio CASSCF/NEVPT2 calculations enable interpretation of the remarkable variation of D and its dependence on the electronic structure and geometry.
Co-reporter:Georgi L. StoychevAlexander A. Auer, Frank Neese
Journal of Chemical Theory and Computation 2017 Volume 13(Issue 2) pp:
Publication Date(Web):December 22, 2016
DOI:10.1021/acs.jctc.6b01041
A procedure was developed to automatically generate auxiliary basis sets (ABSs) for use with the resolution of the identity (RI) approximation, starting from a given orbital basis set (OBS). The goal is to provide an accurate and universal solution for cases where no optimized ABSs are available. In this context, “universal” is understood as the ability of the ABS to be used for Coulomb, exchange, and correlation energy fitting. The generation scheme (denoted AutoAux) works by spanning the product space of the OBS using an even-tempered expansion for each atom in the system. The performance of AutoAux in conjunction with different OBSs [def2-SVP, def2-TZVP, def2-QZVPP, and cc-pwCVnZ (n = D, T, Q, 5)] has been evaluated for elements from H to Rn and compared to existing predefined ABSs. Due to the requirements of simplicity and universality, the generated ABSs are larger than the optimized ones but lead to similar errors in MP2 total energies (on the order of 10–5 to 10–4 Eh/atom).
Co-reporter:Dr. Giovanni Bistoni; Dr. Alexer A. Auer and; Dr. Frank Neese
Chemistry - A European Journal 2017 Volume 23(Issue 4) pp:865-873
Publication Date(Web):2017/01/18
DOI:10.1002/chem.201604127
AbstractThe interaction of Lewis acids and bases in both classical Lewis adducts and frustrated Lewis pairs (FLPs) is investigated to elucidate the role that London dispersion plays in different situations. The analysis comprises 14 different adducts between tris(pentafluorophenyl)borane and a series of phosphines, carbenes, and amines with various substituents, differing in both steric and electronic properties. The domain-based local pair natural orbital coupled-cluster (DLPNO-CCSD(T)) method is used in conjunction with the recently introduced local energy decomposition (LED) analysis to obtain state-of-the-art dissociation energies and, at the same time, a clear-cut definition of the London dispersion component of the interaction, with the ultimate goal of aiding in the development of designing principles for acid/base pairs with well-defined bonding features and reactivity. In agreement with previous DFT investigations, it is found that the London dispersion dominates the interaction energy in FLPs, and is also remarkably strong in Lewis adducts. In these latter systems, its magnitude can be easily modulated by modifying the polarizability of the substituents on the basic center, which is consistent with the recently introduced concept of dispersion energy donors. By counteracting the destabilizing energy contribution associated with the deformation of the monomers, the London dispersion drives the stability of many Lewis adducts.
Co-reporter:Dr. Uttam Chakraborty;Dr. Serhiy Demeshko; Dr. Franc Meyer;M. Sc. Christophe Rebreyend; Dr. Bas de Bruin; Dr. Mihail Atanasov; Dr. Frank Neese;Dr. Bernd Mühldorf; Dr. Robert Wolf
Angewandte Chemie International Edition 2017 Volume 56(Issue 27) pp:7995-7999
Publication Date(Web):2017/06/26
DOI:10.1002/anie.201702454
AbstractThe 15 valence-electron iron(I) complex [CpArFe(IiPr2Me2)] (1, CpAr=C5(C6H4-4-Et)5; IiPr2Me2=1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene) was synthesized in high yield from the FeII precursor [CpArFe(μ-Br)]2. 57Fe Mössbauer and EPR spectroscopic data, magnetic measurements, and ab initio ligand-field calculations indicate an S= 3/2 ground state with a large negative zero-field splitting. As a consequence, 1 features magnetic anisotropy with an effective spin-reversal barrier of Ueff=64 cm−1. Moreover, 1 catalyzes the dehydrogenation of N,N-dimethylamine–borane, affording tetramethyl-1,3-diaza-2,4-diboretane under mild conditions.
Co-reporter:Dr. Frank Neese
Angewandte Chemie International Edition 2017 Volume 56(Issue 37) pp:11003-11010
Publication Date(Web):2017/09/04
DOI:10.1002/anie.201701163
AbstractQuantum chemistry can be used as a powerful link between theory and experiment for studying reactions in all areas of catalysis. The key feature of this approach is the combination of quantum chemistry with a range of high-level spectroscopic methods. This allows for conclusions to be reached that neither theory nor experiment would have been able to obtain in isolation.
Co-reporter:Dr. Uttam Chakraborty;Dr. Serhiy Demeshko; Dr. Franc Meyer;M. Sc. Christophe Rebreyend; Dr. Bas de Bruin; Dr. Mihail Atanasov; Dr. Frank Neese;Dr. Bernd Mühldorf; Dr. Robert Wolf
Angewandte Chemie 2017 Volume 129(Issue 27) pp:8107-8112
Publication Date(Web):2017/06/26
DOI:10.1002/ange.201702454
AbstractDer 15-Valenzelektronen-Komplex [CpArFe(IiPr2Me2)] (1, CpAr=C5(C6H4-4-Et)5; IiPr2Me2=1,3-Diisopropyl-4,5-dimethylimidazolin-2-yliden) ist ausgehend von der Eisen(II)-Vorstufe [CpArFe(μ-Br)]2 in hohen Ausbeuten zugänglich. 57Fe-Mößbauer- und EPR-Daten, magnetische Messungen und Ab-initio-Ligandenfeldrechnungen deuten auf einen S=3/2-Grundzustand mit einer großen negativen Nullfeldaufspaltung hin. Infolgedessen weist 1 magnetische Anisotropie mit einer effektiven Spinumkehrbarriere von Ueff=64 cm−1 auf. Zudem katalysiert 1 die Dehydrierung von Dimethylamin-Boran unter Bildung von Tetramethyl-1,3-diaza-2,4-diboretan unter milden Bedingungen.
Co-reporter: Dr. Frank Neese
Angewandte Chemie 2017 Volume 129(Issue 37) pp:11147-11154
Publication Date(Web):2017/09/04
DOI:10.1002/ange.201701163
AbstractDie Quantenchemie kann als wesentliches Bindeglied zwischen Theorie und Experiment bei der Untersuchung katalytischer Reaktionen in allen Bereichen der Katalyse herangezogen werden. Das Hauptmerkmal dieses Ansatzes besteht darin, dass die Quantenchemie mit einem Arsenal hochwertiger spektroskopischer Methoden kombiniert wird. Auf diese Weise kann man zu Schlussfolgerungen gelangen, die weder durch Theorie noch durch Experiment alleine möglich gewesen wären.
Co-reporter:Adam Kubas;Johannes Noak;Annette Trunschke;Robert Schlögl;Dimitrios Maganas
Chemical Science (2010-Present) 2017 vol. 8(Issue 9) pp:6338-6353
Publication Date(Web):2017/08/21
DOI:10.1039/C7SC01771E
Absorption and multiwavelength resonance Raman spectroscopy are widely used to investigate the electronic structure of transition metal centers in coordination compounds and extended solid systems. In combination with computational methodologies that have predictive accuracy, they define powerful protocols to study the spectroscopic response of catalytic materials. In this work, we study the absorption and resonance Raman spectra of the M1 MoVOx catalyst. The spectra were calculated by time-dependent density functional theory (TD-DFT) in conjunction with the independent mode displaced harmonic oscillator model (IMDHO), which allows for detailed bandshape predictions. For this purpose cluster models with up to 9 Mo and V metallic centers are considered to represent the bulk structure of MoVOx. Capping hydrogens were used to achieve valence saturation at the edges of the cluster models. The construction of model structures was based on a thorough bonding analysis which involved conventional DFT and local coupled cluster (DLPNO-CCSD(T)) methods. Furthermore the relationship of cluster topology to the computed spectral features is discussed in detail. It is shown that due to the local nature of the involved electronic transitions, band assignment protocols developed for molecular systems can be applied to describe the calculated spectral features of the cluster models as well. The present study serves as a reference for future applications of combined experimental and computational protocols in the field of solid-state heterogeneous catalysis.
Co-reporter:Yury Minenkov;Giovanni Bistoni;Christoph Riplinger;Alexander A. Auer;Luigi Cavallo
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 14) pp:9374-9391
Publication Date(Web):2017/04/05
DOI:10.1039/C7CP00836H
In this work, we tested canonical and domain based pair natural orbital coupled cluster methods (CCSD(T) and DLPNO-CCSD(T), respectively) for a set of 32 ligand exchange and association/dissociation reaction enthalpies involving ionic complexes of Li, Be, Na, Mg, Ca, Sr, Ba and Pb(II). Two strategies were investigated: in the former, only valence electrons were included in the correlation treatment, giving rise to the computationally very efficient FC (frozen core) approach; in the latter, all non-ECP electrons were included in the correlation treatment, giving rise to the AE (all electron) approach. Apart from reactions involving Li and Be, the FC approach resulted in non-homogeneous performance. The FC approach leads to very small errors (<2 kcal mol−1) for some reactions of Na, Mg, Ca, Sr, Ba and Pb, while for a few reactions of Ca and Ba deviations up to 40 kcal mol−1 have been obtained. Large errors are both due to artificial mixing of the core (sub-valence) orbitals of metals and the valence orbitals of oxygen and halogens in the molecular orbitals treated as core, and due to neglecting core–core and core–valence correlation effects. These large errors are reduced to a few kcal mol−1 if the AE approach is used or the sub-valence orbitals of metals are included in the correlation treatment. On the technical side, the CCSD(T) and DLPNO-CCSD(T) results differ by a fraction of kcal mol−1, indicating the latter method as the perfect choice when the CPU efficiency is essential. For completely black-box applications, as requested in catalysis or thermochemical calculations, we recommend the DLPNO-CCSD(T) method with all electrons that are not covered by effective core potentials included in the correlation treatment and correlation-consistent polarized core valence basis sets of cc-pwCVQZ(-PP) quality.
Co-reporter:Shengfa Ye, Claudia Kupper, Steffen Meyer, Erik Andris, Rafael Navrátil, Oliver Krahe, Bhaskar Mondal, Mihail Atanasov, Eckhard Bill, Jana Roithová, Franc Meyer, and Frank Neese
Journal of the American Chemical Society 2016 Volume 138(Issue 43) pp:14312-14325
Publication Date(Web):September 28, 2016
DOI:10.1021/jacs.6b07708
In biology, high valent oxo–iron(IV) species have been shown to be pivotal intermediates for functionalization of C–H bonds in the catalytic cycles of a range of O2-activating iron enzymes. This work details an electronic-structure investigation of [FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe–O stretching vibration at 832 ± 3 cm–1. By analyzing the Franck–Condon progression, we can determine the same vibration occurring at 616 ± 10 cm–1 in the E(dxy → dxz,yz) excited state. Both values are similar to those measured for [FeIV(O)(TMC)(NCMe)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm–1. We can unequivocally assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation energy for dxz,yz → dx2-y2 is significantly higher (∼12 000 cm–1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2-y2 orbital to eventually lift it above dz2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet–triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.
Co-reporter:Daniel Aravena, Mihail Atanasov, and Frank Neese
Inorganic Chemistry 2016 Volume 55(Issue 9) pp:4457-4469
Publication Date(Web):April 7, 2016
DOI:10.1021/acs.inorgchem.6b00244
Regularities among electronic configurations for common oxidation states in lanthanide complexes and the low involvement of f orbitals in bonding result in the appearance of several periodic trends along the lanthanide series. These trends can be observed on relatively different properties, such as bonding distances or ionization potentials. Well-known concepts like the lanthanide contraction, the double–double (tetrad) effect, and the similar chemistry along the lanthanide series stem from these regularities. Periodic trends on structural and spectroscopic properties are examined through complete active space self-consistent field (CASSCF) followed by second-order N-electron valence perturbation theory (NEVPT2) including both scalar relativistic and spin–orbit coupling effects. Energies and wave functions from electronic structure calculations are further analyzed in terms of ab initio ligand field theory (AILFT), which allows one to rigorously extract angular overlap model ligand field, Racah, and spin–orbit coupling parameters directly from high-level ab initio calculations. We investigated the elpasolite Cs2NaLnIIICl6 (LnIII = Ce–Nd, Sm–Eu, Tb–Yb) crystals because these compounds have been synthesized for most LnIII ions. Cs2NaLnIIICl6 elpasolites have been also thoroughly characterized with respect to their spectroscopic properties, providing an exceptionally vast and systematic experimental database allowing one to analyze the periodic trends across the lanthanide series. Particular attention was devoted to the apparent discrepancy in metal–ligand covalency trends between theory and spectroscopy described in the literature. Consistent with earlier studies, natural population analysis indicates an increase in covalency along the series, while a decrease in both the nephelauxetic (Racah) and relativistic nephelauxetic (spin–orbit coupling) reduction with increasing atomic number is calculated. These apparently conflicting results are discussed on the basis of AILFT parameters. The AILFT derived parameters faithfully reproduce the underlying multireference electronic structure calculations. The remaining discrepancies with respect to experimentally derived data are mostly due to underestimation of the ligand field splittings, while the dynamic correlation and nephelauxetic effects appears to be adequately covered by CASSCF/NEVPT2.
Co-reporter:Miho Isegawa, Frank Neese, and Dimitrios A. Pantazis
Journal of Chemical Theory and Computation 2016 Volume 12(Issue 5) pp:2272-2284
Publication Date(Web):April 11, 2016
DOI:10.1021/acs.jctc.6b00252
The calculation of redox potentials involves large energetic terms arising from gas phase ionization energies, thermodynamic contributions, and solvation energies of the reduced and oxidized species. In this work we study the performance of a wide range of wave function and density functional theory methods for the prediction of ionization energies and aqueous one-electron oxidation potentials of a set of 19 organic molecules. Emphasis is placed on evaluating methods that employ the computationally efficient local pair natural orbital (LPNO) approach, as well as several implementations of coupled cluster theory and explicitly correlated F12 methods. The electronic energies are combined with implicit solvation models for the solvation energies. With the exception of MP2 and its variants, which suffer from enormous errors arising at least partially from the poor Hartree–Fock reference, ionization energies can be systematically predicted with average errors below 0.1 eV for most of the correlated wave function based methods studies here, provided basis set extrapolation is performed. LPNO methods are the most efficient way to achieve this type of accuracy. DFT methods show in general larger errors and suffer from inconsistent behavior. The only exception is the M06-2X functional which is found to be competitive with the best LPNO-based approaches for ionization energies. Importantly, the limiting factor for the calculation of accurate redox potentials is the solvation energy. The errors in the predicted solvation energies by all continuum solvation models tested in this work dominate the final computed reduction potential, resulting in average errors typically in excess of 0.3 V and hence obscuring the gains that arise from choosing a more accurate electronic structure method.
Co-reporter:Wolfgang B. Schneider, Giovanni Bistoni, Manuel Sparta, Masaaki Saitow, Christoph Riplinger, Alexander A. Auer, and Frank Neese
Journal of Chemical Theory and Computation 2016 Volume 12(Issue 10) pp:4778-4792
Publication Date(Web):August 26, 2016
DOI:10.1021/acs.jctc.6b00523
The local coupled cluster method DLPNO-CCSD(T) allows calculations on systems containing hundreds of atoms to be performed while typically reproducing canonical CCSD(T) energies with chemical accuracy. In this work, we present a scheme for decomposing the DLPNO-CCSD(T) interaction energy between two molecules into physical meaningful contributions, providing a quantification of the most important components of the chemical interaction. The method, called Local Energy Decomposition (LED), is straightforward and requires negligible additional computing time. Both the Hartree–Fock and the correlation energy are decomposed into contributions from localized or pairs of localized occupied orbitals. Assigning these localized orbitals to fragments allows one to differentiate between intra- and intermolecular contributions to the interaction energy. Accordingly, the interaction energy can be decomposed into electronic promotion, electrostatic, exchange, dynamic charge polarization, and dispersion contributions. The LED scheme is applied to a number of test cases ranging from weakly, dispersively bound complexes to systems with strong ionic interactions. The dependence of the results on the one-particle basis set and various technical aspects, such as the localization scheme, are carefully studied in order to ensure that the results do not suffer from technical artifacts. A numerical comparison between the DLPNO-CCSD(T)/LED and the popular symmetry adapted perturbation theory (DFT-SAPT) is made, and the limitations of the proposed scheme are discussed.
Co-reporter:Daniel Aravena, Frank Neese, and Dimitrios A. Pantazis
Journal of Chemical Theory and Computation 2016 Volume 12(Issue 3) pp:1148-1156
Publication Date(Web):February 3, 2016
DOI:10.1021/acs.jctc.5b01048
Improved versions of the segmented all-electron relativistically contracted (SARC) basis sets for the lanthanides are presented. The second-generation SARC2 basis sets maintain efficient construction of their predecessors and their individual adaptation to the DKH2 and ZORA Hamiltonians, but feature exponents optimized with a completely new orbital shape fitting procedure and a slightly expanded f space that results in sizable improvement in CASSCF energies and in significantly more accurate prediction of spin–orbit coupling parameters. Additionally, an extended set of polarization/correlation functions is constructed that is appropriate for multireference correlated calculations and new auxiliary basis sets for use in resolution-of-identity (density-fitting) approximations in combination with both DFT and wave function based treatments. Thus, the SARC2 basis sets extend the applicability of the first-generation DFT-oriented basis sets to routine all-electron wave function-based treatments of lanthanide complexes. The new basis sets are benchmarked with respect to excitation energies, radial distribution functions, optimized geometries, orbital eigenvalues, ionization potentials, and spin–orbit coupling parameters of lanthanide systems and are shown to be suitable for the description of magnetic and spectroscopic properties using both DFT and multireference wave function-based methods.
Co-reporter:Vera Krewald, Frank Neese and Dimitrios A. Pantazis
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 16) pp:10739-10750
Publication Date(Web):07 Jan 2016
DOI:10.1039/C5CP07213A
The redox potential of synthetic oligonuclear transition metal complexes has been shown to correlate with the Lewis acidity of a redox-inactive cation connected to the redox-active transition metals of the cluster via oxo or hydroxo bridges. Such heterometallic clusters are important cofactors in many metalloenzymes, where it is speculated that the redox-inactive constituent ion of the cluster serves to optimize its redox potential for electron transfer or catalysis. A principal example is the oxygen-evolving complex in photosystem II of natural photosynthesis, a Mn4CaO5 cofactor that oxidizes water into dioxygen, protons and electrons. Calcium is critical for catalytic function, but its precise role is not yet established. In analogy to synthetic complexes it has been suggested that Ca2+ fine-tunes the redox potential of the manganese cluster. Here we evaluate this hypothesis by computing the relative redox potentials of substituted derivatives of the oxygen-evolving complex with the cations Sr2+, Gd3+, Cd2+, Zn2+, Mg2+, Sc3+, Na+ and Y3+ for two sequential transitions of its catalytic cycle. The theoretical approach is validated with a series of experimentally well-characterized Mn3AO4 cubane complexes that are structural mimics of the enzymatic cluster. Our results reproduce perfectly the experimentally observed correlation between the redox potential and the Lewis acidities of redox-inactive cations for the synthetic complexes. However, it is conclusively demonstrated that this correlation does not hold for the oxygen evolving complex. In the enzyme the redox potential of the cluster only responds to the charge of the redox-inactive cations and remains otherwise insensitive to their precise identity, precluding redox-tuning of the metal cluster as a primary role for Ca2+ in biological water oxidation.
Co-reporter:Wolfram Ratzke, Lisa Schmitt, Hideto Matsuoka, Christoph Bannwarth, Marius Retegan, Sebastian Bange, Philippe Klemm, Frank Neese, Stefan Grimme, Olav Schiemann, John M. Lupton, and Sigurd Höger
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 22) pp:4802-4808
Publication Date(Web):October 27, 2016
DOI:10.1021/acs.jpclett.6b01907
Metal-free dual singlet–triplet organic light-emitting diode (OLED) emitters can provide direct insight into spin statistics, spin correlations and spin relaxation phenomena, through a comparison of fluorescence to phosphorescence intensity. Remarkably, such materials can also function at room temperature, exhibiting phosphorescence lifetimes of several milliseconds. Using electroluminescence, quantum chemistry, and electron paramagnetic resonance spectroscopy, we investigate the effect of the conjugation pathway on radiative and nonradiative relaxation of the triplet state in phenazine-based compounds and demonstrate that the contribution of the phenazine nπ* excited state is crucial to enabling phosphorescence.
Co-reporter:Adam Kubas, Daniel Berger, Harald Oberhofer, Dimitrios Maganas, Karsten Reuter, and Frank Neese
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 20) pp:4207-4212
Publication Date(Web):October 3, 2016
DOI:10.1021/acs.jpclett.6b01845
Coupled-cluster theory with single, double, and perturbative triple excitations (CCSD(T)) is widely considered to be the “gold standard” of ab initio quantum chemistry. Using the domain-based pair natural orbital local correlation concept (DLPNO-CCSD(T)), these calculations can be performed on systems with hundreds of atoms at an accuracy of ∼99.9% of the canonical CCSD(T) method. This allows for ab initio calculations providing reference adsorption energetics at solid surfaces with an accuracy approaching 1 kcal/mol. This is an invaluable asset, not least for the assessment of density functional theory (DFT) as the prevalent approach for large-scale production calculations in energy or catalysis applications. Here we use DLPNO-CCSD(T) with embedded cluster models to compute entire adsorbate potential energy surfaces for the binding of a set of prototypical closed-shell molecules (H2O, NH3, CH4, CH3OH, CO2) to the rutile TiO2(110) surface. The DLPNO-CCSD(T) calculations show excellent agreement with available experimental data, even for the “infamous” challenge of correctly predicting the CO2 adsorption geometry. The numerical efficiency of the approach is within 1 order of magnitude of hybrid-level DFT calculations, hence blurring the borders between reference and production technique.
Co-reporter:David Schweinfurth; Michael G. Sommer; Mihail Atanasov; Serhiy Demeshko; Stephan Hohloch; Franc Meyer; Frank Neese;Biprajit Sarkar
Journal of the American Chemical Society 2015 Volume 137(Issue 5) pp:1993-2005
Publication Date(Web):January 14, 2015
DOI:10.1021/ja512232f
The azido ligand is one of the most investigated ligands in magnetochemistry. Despite its importance, not much is known about the ligand field of the azido ligand and its influence on magnetic anisotropy. Here we present the electronic structure of a novel five-coordinate Co(II)–azido complex (1), which has been characterized experimentally (magnetically and by electronic d–d absorption spectroscopy) and theoretically (by means of multireference electronic structure methods). Static and dynamic magnetic data on 1 have been collected, and the latter demonstrate slow relaxation of the magnetization in an applied external magnetic field of H = 3000 Oe. The zero-field splitting parameters deduced from static susceptibility and magnetizations (D = −10.7 cm–1, E/D = 0.22) are in excellent agreement with the value of D inferred from an Arrhenius plot of the magnetic relaxation time versus the temperature. Application of the so-called N-electron valence second-order perturbation theory (NEVPT2) resulted in excellent agreement between experimental and computed energies of low-lying d–d transitions. Calculations were performed on 1 and a related four-coordinate Co(II)–azido complex lacking a fifth axial ligand (2). On the basis of these results and contrary to previous suggestions, the N3– ligand is shown to behave as a strong σ and π donor. Magnetostructural correlations show a strong increase in the negative D with increasing Lewis basicity (shortening of the Co–N bond distances) of the axial ligand on the N3– site. The effect on the change in sign of D in going from four-coordinate Co(II) (positive D) to five-coordinate Co(II) (negative D) is discussed in the light of the bonding scheme derived from ligand field analysis of the ab initio results.
Co-reporter:Shang-Da Jiang; Dimitrios Maganas; Nikolaos Levesanos; Eleftherios Ferentinos; Sabrina Haas; Komalavalli Thirunavukkuarasu; J. Krzystek; Martin Dressel; Lapo Bogani; Frank Neese;Panayotis Kyritsis
Journal of the American Chemical Society 2015 Volume 137(Issue 40) pp:12923-12928
Publication Date(Web):September 9, 2015
DOI:10.1021/jacs.5b06716
The high-spin (S = 1) tetrahedral NiII complex [Ni{iPr2P(Se)NP(Se)iPr2}2] was investigated by magnetometry, spectroscopic, and quantum chemical methods. Angle-resolved magnetometry studies revealed the orientation of the magnetization principal axes. The very large zero-field splitting (zfs), D = 45.40(2) cm−1, E = 1.91(2) cm−1, of the complex was accurately determined by far-infrared magnetic spectroscopy, directly observing transitions between the spin sublevels of the triplet ground state. These are the largest zfs values ever determined—directly—for a high-spin NiII complex. Ab initio calculations further probed the electronic structure of the system, elucidating the factors controlling the sign and magnitude of D. The latter is dominated by spin−orbit coupling contributions of the Ni ions, whereas the corresponding effects of the Se atoms are remarkably smaller.
Co-reporter:Martha A. Beckwith; William Ames; Fernando D. Vila; Vera Krewald; Dimitrios A. Pantazis; Claire Mantel; Jacques Pécaut; Marcello Gennari; Carole Duboc; Marie-Noëlle Collomb; Junko Yano; John J. Rehr; Frank Neese;Serena DeBeer
Journal of the American Chemical Society 2015 Volume 137(Issue 40) pp:12815-12834
Publication Date(Web):September 9, 2015
DOI:10.1021/jacs.5b00783
First principle calculations of extended X-ray absorption fine structure (EXAFS) data have seen widespread use in bioinorganic chemistry, perhaps most notably for modeling the Mn4Ca site in the oxygen evolving complex (OEC) of photosystem II (PSII). The logic implied by the calculations rests on the assumption that it is possible to a priori predict an accurate EXAFS spectrum provided that the underlying geometric structure is correct. The present study investigates the extent to which this is possible using state of the art EXAFS theory. The FEFF program is used to evaluate the ability of a multiple scattering-based approach to directly calculate the EXAFS spectrum of crystallographically defined model complexes. The results of these parameter free predictions are compared with the more traditional approach of fitting FEFF calculated spectra to experimental data. A series of seven crystallographically characterized Mn monomers and dimers is used as a test set. The largest deviations between the FEFF calculated EXAFS spectra and the experimental EXAFS spectra arise from the amplitudes. The amplitude errors result from a combination of errors in calculated S02 and Debye–Waller values as well as uncertainties in background subtraction. Additional errors may be attributed to structural parameters, particularly in cases where reliable high-resolution crystal structures are not available. Based on these investigations, the strengths and weaknesses of using first-principle EXAFS calculations as a predictive tool are discussed. We demonstrate that a range of DFT optimized structures of the OEC may all be considered consistent with experimental EXAFS data and that caution must be exercised when using EXAFS data to obtain topological arrangements of complex clusters.
Co-reporter:Shengfa Ye, Genqiang Xue, Itana Krivokapic, Taras Petrenko, Eckhard Bill, Lawrence Que Jr and Frank Neese
Chemical Science 2015 vol. 6(Issue 5) pp:2909-2921
Publication Date(Web):26 Feb 2015
DOI:10.1039/C4SC03268C
High-valent iron(IV)-oxo species are key intermediates in the catalytic cycles of a range of O2-activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([FeIV(O)(L)(NCMe)]2+, 1, L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)FeIV(O)(μ-O)FeIV(OH)(L)]3+, 2) iron(IV) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1, the experimental MCD spectra at 2–10 K are dominated by a broad positive band between 12000 and 18000 cm−1. As the temperature increases up to ∼20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1. The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1, the electronic structure of the FeIVO site is not significantly perturbed by the binding to another iron(IV) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C–H bond activation and O-atom transfer.
Co-reporter:Elizaveta A. Suturina, Dimitrios Maganas, Eckhard Bill, Mihail Atanasov, and Frank Neese
Inorganic Chemistry 2015 Volume 54(Issue 20) pp:9948-9961
Publication Date(Web):October 7, 2015
DOI:10.1021/acs.inorgchem.5b01706
Over the past several decades, tremendous efforts have been invested in finding molecules that display slow relaxation of magnetization and hence act as single-molecule magnets (SMMs). While initial research was strongly focused on polynuclear transition metal complexes, it has become increasingly evident that SMM behavior can also be displayed in relatively simple mononuclear transition metal complexes. One of the first examples of a mononuclear SMM that shows a slow relaxation of the magnetization in the absence of an external magnetic field is the cobalt(II) tetra-thiolate [Co(SPh)4]2–. Fascinatingly, substitution of the donor ligand atom by oxygen or selenium dramatically changes zero-field splitting (ZFS) and relaxation time. Clearly, these large variations call for an in-depth electronic structure investigation in order to develop a qualitative understanding of the observed phenomena. In this work, we present a systematic theoretical study of a whole series of complexes (PPh4)2[Co(XPh)4] (X = O, S, Se) using multireference ab initio methods. To this end, we employ the recently proposed ab initio ligand field theory, which allows us to translate the ab initio results into the framework of ligand field theory. Magneto-structural correlations are then developed that take into account the nature of metal–ligand covalent bonding, ligand spin–orbit coupling, and geometric distortions away from pure tetrahedral symmetry. The absolute value of zero-field splitting increases when the ligand field strength decreases across the series from O to Te. The zero-field splitting of the ground state of the hypothetical [Co(TePh)4]2– complex is computed to be about twice as large as for the well-known (PPh4)2[Co(SPh)4] compound. It is shown that due to the π-anisotropy of the ligand donor atoms (S, Se) magneto-structural correlations in [Co(OPh)4]2– complex differ from [Co(S/SePh)4]2–. In the case of almost isotropic OPh ligand, only variations in the first coordination sphere affect magnetic properties, but in the case of S/SePh ligand, variations in the first and second coordination sphere become equally important for magnetic properties.
Co-reporter:Shelby E. Stavretis, Mihail Atanasov, Andrey A. Podlesnyak, Seth C. Hunter, Frank Neese, and Zi-Ling Xue
Inorganic Chemistry 2015 Volume 54(Issue 20) pp:9790-9801
Publication Date(Web):October 2, 2015
DOI:10.1021/acs.inorgchem.5b01505
Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe(TPP)X] (X = F, Br, I; H2TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are D = 4.49(9) cm–1 for tetragonal polycrystalline [Fe(TPP)F], and D = 8.8(2) cm–1, E = 0.1(2) cm–1 and D = 13.4(6) cm–1, E = 0.3(6) cm–1 for monoclinic polycrystalline [Fe(TPP)Br] and [Fe(TPP)I], respectively. Along with our recent report of the ZFS value of D = 6.33(8) cm–1 for tetragonal polycrystalline [Fe(TPP)Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe(TPP)X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting D of the 6A1 ground state. D was calculated from wave functions of the electronic multiplets spanned by the d5 configuration of Fe(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of D from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the S = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX63– complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of D and decrease of the π- and σ-antibonding energies eλX (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. D parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental D and spectroscopic transitions for high-spin Fe(III) complexes are proposed.
Co-reporter:Steffen Meyer, Oliver Krahe, Claudia Kupper, Iris Klawitter, Serhiy Demeshko, Eckhard Bill, Frank Neese, and Franc Meyer
Inorganic Chemistry 2015 Volume 54(Issue 20) pp:9770-9776
Publication Date(Web):October 7, 2015
DOI:10.1021/acs.inorgchem.5b01446
A disulfide-bridged diiron complex with [Fe–S–S–Fe] core, which represents an isomer of the common biological [2Fe–2S] ferredoxin-type clusters, was synthesized using strongly σ-donating macrocyclic tetracarbene capping ligands. Though the complex is quite labile in solution, single crystals were obtained, and the structure was elucidated by X-ray diffraction. The electron-rich iron–sulfur core is found to show rather unusual magnetic and electronic properties. Experimental data and density functional theory studies indicate extremely strong antiferromagnetic coupling (−J > 800 cm–1) between two low-spin iron(III) ions via the S22– bridge, and the intense near-IR absorption characteristic for the [Fe–S–S–Fe] core was assigned to a S → Fe ligand-to-metal charge transfer transition.
Co-reporter:Dimitrios G. Liakos and Frank Neese
Journal of Chemical Theory and Computation 2015 Volume 11(Issue 9) pp:4054-4063
Publication Date(Web):July 24, 2015
DOI:10.1021/acs.jctc.5b00359
The recently developed domain-based local pair natural orbital coupled cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) delivers results that are closely approaching those of the parent canonical coupled cluster method at a small fraction of the computational cost. A recent extended benchmark study established that, depending on the three main truncation thresholds, it is possible to approach the canonical CCSD(T) results within 1 kJ (default setting, TightPNO), 1 kcal/mol (default setting, NormalPNO), and 2–3 kcal (default setting, LoosePNO). Although thresholds for calculations with TightPNO are 2–4 times slower than those based on NormalPNO thresholds, they are still many orders of magnitude faster than canonical CCSD(T) calculations, even for small and medium sized molecules where there is little locality. The computational effort for the coupled cluster step scales nearly linearly with system size. Since, in many instances, the coupled cluster step in DLPNO-CCSD(T) is cheaper or at least not much more expensive than the preceding Hartree–Fock calculation, it is useful to compare the method against modern density functional theory (DFT), which requires an effort comparable to that of Hartree–Fock theory (at least if Hartree–Fock exchange is part of the functional definition). Double hybrid density functionals (DHDF’s) even require a MP2-like step. The purpose of this article is to evaluate the cost vs accuracy ratio of DLPNO-CCSD(T) against modern DFT (including the PBE, B3LYP, M06-2X, B2PLYP, and B2GP-PLYP functionals and, where applicable, their van der Waals corrected counterparts). To eliminate any possible bias in favor of DLPNO-CCSD(T), we have chosen established benchmark sets that were specifically proposed for evaluating DFT functionals. It is demonstrated that DLPNO-CCSD(T) with any of the three default thresholds is more accurate than any of the DFT functionals. Furthermore, using the aug-cc-pVTZ basis set and the LoosePNO default settings, DLPNO-CCSD(T) is only about 1.2 times slower than B3LYP. With NormalPNO thresholds, DLPNO-CCSD(T) is about a factor of 2 slower than B3LYP and shows a mean absolute deviation of less than 1 kcal/mol to the reference values for the four different data sets used. Our conclusion is that coupled cluster energies can indeed be obtained at near DFT cost.
Co-reporter:Wulf Thimm; Christian Gradert; Henning Broda; Frank Wennmohs; Frank Neese;Felix Tuczek
Inorganic Chemistry 2015 Volume 54(Issue 19) pp:9248-9255
Publication Date(Web):June 24, 2015
DOI:10.1021/acs.inorgchem.5b00787
A series of density functional theory (DFT) calculations on the full [MoHIPTN3N] catalyst are performed to obtain an energy profile of the Schrock cycle. This is a continuation of our earlier investigation of this cycle in which the bulky hexaisopropyterphenyl (HIPT) substituents of the ligand were replaced by hydrogen atoms (Angew. Chem., Int. Ed. 2005, 44, 5639). In an effort to provide a treatment that is as converged as possible from a quantum-chemical point of view, the present study now fully takes the HIPT moieties into account. Moreover, structures and energies are calculated with a near-saturated basis set, leading to models with 280 atoms and 4850 basis functions. Solvent and scalar relativistic effects have been treated using the conductor-like screening model and zeroth-order regular approximation, respectively. Free reaction enthalpies are evaluated using the PBE and B3LYP functionals. A comparison to the available experimental data reveals much better agreement with the experiment than preceding DFT treatments of the Schrock cycle. In particular, free reaction enthalpies of reduction steps and NH3/N2 exchange are now excellently reproduced.
Co-reporter:Ondřej Demel and Jiří Pittner and Frank Neese
Journal of Chemical Theory and Computation 2015 Volume 11(Issue 7) pp:3104-3114
Publication Date(Web):June 11, 2015
DOI:10.1021/acs.jctc.5b00334
This paper reports the development of a local variant of Mukherjee’s state-specific multireference coupled cluster method based on the pair natural orbital approach (LPNO-MkCC). The current implementation is restricted to single and double excitations. The performance of the LPNO-MkCCSD method was tested on calculations of naphthyne isomers, tetramethyleneethane, and β-carotene molecules. The results show that 99.7–99.8% of correlation energy was recovered with respect to the MkCC method based on canonical orbitals. Moreover, the errors of relative energies between different isomers or along a potential energy curve (with respect to the canonical method) are below 0.4 kcal/mol, safely within the chemical accuracy. The computational efficiency of our implementation of LPNO-MkCCSD in the ORCA program allows calculation of the β-carotene molecule (96 atoms and 1984 basis functions) on a single CPU core.
Co-reporter:Dimitrios G. Liakos and Frank Neese
Journal of Chemical Theory and Computation 2015 Volume 11(Issue 5) pp:2137-2143
Publication Date(Web):April 20, 2015
DOI:10.1021/acs.jctc.5b00265
In this study the question of what is the last unbranched alkane that prefers a linear conformation over a folded one is revisited from a theoretical point of view. Geometries have been optimized carefully using the most accurate theoretical approach available to date for such systems, namely, doubly hybrid density functional theory in conjunction with larger quadruple-ζ quality basis sets. The resulting geometries deviate significantly from previously reported ones and have a significant impact on the predicted energetics. Electronic energies were calculated using the efficient and accurate domain local pair natural orbital coupled cluster method with single-, double-, and triple substitutions (DLPNO-CCSD(T)) electronic structure method. Owing to the method’s efficiency, we were able to employ up to quadruple-ζ quality basis sets for all hydrocarbons up to C19H40. In conjunction with carefully designed basis set extrapolation techniques, it is estimated that the electronic energies reported in this study deviate less than 1 kJ/mol from the canonical CCSD(T) basis set limit. Thermodynamic corrections were calculated with the PW6B95-D3 functional and the def2-QZVP basis set. Our prediction is that the last linear conformer is either C16H34 or C17H36 with the latter being more probable. C18H38 can be safely ruled out as the most stable isomer at 100 K. These findings are in agreement with the elegant experimental studies of Suhm and co-workers. Deviations between the current and previous theoretical results are analyzed in detail.
Co-reporter:Dimitrios G. Liakos, Manuel Sparta, Manoj K. Kesharwani, Jan M. L. Martin, and Frank Neese
Journal of Chemical Theory and Computation 2015 Volume 11(Issue 4) pp:1525-1539
Publication Date(Web):March 10, 2015
DOI:10.1021/ct501129s
The domain based local pair natural orbital coupled cluster method with single-, double-, and perturbative triple excitations (DLPNO–CCSD(T)) is an efficient quantum chemical method that allows for coupled cluster calculations on molecules with hundreds of atoms. Because coupled-cluster theory is the method of choice if high-accuracy is needed, DLPNO–CCSD(T) is very promising for large-scale chemical application. However, the various approximations that have to be introduced in order to reach near linear scaling also introduce limited deviations from the canonical results. In the present work, we investigate how far the accuracy of the DLPNO–CCSD(T) method can be pushed for chemical applications. We also address the question at which additional computational cost improvements, relative to the previously established default scheme, come. To answer these questions, a series of benchmark sets covering a broad range of quantum chemical applications including reaction energies, hydrogen bonds, and other noncovalent interactions, conformer energies, and a prototype organometallic problem were selected. An accuracy of 1 kcal/mol or better can readily be obtained for all data sets using the default truncation scheme, which corresponds to the stated goal of the original implementation. Tightening of the three thresholds that control DLPNO leads to mean absolute errors and standard deviations from the canonical results of less than 0.25 kcal/mol (<1 kJ/mol). The price one has then to pay is an increased computational time by a factor close to 3. The applicability of the method is shown to be independent of the nature of the reaction. On the basis of the careful analysis of the results, three different sets of truncation thresholds (termed “LoosePNO”, “NormalPNO”, and “TightPNO”) have been chosen for “black box” use of DLPNO–CCSD(T). This will allow users of the method to optimally balance performance and accuracy.
Co-reporter:Barbara Kirchner and Frank Neese
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 22) pp:14268-14269
Publication Date(Web):31 Mar 2015
DOI:10.1039/C5CP90040A
A graphical abstract is available for this content
Co-reporter:Vera Krewald;Dimitrios A. Pantazis
Israel Journal of Chemistry 2015 Volume 55( Issue 11-12) pp:1219-1232
Publication Date(Web):
DOI:10.1002/ijch.201500051
Abstract
A frequent challenge when dealing with multinuclear transition metal clusters in biology is to determine the absolute oxidation states of the individual metal ions and to identify how they evolve during catalytic turnover. The oxygen-evolving complex of biological photosynthesis, an active site that harbors an oxo-bridged Mn4Ca cluster as the water-oxidizing species, offers a prime example of such a challenge that withstood satisfactory resolution for decades. A multitude of experimental studies have approached this question and have offered insights from different angles, but they were also accompanied by incomplete or inconclusive interpretations. Only very recently, through a combination of experiment and theory, has a definitive assignment of the individual Mn oxidation states been achieved for all observable catalytic states of the complex. Here we review the information obtained by structural and spectroscopic methods, describe the interpretation and synthesis achieved through quantum chemistry, and summarize our current understanding of the electronic structure of nature’s water splitting catalyst.
Co-reporter:Mihail Atanasov, Daniel Aravena, Elizaveta Suturina, Eckhard Bill, Dimitrios Maganas, Frank Neese
Coordination Chemistry Reviews 2015 s 289–290() pp: 177-214
Publication Date(Web):
DOI:10.1016/j.ccr.2014.10.015
Co-reporter:Manuel Sparta and Frank Neese
Chemical Society Reviews 2014 vol. 43(Issue 14) pp:5032-5041
Publication Date(Web):27 Mar 2014
DOI:10.1039/C4CS00050A
The scope of this review is to provide a brief overview of the chemical applications carried out by local pair natural orbital coupled-electron pair and coupled-cluster methods. Benchmark tests reveal that these methods reproduce, with excellent accuracy, their canonical counterparts. At the same time, the speed up achieved by exploiting the locality of the electron correlation permits us to tackle chemical systems that, due to their size, would normally only be addressable with density functional theory. This review covers a broad variety of the chemical applications e.g. simulation of transition metal catalyzed reactions, estimation of weak interactions, and calculation of lattice properties in molecular crystals. This demonstrates that modern implementations of wavefunction-based correlated methods are playing an increasingly important role in applied computational chemistry.
Co-reporter:Thomas U. Nick; Wankyu Lee; Simone Koßmann; Frank Neese; JoAnne Stubbe;Marina Bennati
Journal of the American Chemical Society 2014 Volume 137(Issue 1) pp:289-298
Publication Date(Web):December 16, 2014
DOI:10.1021/ja510513z
Ribonucleotide reductases (RNRs) catalyze the conversion of ribonucleotides to deoxyribonucleotides in all organisms. In all Class Ia RNRs, initiation of nucleotide diphosphate (NDP) reduction requires a reversible oxidation over 35 Å by a tyrosyl radical (Y122•, Escherichia coli) in subunit β of a cysteine (C439) in the active site of subunit α. This radical transfer (RT) occurs by a specific pathway involving redox active tyrosines (Y122 ⇆ Y356 in β to Y731 ⇆ Y730 ⇆ C439 in α); each oxidation necessitates loss of a proton coupled to loss of an electron (PCET). To study these steps, 3-aminotyrosine was site-specifically incorporated in place of Y356-β, Y731- and Y730-α, and each protein was incubated with the appropriate second subunit β(α), CDP and effector ATP to trap an amino tyrosyl radical (NH2Y•) in the active α2β2 complex. High-frequency (263 GHz) pulse electron paramagnetic resonance (EPR) of the NH2Y•s reported the gx values with unprecedented resolution and revealed strong electrostatic effects caused by the protein environment. 2H electron–nuclear double resonance (ENDOR) spectroscopy accompanied by quantum chemical calculations provided spectroscopic evidence for hydrogen bond interactions at the radical sites, i.e., two exchangeable H bonds to NH2Y730•, one to NH2Y731• and none to NH2Y356•. Similar experiments with double mutants α-NH2Y730/C439A and α-NH2Y731/Y730F allowed assignment of the H bonding partner(s) to a pathway residue(s) providing direct evidence for colinear PCET within α. The implications of these observations for the PCET process within α and at the interface are discussed.
Co-reporter:Christopher J. Pollock ; Mario Ulises Delgado-Jaime ; Mihail Atanasov ; Frank Neese ;Serena DeBeer
Journal of the American Chemical Society 2014 Volume 136(Issue 26) pp:9453-9463
Publication Date(Web):June 10, 2014
DOI:10.1021/ja504182n
The mainline feature in metal Kβ X-ray emission spectroscopy (XES) has long been recognized as an experimental marker for the spin state of the metal center. However, even within a series of metal compounds with the same nominal oxidation and spin state, significant changes are observed that cannot be explained on the basis of overall spin. In this work, the origin of these effects is explored, both experimentally and theoretically, in order to develop the chemical information content of Kβ mainline XES. Ligand field expressions are derived that describe the behavior of Kβ mainlines for first row transition metals with any dn count, allowing for a detailed analysis of the factors governing mainline shape. Further, due to limitations associated with existing computational approaches, we have developed a new methodology for calculating Kβ mainlines using restricted active space configuration interaction (RAS–CI) calculations. This approach eliminates the need for empirical parameters and provides a powerful tool for investigating the effects that chemical environment exerts on the mainline spectra. On the basis of a detailed analysis of the intermediate and final states involved in these transitions, we confirm the known sensitivity of Kβ mainlines to metal spin state via the 3p–3d exchange coupling. Further, a quantitative relationship between the splitting of the Kβ mainline features and the metal–ligand covalency is established. Thus, this study furthers the quantitative electronic structural information that can be extracted from Kβ mainline spectroscopy.
Co-reporter:Wenjing Liu, Jonathan H. Christian, Rami Al-Oweini, Bassem S. Bassil, Johan van Tol, Mihail Atanasov, Frank Neese, Naresh S. Dalal, and Ulrich Kortz
Inorganic Chemistry 2014 Volume 53(Issue 17) pp:9274-9283
Publication Date(Web):August 19, 2014
DOI:10.1021/ic501385r
Two monochromium(III)-containing heteropolytungstates, [CrIII(HPVW7O28)2]13- (1a) and [CrIII(HAsVW7O28)2]13- (2a), were prepared via simple, one-pot reactions in aqueous, basic medium, by reaction of the composing elements, and then isolated as hydrated sodium salts, Na13[CrIII(HPVW7O28)2]·47H2O (1) and Na13[CrIII(HAsVW7O28)2]·52H2O (2). Polyanions 1a and 2a comprise an octahedrally coordinated CrIII ion, sandwiched by two {PW7} or {AsW7} units. Both compounds 1 and 2 were fully characterized in the solid state by single-crystal XRD, IR spectroscopy, thermogravimetric and elemental analyses, magnetic susceptibility, and EPR measurements. Magnetic studies on 1 and 2 demonstrated that both compounds exhibit appreciable deviation from typical paramagnetic behavior, and have a ground state S = 3/2, as expected for a CrIII ion, but with an exceptionally large zero-field uniaxial anisotropy parameter (D). EPR measurements on powder and single-crystal samples of 1 and 2 using 9.5, 34.5, and 239.2 GHz frequencies and over 4–295 K temperature fully support the magnetization results and show that D = +2.4 cm–1, the largest and sign-assigned D-value so far reported for an octahedral CrIII-containing, molecular compound. Ligand field analysis of results from CASSCF and NEVPT2-correlated electronic structure calculations on Cr(OH)63– model complexes allowed to unravel the crucial role of the second coordination sphere of CrIII for the unusually large magnetic anisotropy reflected by the experimental value of D. The newly developed theoretical modeling, combined with the synthetic procedure for producing such unusual magnetic molecules in a well-defined and essentially magnetically isolated environment, appears to be a versatile new research area.
Co-reporter:Amanda E. King, Michael Nippe, Mihail Atanasov, Teera Chantarojsiri, Curtis A. Wray, Eckhard Bill, Frank Neese, Jeffrey R. Long, and Christopher J. Chang
Inorganic Chemistry 2014 Volume 53(Issue 21) pp:11388-11395
Publication Date(Web):August 6, 2014
DOI:10.1021/ic5010177
The ubiquity of vanadium oxo complexes in the V+ and IV+ oxidation states has contributed to a comprehensive understanding of their electronic structure and reactivity. However, despite being predicted to be stable by ligand-field theory, the isolation and characterization of a well-defined terminal mononuclear vanadium(III) oxo complex has remained elusive. We present the synthesis and characterization of a unique terminal mononuclear vanadium(III) oxo species supported by the pentadentate polypyridyl ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine (PY5Me2). Exposure of [VII(NCCH3)(PY5Me2)]2+ (1) to either dioxygen or selected O-atom-transfer reagents yields [VIV(O)(PY5Me2)]2+ (2). The metal-centered one-electron reduction of this vanadium(IV) oxo complex furnishes a stable, diamagnetic [VIII(O)(PY5Me2)]+ (3) species. The vanadium(III) oxo species is unreactive toward H- and O-atom transfer but readily reacts with protons to form a putative vanadium hydroxo complex. Computational results predict that further one-electron reduction of the vanadium(III) oxo species will result in ligand-based reduction, even though pyridine is generally considered to be a poor π-accepting ligand. These results have implications for future efforts toward low-valent vanadyl chemistry, particularly with regard to the isolation and study of formal vanadium(II) oxo species.
Co-reporter:Manuel Sparta, Christoph Riplinger, and Frank Neese
Journal of Chemical Theory and Computation 2014 Volume 10(Issue 3) pp:1099-1108
Publication Date(Web):January 28, 2014
DOI:10.1021/ct400917j
Since the development of chiral phosphino-oxazoline iridium catalysts, which hydrogenate unfunctionalized alkenes enantioselectively, the asymmetric hydrogenation of prochiral olefins has become important in the production of chiral compounds. For the last 10 years, details of the mechanism, including formal oxidation state assignment of the metal center and the nature of intermediates and transition states have been debated. Various contributions have been given from a theoretical point of view, but due to the size of the structures, these have been forced to rely on density functional theory (DFT) methods. In our investigation of the catalytic cycle, we employ both DFT and a correlated ab initio method, namely, the newly implemented domain-based local pair natural orbital coupled-cluster theory with single and double excitations and the inclusion of perturbative triples correction (DLPNO-CCSD(T)). Our results show that the most likely active paths involve the formation of an intermediate IrV species. Furthermore, we have been able to predict the absolute configuration of the major products, and where comparison to experiment is possible, the results of our calculations agree with the enantiomeric excess obtained from hydrogenating five prochiral substrates. This work also shows that it is now possible to study catalytic reactions with untruncated models (having up to 88 atoms) at the CCSD(T) level of theory.
Co-reporter:Dimitrios Maganas, Serena DeBeer, and Frank Neese
Inorganic Chemistry 2014 Volume 53(Issue 13) pp:6374-6385
Publication Date(Web):May 28, 2014
DOI:10.1021/ic500197v
X-ray metal L-edge spectroscopy has proven to be a powerful technique for investigating the electronic structure of transition-metal centers in coordination compounds and extended solid systems. We have recently proposed the Restricted Open-Shell Configuration Interaction Singles (ROCIS) method and its density functional theory variant (DFT/ROCIS) as methods of general applicability for interpreting such spectra. In this work, we apply the ROCIS and DFT/ROCIS methods for the investigation of cluster systems in order to interpret the Ca and Ti L-edge spectra of CaF2 and TiO2 (rutile and anatase), respectively. Cluster models with up to 23 metallic centers are considered together with the hydrogen saturation and embedding techniques to represent the extended ionic and covalent bulk environments of CaF2 and TiO2. The experimentally probed metal coordination environment is discussed in detail. The influence of local as well as nonlocal effects on the intensity mechanism is investigated. In addition, the physical origin of the observed spectral features is qualitatively and quantitatively discussed through decomposition of the dominant relativistic states in terms of leading individual 2p–3d excitations. This contribution serves as an important reference for future applications of ROCIS and DFT/ROCIS methods in the field of metal L-edge spectroscopy in solid-state chemistry.
Co-reporter:Marius Retegan, Nicholas Cox, Dimitrios A. Pantazis, and Frank Neese
Inorganic Chemistry 2014 Volume 53(Issue 21) pp:11785-11793
Publication Date(Web):October 23, 2014
DOI:10.1021/ic502081c
The interpretation of electron paramagnetic resonance spectra of polynuclear transition metal complexes in terms of individual contributions from each paramagnetic center can be greatly facilitated by the availability of theoretical methods that enable the reliable prediction of local spectroscopic parameters. In this work we report an approach that enables the application of multireference ab initio methods for the calculation of local zero field splitting tensors, one of the leading terms in the spin Hamiltonian for exchange-coupled systems of high nuclearity. The method referred to as local complete active space configuration interaction (L-CASCI) represents a multireference calculation with an active space composed of local orbitals of the center of interest. By successive permutation of the active space to include the localized orbitals corresponding to a particular center of the complex, all on-site parameters can be easily obtained at a high-level of theory with a corresponding low computational cost. Benchmark calculations on synthetic complexes confirm the validity of the approach. As an example of the applicability of the L-CASCI method to large systems, we determine the local anisotropy of the Mn(III) ion of the tetranuclear manganese cluster of photosystem II in both structural forms of its S2 state.
Co-reporter:Caiyun Geng, Shengfa Ye and Frank Neese
Dalton Transactions 2014 vol. 43(Issue 16) pp:6079-6086
Publication Date(Web):09 Jan 2014
DOI:10.1039/C3DT53051E
In this work, the reactions of C–H bond activation by two series of iron-oxo (1 (FeIV), 2 (FeV), 3 (FeVI)) and -nitrido model complexes (4 (FeIV), 5 (FeV), 6 (FeVI)) with a nearly identical coordination geometry but varying iron oxidation states ranging from IV to VI were comprehensively investigated using density functional theory. We found that in a distorted octahedral coordination environment, the iron-oxo species and their isoelectronic nitrido analogues feature totally different intrinsic reactivities toward C–H bond cleavage. In the case of the iron-oxo complexes, the reaction barrier monotonically decreases as the iron oxidation state increases, consistent with the gradually enhanced electrophilicity across the series. The iron-nitrido complex is less reactive than its isoelectronic iron-oxo species, and more interestingly, a counterintuitive reactivity pattern was observed, i.e. the activation barriers essentially remain constant independent of the iron oxidation states. The detailed analysis using the Polanyi principle demonstrates that the different reactivities between these two series originate from the distinct thermodynamic driving forces, more specifically, the bond dissociation energies (BDEE–Hs, E = O, N) of the nascent E–H bonds in the FeE–H products. Further decomposition of the BDEE–Hs into the electron and proton affinity components shed light on how the oxidation states modulate the BDEE–Hs of the two series.
Co-reporter:Dimitrios Maganas, Michael Roemelt, Thomas Weyhermüller, Raoul Blume, Michael Hävecker, Axel Knop-Gericke, Serena DeBeer, Robert Schlögl and Frank Neese
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 1) pp:264-276
Publication Date(Web):24 Oct 2013
DOI:10.1039/C3CP52711E
A series of mononuclear V(V), V(IV) and V(III) complexes were investigated by V L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The spectra show significant sensitivity to the vanadium oxidation state and the coordination environment surrounding the vanadium center. The L-edge spectra are interpreted with the aid of the recently developed Density Functional Theory/Restricted Open Shell Configuration Interaction Singles (DFT/ROCIS) method. This method is calibrated for the prediction of vanadium L-edges with different hybrid density functionals and basis sets. For the B3LYP/def2-TZVP(-f) and BHLYP/def2-TZVP(-f) functional/basis-set combinations, good to excellent agreement between calculated and experimental spectra is obtained. A treatment of the spin–orbit coupling interaction to all orders is achieved by quasi-degenerate perturbation theory (QDPT), in conjunction with DFT/ROCIS for the calculation of the molecular multiplets while accounting for dynamic correlation and anisotropic covalency. The physical origin of the observed spectral features is discussed qualitatively and quantitatively in terms of spin multiplicities, magnetic sublevels and individual 2p to 3d core level excitations. This investigation is an important prerequisite for future applications of the DFT/ROCIS method to vanadium L-edge absorption spectroscopy and vanadium-based heterogeneous catalysts.
Co-reporter:Oliver Krahe;Dr. Eckhard Bill ;Dr. Frank Neese
Angewandte Chemie International Edition 2014 Volume 53( Issue 33) pp:8727-8731
Publication Date(Web):
DOI:10.1002/anie.201403402
Abstract
Cryogenically trapped FeV nitride complexes with cyclam-based ligands were found to decay by bimolecular reactions, forming exclusively FeII compounds. Characterization of educts and products by Mössbauer spectroscopy, mass spectrometry, and spectroscopy-oriented DFT calculations showed that the reaction mechanism is reductive nitride coupling and release of dinitrogen (2 FeVNFeII-NN-FeII2 FeII+N2). The reaction pathways, representing an “inverse” of the Haber–Bosch reaction, were computationally explored in detail, also to judge the feasibility of yielding catalytically competent FeV(N). Implications for the photolytic cleavage of FeIII azides used to generate high-valent Fe nitrides are discussed.
Co-reporter:Oliver Krahe;Dr. Eckhard Bill ;Dr. Frank Neese
Angewandte Chemie 2014 Volume 126( Issue 33) pp:8872-8876
Publication Date(Web):
DOI:10.1002/ange.201403402
Abstract
Kryogen abgefangene FeV-Nitrid-Komplexe mit Cyclam-basierten Liganden zerfallen in bimolekularen Reaktionen und bilden dabei ausschließlich FeII-Verbindungen. Die Charakterisierung der Ausgangsverbindungen und Produkte durch Mößbauer-Spektroskopie, Massenspektrometrie und DFT-Rechnungen ergab, dass die Reaktion über eine reduktive Nitrid-Kupplung und die Freisetzung von Distickstoff verläuft (2 FeVN FeII-NN-FeII 2 FeII+N2). Der Reaktionspfad, der einer “inversen” Haber-Bosch-Reaktion entspricht, wurde mit Computermethoden erforscht, auch um zu klären, ob die Umsetzung katalytisch aktives FeV(N) liefern könnte. Die Ergebnisse bieten Einblick in die photolytische Spaltung von FeIII-Aziden, die zur Erzeugung von hochvalenten Fe-Nitriden genutzt wird.
Co-reporter:Dmytro Bykov;Matthias Plog
JBIC Journal of Biological Inorganic Chemistry 2014 Volume 19( Issue 1) pp:97-112
Publication Date(Web):2014 January
DOI:10.1007/s00775-013-1065-6
In this article, we consider, in detail, the second half-cycle of the six-electron nitrite reduction mechanism catalyzed by cytochrome c nitrite reductase. In total, three electrons and four protons must be provided to reach the final product, ammonia, starting from the HNO intermediate. According to our results, the first event in this half-cycle is the reduction of the HNO intermediate, which is accomplished by two PCET reactions. Two isomeric radical intermediates, HNOH• and H2NO•, are formed. Both intermediates are readily transformed into hydroxylamine, most likely through intramolecular proton transfer from either Arg114 or His277. An extra proton must enter the active site of the enzyme to initiate heterolytic cleavage of the N–O bond. As a result of N–O bond cleavage, the H2N+ intermediate is formed. The latter readily picks up an electron, forming H2N+•, which in turn reacts with Tyr218. Interestingly, evidence for Tyr218 activity was provided by the mutational studies of Lukat (Biochemistry 47:2080, 2008), but this has never been observed in the initial stages of the overall reduction process. According to our results, an intramolecular reaction with Tyr218 in the final step of the nitrite reduction process leads directly to the final product, ammonia. Dissociation of the final product proceeds concomitantly with a change in spin state, which was also observed in the resonance Raman investigations of Martins et al. (J Phys Chem B 114:5563, 2010).
Co-reporter:Dimitrios Maganas ; Paw Kristiansen ; Laurent-Claudius Duda ; Axel Knop-Gericke ; Serena DeBeer ; Robert Schlögl
The Journal of Physical Chemistry C 2014 Volume 118(Issue 35) pp:20163-20175
Publication Date(Web):August 11, 2014
DOI:10.1021/jp505628y
The fundamental problem of the symmetry breaking in the resonant inelastic X-ray scattering (RIXS) of the CO2 gas molecule is studied. The measurements were performed under catalytically relevant conditions within an in-house constructed reaction cell. The experimental RIXS plane is constructed from a sequence of resonances, covering the near-edge X-ray absorption fine structure (NEXAFS) spectrum up to 539 eV. The spectra show significant sensitivity with respect to the excitation frequency. The NEXAFS absorption spectrum, as well as the corresponding RIXS spectra, is interpreted with the aid of multireference configuration interaction (MRCI) theory. In this framework, the configuration interaction space spans the space of the intermediate and final states with single and single–double excitations. The dynamic character of the RIXS spectra is investigated by considering the electronic–nuclear vibrational coupling with the bending and antisymmetric stretching vibrations in the important intermediate excited states. In addition, the vibronic coupling mechanism involving the Renner–Teller effect and the core–hole localization pseudo-Jahn–Teller effect of the intermediate states is fully considered. The physical origin of the observed spectral features is discussed qualitatively and quantitatively in terms of individual core-to-valence excitations and valence-to-core decays, respectively. The computational protocol presented here, based on multireference wave function ab initio theory, serves as an important reference for future theoretical and experimental applications of RIXS spectroscopy.
Co-reporter:Dr. Christoph Riplinger;Dr. Eckhard Bill;Dr. Andreas Daiber;Dr. Volker Ullrich;Dr. Hirofumi Shoun;Dr. Frank Neese
Chemistry - A European Journal 2014 Volume 20( Issue 6) pp:1602-1614
Publication Date(Web):
DOI:10.1002/chem.201302443
Abstract
Cytochrome P450 NO reductase is an unusual member of the cytochrome P450 superfamily. It catalyzes the reduction of nitric oxide to nitrous oxide. The reaction intermediates were studied in detail by a combination of experimental and computational methods. They have been characterized experimentally by UV/Vis, EPR, Mössbauer, and MCD spectroscopy. In conjunction with quantum mechanics/molecular mechanics (QM/MM) calculations, we sought to characterize the resting state and the two detectable intermediates in detail and to elucidate the nature of the key intermediate I of the reaction. Six possible candidates were taken into account for the unknown key intermediate in the computational study, differing in protonation state and electronic structure. Two out of the six candidates could be identified as putative intermediates I with the help of the spectroscopic data: singlet diradicals FeIII-NHO.− and FeIII-NHOH.. In a companion publication (C. Riplinger, F. Neese, ChemPhysChem 2011, 12, 3192) we have used QM/MM models based on these structures and performed a kinetic simulation. The combination of these two studies shows the nature of the key intermediate to be the singlet diradical, FeIII-NHOH..
Co-reporter:Mario Kampa ; Maria-Eirini Pandelia ; Wolfgang Lubitz ; Maurice van Gastel
Journal of the American Chemical Society 2013 Volume 135(Issue 10) pp:3915-3925
Publication Date(Web):February 12, 2013
DOI:10.1021/ja3115899
The light-induced Ni–L state of [NiFe] hydrogenases is well suited to investigate the identity of the amino acid base that functions as a proton acceptor in the hydrogen turnover cycle in this important class of enzymes. Density functional theory calculations have been performed on large models that include the complete [NiFe] center and parts of the second coordination sphere. Combined with experimental data, in particular from electron paramagnetic resonance and Fourier transform infrared (FTIR) spectroscopy, the calculations indicate that the hydride ion, which is located in the bridging position between nickel and iron in the Ni–C state, dissociates upon illumination as a proton and binds to a nearby thiolate base. Moreover, the formation of a functionally relevant nickel–iron bond upon dissociation of the hydride is unequivocally observed and is in full agreement with the observed g values, ligand hyperfine coupling constants, and FTIR stretching frequencies. This metal–metal bond can be protonated and thus functions like a base. The nickel–iron bond is important for all intermediates with an empty bridge in the catalytic cycle, and the electron pair that constitutes this bond thus plays a crucial role in the hydrogen evolution catalyzed by the enzyme.
Co-reporter:Joseph M. Zadrozny, Mihail Atanasov, Aimee M. Bryan, Chun-Yi Lin, Brian D. Rekken, Philip P. Power, Frank Neese and Jeffrey R. Long
Chemical Science 2013 vol. 4(Issue 1) pp:125-138
Publication Date(Web):25 Oct 2012
DOI:10.1039/C2SC20801F
A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), and Fe(OAr′)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe[N(H)Ar#]2 (6), is bent with an N–Fe–N angle of 140.9(2)° (Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 ≫ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff = 181, 146, 109, 104, and 43 cm−1 for 1–5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous D∞h geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1–6, but ultimately spin–orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1–5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.
Co-reporter:Mihail Atanasov, Joseph M. Zadrozny, Jeffrey R. Long and Frank Neese
Chemical Science 2013 vol. 4(Issue 1) pp:139-156
Publication Date(Web):25 Oct 2012
DOI:10.1039/C2SC21394J
The electronic structure and magnetic anisotropy of six complexes of high-spin FeII with linear FeX2 (X = C, N, O) cores, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), Fe[O(Ar′)]2 (5), and Fe[N(t-Bu)2]2 (7) [Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2], and one bent (FeN2) complex, Fe[N(H)Ar#]2 (6), have been studied theoretically using complete active space self-consistent field (CASSCF) wavefunctions in conjunction with N-Electron Valence Perturbation Theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for the treatment of magnetic field and spin-dependent relativistic effects. Mössbauer studies on compound 2 indicate an internal magnetic field of unprecedented magnitude (151.7 T) at the FeII nucleus. This has been interpreted as arising from first order angular momentum of the 5Δ ground state of FeII center (J. Am. Chem. Soc. 2004, 126, 10206). Using geometries from X-ray structural data, ligand field parameters for the Fe-ligand bonds were extracted using a 1:1 mapping of the angular overlap model onto multireference wavefunctions. The results demonstrate that the metal–ligand bonding in these complexes is characterized by: (i) strong 3dz2–4s mixing (in all complexes), (ii) π-bonding anisotropy involving the strong π-donor amide ligands (in 1, 3–4, 6, and 7) and (iii) orbital mixings of the σ–π type for Fe–O bonds (misdirected valence in 5). The interplay of all three effects leads to an appreciable symmetry lowering and splitting of the 5Δ (3dxy, 3dx2−y2) ground state. The strengths of the effects increase in the order 1 < 5 < 7 ∼ 6. However, the differential bonding effects are largely overruled by first-order spin–orbit coupling, which leads to a nearly non-reduced orbital contribution of L = 1 to yield a net magnetic moment of about 6 μB. This unique spin–orbital driven magnetism is significantly modulated by geometric distortion effects: static distortions for the bent complex 6 and dynamic vibronic coupling effects of the Renner–Teller type of increasing strength for the series 1–5.Ab initio calculations based on geometries from X-ray data for 1 and 2 reproduce the magnetic data exceptionally well. Magnetic sublevels and wavefunctions were calculated employing a dynamic Renner–Teller vibronic coupling model with vibronic coupling parameters adjusted from the ab initio results on a small Fe(CH3)2 truncated model complex. The model reproduces the observed reduction of the orbital moments and quantitatively reproduces the magnetic susceptibility data of 3–5 after introduction of the vibronic coupling strength (f) as a single adjustable parameter. Its value varies in a narrow range (f = 0.142 ± 0.015) across the series. The results indicate that the systems are near the borderline of the transition from a static to a dynamic Renner–Teller effect. Renner–Teller vibronic activity is used to explain the large reduction of the spin-reversal barrier Ueff along the series from 1 to 5. Based upon the theoretical analysis, guidelines for generating new single-molecule magnets with enhanced magnetic anisotropies and longer relaxation times are formulated.
Co-reporter:David Schweinfurth, J. Krzystek, Igor Schapiro, Serhiy Demeshko, Johannes Klein, Joshua Telser, Andrew Ozarowski, Cheng-Yong Su, Franc Meyer, Mihail Atanasov, Frank Neese, and Biprajit Sarkar
Inorganic Chemistry 2013 Volume 52(Issue 12) pp:6880-6892
Publication Date(Web):May 23, 2013
DOI:10.1021/ic3026123
The coordination complexes of Ni(II) with the tripodal ligands tpta (tris[(1-phenyl-1H-1,2,3-triazol-4-yl)methyl]amine), tbta ([(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine), and tdta (tris[(1-(2,6-diisopropyl-phenyl)-1H-1,2,3-triazol-4-yl)methyl]amine) and the bidentate ligand pyta (1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-triazole), [Ni(tpta)2](BF4)2 (1), [Ni(tbta)2](BF4)2 (2), [Ni(tdta)2](BF4)2 (3), and [Ni(pyta)3](BF4)2 (4), were synthesized from Ni(BF4)2·6H2O and the corresponding ligands. Complexes 2 and 4 were also characterized structurally using X-ray diffraction and magnetically via susceptibility measurements. Structural characterization of 2 that contains the potentially tetradentate, tripodal tbta ligand revealed that the Ni(II) center in that complex is in a distorted octahedral environment, being surrounded by two of the tripodal ligands. Each of those ligands coordinate to the Ni(II) center through the central amine nitrogen atom and two of the triazole nitrogen donors; the Ni–N(amine) distances being longer than Ni–N(triazole) distances. In case of 4, three of the bidentate ligands pyta bind to the Ni(II) center with the binding of the triazole nitrogen atoms being stronger than those of the pyridine. Temperature dependent susceptibility measurements on 2 and 4 revealed a room temperature χMT value of 1.18 and 1.20 cm3 K mol–1, respectively, indicative of S = 1 systems. High-frequency and -field EPR (HFEPR) measurements were performed on all the complexes to accurately determine their g-tensors and the all-important zero-field splitting (zfs) parameters D and E. Interpretation of the optical d–d absorption spectra using ligand field theory revealed the B and Dq values for these complexes. Quantum chemical calculations based on the X-ray and DFT optimized geometries and their ligand field analysis have been used to characterize the metal–ligand bonding and its influence on the magnitude and sign of the zfs parameters. This is the first time that such extensive HFEPR, LFT, and advanced computational studies are being reported on a series of mononuclear, distorted octahedral Ni(II) complexes containing different kinds of nitrogen donating ligands in the same complex.
Co-reporter:Fernande Grandjean ;Gary J. Long
Inorganic Chemistry 2013 Volume 52(Issue 22) pp:13123-13131
Publication Date(Web):October 31, 2013
DOI:10.1021/ic402013n
The iron-57 Mössbauer spectra of the linear, two-coordinate complexes, [K(crypt-222)][Fe(C(SiMe3)3)2], 1, and Fe(C(SiMe3)3)2, 2, were measured between 5 and 295 K under zero applied direct current (dc) field. These spectra were analyzed with a relaxation profile that models the relaxation of the hyperfine field associated with the inversion of the iron cation spin. Because of the lifetime of the measurement (10–8 to 10–9 s), iron-57 Mössbauer spectroscopy yielded the magnetization dynamics of 1 and 2 on a significantly faster time scale than was previously possible with alternating current (ac) magnetometry. From the modeling of the Mössbauer spectral profiles, Arrhenius plots between 5 and 295 K were obtained for both 1 and 2. The high-temperature regimes revealed Orbach relaxation processes with Ueff = 246(3) and 178(9) cm–1 for 1 and 2, respectively, effective relaxation barriers which are in agreement with magnetic measurements and supporting ab initio calculations. In 1, two distinct high-temperature regimes of magnetic relaxation are observed with mechanisms that correspond to two distinct single-excitation Orbach processes within the ground-state spin–orbit coupled manifold of the iron(I) ion. For 2, Mössbauer spectroscopy yields the temperature dependence of the magnetic relaxation in zero applied dc field, a relaxation that could not be observed with zero-field ac magnetometry. The ab initio calculated Mössbauer hyperfine parameters of both 1 and 2 are in excellent agreement with the observed hyperfine parameters.
Co-reporter:Genqiang Xue, Caiyun Geng, Shengfa Ye, Adam T. Fiedler, Frank Neese, and Lawrence Que Jr.
Inorganic Chemistry 2013 Volume 52(Issue 7) pp:3976-3984
Publication Date(Web):March 15, 2013
DOI:10.1021/ic3027896
Complexes 1–OH and 1–F are related complexes that share similar [X–FeIII–O–FeIV═O]3+ core structures with a total spin S of 1/2, which arises from antiferromagnetic coupling of an S = 5/2 FeIII–X site and an S = 2 FeIV═O site. EXAFS analysis shows that 1–F has a nearly linear FeIII–O–FeIV core compared to that of 1–OH, which has an Fe–O–Fe angle of ∼130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C–H bond cleavage than 2, a related complex with a [OH–FeIV–O–FeIV═O]4+ core having individual S = 1 FeIV units. Interestingly, 1–F is 10-fold more reactive than 1–OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing 1–OH and 1–F strongly suggest that the H-bond in 1–OH does not significantly change the electrophilicity of the reactive FeIV═O unit and that the lower reactivity of 1–OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron(IV) moiety.
Co-reporter:Vera Krewald ; Benedikt Lassalle-Kaiser ; Thaddeus T. Boron III; Christopher J. Pollock ; Jan Kern ; Martha A. Beckwith ; Vittal K. Yachandra ; Vincent L. Pecoraro ; Junko Yano ; Frank Neese ;Serena DeBeer
Inorganic Chemistry 2013 Volume 52(Issue 22) pp:12904-12914
Publication Date(Web):October 25, 2013
DOI:10.1021/ic4008203
In nature, the protonation of oxo bridges is a commonly encountered mechanism for fine-tuning chemical properties and reaction pathways. Often, however, the protonation states are difficult to establish experimentally. This is of particular importance in the oxygen evolving complex of photosystem II, where identification of the bridging oxo protonation states is one of the essential requirements toward unraveling the mechanism. In order to establish a combined experimental and theoretical protocol for the determination of protonation states, we have systematically investigated a series of Mn model complexes by Mn K pre-edge X-ray absorption spectroscopy. An ideal test case for selective bis-μ-oxo-bridge protonation in a Mn dimer is represented by the system [MnIV2(salpn)2(μ-OHn)2]n+. Although the three species [MnIV2(salpn)2(μ-O)2], [MnIV2(salpn)2(μ-O)(μ-OH)]+ and [MnIV2(salpn)2(μ-OH)2]2+ differ only in the protonation of the oxo bridges, they exhibit distinct differences in the pre-edge region while maintaining the same edge energy. The experimental spectra are correlated in detail to theoretically calculated spectra. A time-dependent density functional theory approach for calculating the pre-edge spectra of molecules with multiple metal centers is presented, using both high spin (HS) and broken symmetry (BS) electronic structure solutions. The most intense pre-edge transitions correspond to an excitation of the Mn 1s core electrons into the unoccupied orbitals of local eg character (dz2 and dxy based in the chosen coordinate system). The lowest energy experimental feature is dominated by excitations of 1s-α electrons, and the second observed feature is primarily attributed to 1s-β electron excitations. The observed energetic separation is due to spin polarization effects in spin-unrestricted density functional theory and models final state multiplet effects. The effects of spin polarization on the calculated Mn K pre-edge spectra, in both the HS and BS solutions, are discussed in terms of the strength of the antiferromagnetic coupling and associated changes in the covalency of Mn–O bonds. The information presented in this paper is complemented with the X-ray emission spectra of the same compounds published in an accompanying paper. Taken together, the two studies provide the foundation for a better understanding of the X-ray spectroscopic data of the oxygen evolving complex (OEC) in photosystem II.
Co-reporter:Igor Schapiro, Kantharuban Sivalingam, and Frank Neese
Journal of Chemical Theory and Computation 2013 Volume 9(Issue 8) pp:3567-3580
Publication Date(Web):July 12, 2013
DOI:10.1021/ct400136y
The multireference n-electron Valence State Perturbation Theory is applied to a benchmark set of 28 organic molecules compiled by Schreiber et al. J. Chem. Phys. (2008) 128, 13. Different types of low-lying vertical excitation energies are computed using the same geometries and TZVP basis set as in the original work. The previously published coupled cluster CC3 results are used as a reference. The complete active space second order perturbation theory (CASPT2) results, as well as the results of second order N-electron valence perturbation theory (NEVPT2) (both in their single-state variants) are evaluated against this reference set, which includes 153 singlet and 72 triplet vertical transition energies. NEVPT2 calculations are carried out in two variants: the partially contracted (PC) and the strongly contracted (SC) scheme. The statistical evaluation with respect to CC3 is found to be similar for both: the mean unsigned deviations is 0.28 eV for singlets and 0.16 eV for triplets for PC-NEVPT2, while it is 0.23 and 0.17 eV for SC-NEVPT2, respectively. Further analysis has shown that deficiencies in the zeroth-order wave functions, in particular for the subset of π → π* singlet excitations, are responsible for the largest deviations from CC3. Those states have either a charge transfer or an ionic character. For the remaining singlet and all triplet excitations the general trend was established that NEVPT2 tends to slightly overestimate excitation energies while CASPT2 slightly underestimates them. However, overall, both methods are of very similar accuracy provided that the IPEA shift is used in the CASPT2 method. Interestingly, the conclusions reached in this study are independent of the orbital canonicalization scheme used in the NEVPT2 calculation.
Co-reporter:Shengfa Ye, Cai-Yun Geng, Sason Shaik and Frank Neese
Physical Chemistry Chemical Physics 2013 vol. 15(Issue 21) pp:8017-8030
Publication Date(Web):01 May 2013
DOI:10.1039/C3CP00080J
This perspective discusses the principles of the multistate scenario often encountered in transition metal catalyzed reactions, and is organized as follows. First, several important theoretical concepts (physical versus formal oxidation states, orbital interactions, use of (spin) natural and corresponding orbitals, exchange enhanced reactivity and the connection between valence bond and molecular orbital based electronic structure analysis) are presented. These concepts are then used to analyze the electronic structure changes occurring in the reaction of C–H bond oxidation by FeIVoxo species. The analysis reveals that the energy separation and the overlap between the electron donating orbitals and electron accepting orbitals of the FeIVoxo complexes dictate the reaction stereochemistry, and that the manner in which the exchange interaction changes depends on the identity of these orbitals. The electronic reorganization of the FeIVoxo species during the reaction is thoroughly analyzed and it is shown that the FeIVoxo reactant develops oxyl radical character, which interacts effectively with the σCH orbital of the alkane. The factors that determine the energy barrier for the reaction are discussed in terms of molecular orbital and valence bond concepts.
Co-reporter:Dimitrios Maganas, Michael Roemelt, Michael Hävecker, Annette Trunschke, Axel Knop-Gericke, Robert Schlögl and Frank Neese
Physical Chemistry Chemical Physics 2013 vol. 15(Issue 19) pp:7260-7276
Publication Date(Web):14 Mar 2013
DOI:10.1039/C3CP50709B
A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal–ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.
Co-reporter:Michael Roemelt and Frank Neese
The Journal of Physical Chemistry A 2013 Volume 117(Issue 14) pp:3069-3083
Publication Date(Web):March 19, 2013
DOI:10.1021/jp3126126
A spin-adapted configuration interaction with singles method that is based on a restricted open-shell reference function (ROCIS) with general total spin S is presented. All excited configuration state functions (CSFs) are generated with the aid of a spin-free second quantization formalism that only leads to CSFs within the first order interacting space. By virtue of the CSF construction, the formalism involves higher than singly excited determinants but not higher than singly excited configurations. Matrix elements between CSFs are evaluated on the basis of commutator relationships using a symbolic algebra program. The final equations were, however, hand-coded in order to maximize performance. The method can be applied to fairly large systems with more than 100 atoms in reasonable wall-clock times and also parallelizes well. Test calculations demonstrate that the approach is far superior to UHF-based configuration interaction with single excitations but necessarily falls somewhat short of quantitative accuracy due to the lack of dynamic correlation contributions. In order to implicitly account for dynamic correlation in a crude way, the program optionally allows for the use of Kohn–Sham orbitals in combination with a modest downscaling of two-electron integrals (DFT/ROCIS). All two-electron integrals of Kohn–Sham orbitals that appear in the Hamiltonian matrix are reduced by a total of three scaling parameters that are suitable for a wide range of molecules. Test calculations on open-shell organic radicals as well as transition metal complexes demonstrate the wide applicability of the method and its ability to calculate the electronic spectra of large molecular systems.
Co-reporter:Dr. Tobias Krämer;Dr. Mario Kampa; Dr. Dr. Wolfgang Lubitz;Dr. Maurice van Gastel; Dr. Frank Neese
ChemBioChem 2013 Volume 14( Issue 14) pp:1898-1905
Publication Date(Web):
DOI:10.1002/cbic.201300104
Abstract
[NiFe] hydrogenases catalyze the reversible oxidation of dihydrogen. The corresponding catalytic cycle involves a formidable number of redox states of the Ni-Fe active site; these can be distinguished experimentally by the IR stretching frequencies of their CN and CO ligands coordinated to iron. These spectroscopic fingerprints serve as sensitive probes for the intrinsic electronic structure of the metal core and, indirectly, for the structural composition of the active site. In this study, density functional theory (DFT) was used to calculate vibrational frequencies, by focusing on the EPR-silent intermediate states that contain divalent metal centers. By using the well-characterized Ni-C and Ni-B states as references, we identified candidates for the Ni-SIr, Ni-SIa, and Ni-R states by matching the predicted relative frequency shifts with experimental results. The Ni-SIr and Ni-SIa states feature a water molecule loosely bound to nickel and a formally vacant bridge. Both states are connected to each other through protonation equilibria; that is, in the Ni-SIa state one of the terminal thiolates is protonated, whereas in Ni-SIr this thiolate is unprotonated. For the reduced Ni-R state two feasible models emerged: in one, H2 coordinates side-on to nickel, and the second features a hydride bridge and a protonated thiolate. The Ni-SU state remains elusive as no unequivocal correspondence between the experimental data and calculated frequencies of the models was found, thus indicating that a larger structural rearrangement might occur upon reduction from Ni-A to Ni-SU and that the bridging ligand might dissociate.
Co-reporter:Oliver Krahe;Dr. Frank Neese;Dr. Marianne Engeser
ChemPlusChem 2013 Volume 78( Issue 9) pp:1053-1057
Publication Date(Web):
DOI:10.1002/cplu.201300182
Abstract
The FeIII azide complexes [FeIII(N3)cyclam-ac]PF6 (1⋅PF6), [FeIII(N3)Me3cyclam-ac]PF6 (2⋅PF6), and trans-[FeIII(N3)2cyclam]ClO4 (3⋅ClO4) (cyclam=1,4,8,11-tetraazacyclotetradecane; cyclam-ac=1,4,8,11-tetraazacyclotetradecane-1-acetate; Me3cyclam-ac=4,8,11-trimethyl-1,4,8,11-tetraazacyclotetra-decane-1-acetate) are studied in the gas phase with special emphasis on the formation of high-valent iron nitrides by collision-induced dissociation. Whereas the azide complex with unsubstituted cyclam-acetate 1 as major fragmentation expels N2 to form a high-valent FeV nitride complex, a similar process is not observed for its methyl-substituted congener. In contrast, loss of an azide radical results in iron reduction to FeII. Thus, the gas-phase behavior is parallel to the results obtained in spectroscopic studies of photolyzed frozen solution. The diazide complex 3 mainly fragments via consecutive losses of HN3 without change in the iron oxidation state. However, small amounts of dinitrogen loss and thus FeV nitride formation are also observed. While it is assumed that the FeV nitride complex detected by Mössbauer spectroscopy in frozen solution is still coordinated by an azide in the trans position to the nitride, both the complex [FeV(N)(N3)(cyclam)]+still bearing an intact second azide and the coordinatively unsaturated [FeV(N)(cyclam-H)]+ are observed in the gas phase.
Co-reporter:Maria-Eirini Pandelia;Pascale Infossi;Eckhard Bill;Marie-Thérèse Giudici-Orticoni;Robert Izsak;Dmytro Bykov;Wolfgang Lubitz
PNAS 2013 Volume 110 (Issue 28 ) pp:E2539
Publication Date(Web):2013-07-09
DOI:10.1073/pnas.1306038110
Co-reporter:Tomislav Argirević ; Christoph Riplinger ; JoAnne Stubbe ; Frank Neese ;Marina Bennati
Journal of the American Chemical Society 2012 Volume 134(Issue 42) pp:17661-17670
Publication Date(Web):October 16, 2012
DOI:10.1021/ja3071682
Escherichia coli class I ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to deoxynucleotides and is composed of two subunits: α2 and β2. β2 contains a stable di-iron tyrosyl radical (Y122•) cofactor required to generate a thiyl radical (C439•) in α2 over a distance of 35 Å, which in turn initiates the chemistry of the reduction process. The radical transfer process is proposed to occur by proton-coupled electron transfer (PCET) via a specific pathway: Y122 ⇆ W48[?] ⇆ Y356 in β2, across the subunit interface to Y731 ⇆ Y730 ⇆ C439 in α2. Within α2 a colinear PCET model has been proposed. To obtain evidence for this model, 3-amino tyrosine (NH2Y) replaced Y730 in α2, and this mutant was incubated with β2, cytidine 5′-diphosphate, and adenosine 5′-triphosphate to generate a NH2Y730• in D2O. [2H]-Electron–nuclear double resonance (ENDOR) spectra at 94 GHz of this intermediate were obtained, and together with DFT models of α2 and quantum chemical calculations allowed assignment of the prominent ENDOR features to two hydrogen bonds likely associated with C439 and Y731. A third proton was assigned to a water molecule in close proximity (2.2 Å O–H···O distance) to residue 730. The calculations also suggest that the unusual g-values measured for NH2Y730• are consistent with the combined effect of the hydrogen bonds to Cys439 and Tyr731, both nearly perpendicular to the ring plane of NH2Y730. The results provide the first experimental evidence for the hydrogen-bond network between the pathway residues in α2 of the active RNR complex, for which no structural data are available.
Co-reporter:Mihail Atanasov, Peter Comba, Stefan Helmle, Dennis Müller, and Frank Neese
Inorganic Chemistry 2012 Volume 51(Issue 22) pp:12324-12335
Publication Date(Web):October 26, 2012
DOI:10.1021/ic3016047
The synthesis, single-crystal X-ray structures, electronic absorption spectra, and magnetic properties of six NiII complexes with a tetradentate (L1) and three pentadentate (L2, L3, L4) bispidine ligands (3,7-diazabicyclo[3.3.1]nonane derivatives), Ni(L1·H2O)(OH2)2](PF6)2, [Ni(L1·H2O)(O2NO)]NO3, [Ni(L1·H2O)(OOCCH3)]PF6, [Ni(L2·H2O)NCMe](PF6)2, [Ni(L3·H2O)OH2](PF6)2, and [Ni(L4·H2O)NCMe](PF6)2 are reported. The Ni–donor bonding to pyridine and tertiary amine groups and oxygen- or nitrogen-bound coligands, completing the octahedral coordination sphere of NiII, is analyzed using a combination of ab initio electronic structure calculations (complete active space self-consistent field, CASSCF, followed by N-electron valence perturbation theory, NEVPT2) and angular overlap ligand field analysis. Magnetic properties are rationalized with an analysis of the magnetic anisotropy in terms of zero-field splitting and g-tensor parameters, obtained from first principles, and their correlation with the NiII–donor bonding parameters from the ligand field analysis of the ab initio results. A two-dimensional spectrochemical series of the ligands considered, according to their σ and π bonding to NiII, is also derived.
Co-reporter:Dr. Dimitrios A. Pantazis;Dr. William Ames;Dr. Nicholas Cox;Dr. Wolfgang Lubitz ;Dr. Frank Neese
Angewandte Chemie International Edition 2012 Volume 51( Issue 39) pp:
Publication Date(Web):
DOI:10.1002/anie.201206873
Co-reporter:Dr. Dimitrios A. Pantazis;Dr. William Ames;Dr. Nicholas Cox;Dr. Wolfgang Lubitz ;Dr. Frank Neese
Angewandte Chemie International Edition 2012 Volume 51( Issue 39) pp:9935-9940
Publication Date(Web):
DOI:10.1002/anie.201204705
Co-reporter:Dmytro Bykov
Inorganic Chemistry () pp:
Publication Date(Web):
DOI:10.1021/acs.inorgchem.5b01506
In this Forum Article, an extensive discussion of the mechanism of six-electron, seven-proton nitrite reduction by the cytochrome c nitrite reductase enzyme is presented. On the basis of previous studies, the entire mechanism is summarized and a unified picture of the most plausible sequence of elementary steps is presented. According to this scheme, the mechanism can be divided into five functional stages. The first phase of the reaction consists of substrate binding and N–O bond cleavage. Here His277 plays a crucial role as a proton donor. In this step, the N–O bond is cleaved heterolytically through double protonation of the substrate. The second phase of the mechanism consists of two proton-coupled electron-transfer events, leading to an HNO intermediate. The third phase involves the formation of hydroxylamine, where Arg114 provides the necessary proton for the reaction. The second N–O bond is cleaved in the fourth phase of the mechanism, again triggered by proton transfer from His277. The Tyr218 side chain governs the fifth and last phase of the mechanism. It consists of radical transfer and ammonia formation. Thus, this mechanism implies that all conserved active-site side chains work in a concerted way in order to achieve this complex chemical transformation from nitrite to ammonia. The Forum Article also provides a detailed discussion of the density functional theory based cluster model approach to bioinorganic reactivity. A variety of questions are considered: the resting state of enzyme and substrate binding modes, interaction with the metal site and with active-site side chains, electron- and proton-transfer events, substrate dissociation, etc.
Co-reporter:Yury Minenkov, Giovanni Bistoni, Christoph Riplinger, Alexander A. Auer, Frank Neese and Luigi Cavallo
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 14) pp:NaN9391-9391
Publication Date(Web):2017/03/07
DOI:10.1039/C7CP00836H
In this work, we tested canonical and domain based pair natural orbital coupled cluster methods (CCSD(T) and DLPNO-CCSD(T), respectively) for a set of 32 ligand exchange and association/dissociation reaction enthalpies involving ionic complexes of Li, Be, Na, Mg, Ca, Sr, Ba and Pb(II). Two strategies were investigated: in the former, only valence electrons were included in the correlation treatment, giving rise to the computationally very efficient FC (frozen core) approach; in the latter, all non-ECP electrons were included in the correlation treatment, giving rise to the AE (all electron) approach. Apart from reactions involving Li and Be, the FC approach resulted in non-homogeneous performance. The FC approach leads to very small errors (<2 kcal mol−1) for some reactions of Na, Mg, Ca, Sr, Ba and Pb, while for a few reactions of Ca and Ba deviations up to 40 kcal mol−1 have been obtained. Large errors are both due to artificial mixing of the core (sub-valence) orbitals of metals and the valence orbitals of oxygen and halogens in the molecular orbitals treated as core, and due to neglecting core–core and core–valence correlation effects. These large errors are reduced to a few kcal mol−1 if the AE approach is used or the sub-valence orbitals of metals are included in the correlation treatment. On the technical side, the CCSD(T) and DLPNO-CCSD(T) results differ by a fraction of kcal mol−1, indicating the latter method as the perfect choice when the CPU efficiency is essential. For completely black-box applications, as requested in catalysis or thermochemical calculations, we recommend the DLPNO-CCSD(T) method with all electrons that are not covered by effective core potentials included in the correlation treatment and correlation-consistent polarized core valence basis sets of cc-pwCVQZ(-PP) quality.
Co-reporter:Shengfa Ye, Genqiang Xue, Itana Krivokapic, Taras Petrenko, Eckhard Bill, Lawrence Que Jr and Frank Neese
Chemical Science (2010-Present) 2015 - vol. 6(Issue 5) pp:NaN2921-2921
Publication Date(Web):2015/02/26
DOI:10.1039/C4SC03268C
High-valent iron(IV)-oxo species are key intermediates in the catalytic cycles of a range of O2-activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([FeIV(O)(L)(NCMe)]2+, 1, L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)FeIV(O)(μ-O)FeIV(OH)(L)]3+, 2) iron(IV) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1, the experimental MCD spectra at 2–10 K are dominated by a broad positive band between 12000 and 18000 cm−1. As the temperature increases up to ∼20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1. The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1, the electronic structure of the FeIVO site is not significantly perturbed by the binding to another iron(IV) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C–H bond activation and O-atom transfer.
Co-reporter:Joseph M. Zadrozny, Mihail Atanasov, Aimee M. Bryan, Chun-Yi Lin, Brian D. Rekken, Philip P. Power, Frank Neese and Jeffrey R. Long
Chemical Science (2010-Present) 2013 - vol. 4(Issue 1) pp:NaN138-138
Publication Date(Web):2012/10/25
DOI:10.1039/C2SC20801F
A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), and Fe(OAr′)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe[N(H)Ar#]2 (6), is bent with an N–Fe–N angle of 140.9(2)° (Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 ≫ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff = 181, 146, 109, 104, and 43 cm−1 for 1–5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous D∞h geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1–6, but ultimately spin–orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1–5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.
Co-reporter:Mihail Atanasov, Joseph M. Zadrozny, Jeffrey R. Long and Frank Neese
Chemical Science (2010-Present) 2013 - vol. 4(Issue 1) pp:NaN156-156
Publication Date(Web):2012/10/25
DOI:10.1039/C2SC21394J
The electronic structure and magnetic anisotropy of six complexes of high-spin FeII with linear FeX2 (X = C, N, O) cores, Fe[N(SiMe3)(Dipp)]2 (1), Fe[C(SiMe3)3]2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*]2 (4), Fe[O(Ar′)]2 (5), and Fe[N(t-Bu)2]2 (7) [Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pri2)2; Ar* = C6H3-2,6-(C6H2-2,4,6-Pri2)2; Ar# = C6H3-2,6-(C6H2-2,4,6-Me3)2], and one bent (FeN2) complex, Fe[N(H)Ar#]2 (6), have been studied theoretically using complete active space self-consistent field (CASSCF) wavefunctions in conjunction with N-Electron Valence Perturbation Theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for the treatment of magnetic field and spin-dependent relativistic effects. Mössbauer studies on compound 2 indicate an internal magnetic field of unprecedented magnitude (151.7 T) at the FeII nucleus. This has been interpreted as arising from first order angular momentum of the 5Δ ground state of FeII center (J. Am. Chem. Soc. 2004, 126, 10206). Using geometries from X-ray structural data, ligand field parameters for the Fe-ligand bonds were extracted using a 1:1 mapping of the angular overlap model onto multireference wavefunctions. The results demonstrate that the metal–ligand bonding in these complexes is characterized by: (i) strong 3dz2–4s mixing (in all complexes), (ii) π-bonding anisotropy involving the strong π-donor amide ligands (in 1, 3–4, 6, and 7) and (iii) orbital mixings of the σ–π type for Fe–O bonds (misdirected valence in 5). The interplay of all three effects leads to an appreciable symmetry lowering and splitting of the 5Δ (3dxy, 3dx2−y2) ground state. The strengths of the effects increase in the order 1 < 5 < 7 ∼ 6. However, the differential bonding effects are largely overruled by first-order spin–orbit coupling, which leads to a nearly non-reduced orbital contribution of L = 1 to yield a net magnetic moment of about 6 μB. This unique spin–orbital driven magnetism is significantly modulated by geometric distortion effects: static distortions for the bent complex 6 and dynamic vibronic coupling effects of the Renner–Teller type of increasing strength for the series 1–5.Ab initio calculations based on geometries from X-ray data for 1 and 2 reproduce the magnetic data exceptionally well. Magnetic sublevels and wavefunctions were calculated employing a dynamic Renner–Teller vibronic coupling model with vibronic coupling parameters adjusted from the ab initio results on a small Fe(CH3)2 truncated model complex. The model reproduces the observed reduction of the orbital moments and quantitatively reproduces the magnetic susceptibility data of 3–5 after introduction of the vibronic coupling strength (f) as a single adjustable parameter. Its value varies in a narrow range (f = 0.142 ± 0.015) across the series. The results indicate that the systems are near the borderline of the transition from a static to a dynamic Renner–Teller effect. Renner–Teller vibronic activity is used to explain the large reduction of the spin-reversal barrier Ueff along the series from 1 to 5. Based upon the theoretical analysis, guidelines for generating new single-molecule magnets with enhanced magnetic anisotropies and longer relaxation times are formulated.
Co-reporter:Caiyun Geng, Shengfa Ye and Frank Neese
Dalton Transactions 2014 - vol. 43(Issue 16) pp:NaN6086-6086
Publication Date(Web):2014/01/09
DOI:10.1039/C3DT53051E
In this work, the reactions of C–H bond activation by two series of iron-oxo (1 (FeIV), 2 (FeV), 3 (FeVI)) and -nitrido model complexes (4 (FeIV), 5 (FeV), 6 (FeVI)) with a nearly identical coordination geometry but varying iron oxidation states ranging from IV to VI were comprehensively investigated using density functional theory. We found that in a distorted octahedral coordination environment, the iron-oxo species and their isoelectronic nitrido analogues feature totally different intrinsic reactivities toward C–H bond cleavage. In the case of the iron-oxo complexes, the reaction barrier monotonically decreases as the iron oxidation state increases, consistent with the gradually enhanced electrophilicity across the series. The iron-nitrido complex is less reactive than its isoelectronic iron-oxo species, and more interestingly, a counterintuitive reactivity pattern was observed, i.e. the activation barriers essentially remain constant independent of the iron oxidation states. The detailed analysis using the Polanyi principle demonstrates that the different reactivities between these two series originate from the distinct thermodynamic driving forces, more specifically, the bond dissociation energies (BDEE–Hs, E = O, N) of the nascent E–H bonds in the FeE–H products. Further decomposition of the BDEE–Hs into the electron and proton affinity components shed light on how the oxidation states modulate the BDEE–Hs of the two series.
Co-reporter:Shengfa Ye, Cai-Yun Geng, Sason Shaik and Frank Neese
Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 21) pp:NaN8030-8030
Publication Date(Web):2013/05/01
DOI:10.1039/C3CP00080J
This perspective discusses the principles of the multistate scenario often encountered in transition metal catalyzed reactions, and is organized as follows. First, several important theoretical concepts (physical versus formal oxidation states, orbital interactions, use of (spin) natural and corresponding orbitals, exchange enhanced reactivity and the connection between valence bond and molecular orbital based electronic structure analysis) are presented. These concepts are then used to analyze the electronic structure changes occurring in the reaction of C–H bond oxidation by FeIVoxo species. The analysis reveals that the energy separation and the overlap between the electron donating orbitals and electron accepting orbitals of the FeIVoxo complexes dictate the reaction stereochemistry, and that the manner in which the exchange interaction changes depends on the identity of these orbitals. The electronic reorganization of the FeIVoxo species during the reaction is thoroughly analyzed and it is shown that the FeIVoxo reactant develops oxyl radical character, which interacts effectively with the σCH orbital of the alkane. The factors that determine the energy barrier for the reaction are discussed in terms of molecular orbital and valence bond concepts.
Co-reporter:Dimitrios Maganas, Michael Roemelt, Michael Hävecker, Annette Trunschke, Axel Knop-Gericke, Robert Schlögl and Frank Neese
Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 19) pp:NaN7276-7276
Publication Date(Web):2013/03/14
DOI:10.1039/C3CP50709B
A detailed study of the electronic and geometric structure of V2O5 and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V2O5. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L2,3-edge NEXAFS spectra of V2O5 were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V2O5 was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal–ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids.
Co-reporter:Dimitrios Maganas, Michael Roemelt, Thomas Weyhermüller, Raoul Blume, Michael Hävecker, Axel Knop-Gericke, Serena DeBeer, Robert Schlögl and Frank Neese
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 1) pp:NaN276-276
Publication Date(Web):2013/10/24
DOI:10.1039/C3CP52711E
A series of mononuclear V(V), V(IV) and V(III) complexes were investigated by V L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The spectra show significant sensitivity to the vanadium oxidation state and the coordination environment surrounding the vanadium center. The L-edge spectra are interpreted with the aid of the recently developed Density Functional Theory/Restricted Open Shell Configuration Interaction Singles (DFT/ROCIS) method. This method is calibrated for the prediction of vanadium L-edges with different hybrid density functionals and basis sets. For the B3LYP/def2-TZVP(-f) and BHLYP/def2-TZVP(-f) functional/basis-set combinations, good to excellent agreement between calculated and experimental spectra is obtained. A treatment of the spin–orbit coupling interaction to all orders is achieved by quasi-degenerate perturbation theory (QDPT), in conjunction with DFT/ROCIS for the calculation of the molecular multiplets while accounting for dynamic correlation and anisotropic covalency. The physical origin of the observed spectral features is discussed qualitatively and quantitatively in terms of spin multiplicities, magnetic sublevels and individual 2p to 3d core level excitations. This investigation is an important prerequisite for future applications of the DFT/ROCIS method to vanadium L-edge absorption spectroscopy and vanadium-based heterogeneous catalysts.
Co-reporter:Barbara Kirchner and Frank Neese
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 22) pp:NaN14269-14269
Publication Date(Web):2015/03/31
DOI:10.1039/C5CP90040A
A graphical abstract is available for this content
Co-reporter:Vera Krewald, Frank Neese and Dimitrios A. Pantazis
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 16) pp:NaN10750-10750
Publication Date(Web):2016/01/07
DOI:10.1039/C5CP07213A
The redox potential of synthetic oligonuclear transition metal complexes has been shown to correlate with the Lewis acidity of a redox-inactive cation connected to the redox-active transition metals of the cluster via oxo or hydroxo bridges. Such heterometallic clusters are important cofactors in many metalloenzymes, where it is speculated that the redox-inactive constituent ion of the cluster serves to optimize its redox potential for electron transfer or catalysis. A principal example is the oxygen-evolving complex in photosystem II of natural photosynthesis, a Mn4CaO5 cofactor that oxidizes water into dioxygen, protons and electrons. Calcium is critical for catalytic function, but its precise role is not yet established. In analogy to synthetic complexes it has been suggested that Ca2+ fine-tunes the redox potential of the manganese cluster. Here we evaluate this hypothesis by computing the relative redox potentials of substituted derivatives of the oxygen-evolving complex with the cations Sr2+, Gd3+, Cd2+, Zn2+, Mg2+, Sc3+, Na+ and Y3+ for two sequential transitions of its catalytic cycle. The theoretical approach is validated with a series of experimentally well-characterized Mn3AO4 cubane complexes that are structural mimics of the enzymatic cluster. Our results reproduce perfectly the experimentally observed correlation between the redox potential and the Lewis acidities of redox-inactive cations for the synthetic complexes. However, it is conclusively demonstrated that this correlation does not hold for the oxygen evolving complex. In the enzyme the redox potential of the cluster only responds to the charge of the redox-inactive cations and remains otherwise insensitive to their precise identity, precluding redox-tuning of the metal cluster as a primary role for Ca2+ in biological water oxidation.
Co-reporter:Manuel Sparta and Frank Neese
Chemical Society Reviews 2014 - vol. 43(Issue 14) pp:NaN5041-5041
Publication Date(Web):2014/03/27
DOI:10.1039/C4CS00050A
The scope of this review is to provide a brief overview of the chemical applications carried out by local pair natural orbital coupled-electron pair and coupled-cluster methods. Benchmark tests reveal that these methods reproduce, with excellent accuracy, their canonical counterparts. At the same time, the speed up achieved by exploiting the locality of the electron correlation permits us to tackle chemical systems that, due to their size, would normally only be addressable with density functional theory. This review covers a broad variety of the chemical applications e.g. simulation of transition metal catalyzed reactions, estimation of weak interactions, and calculation of lattice properties in molecular crystals. This demonstrates that modern implementations of wavefunction-based correlated methods are playing an increasingly important role in applied computational chemistry.
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