Co-reporter:Hasan Pašalić;Adelia J. A. Aquino;Daniel Tunega
Journal of Molecular Modeling 2017 Volume 23( Issue 4) pp:
Publication Date(Web):2017 April
DOI:10.1007/s00894-017-3302-3
Cation–π interactions were systematically investigated for the adsorption of H+ and alkali metal cations M+ to pyrene by means of Møller–Plesset perturbation theory (MP2) and density functional theory (DFT). The main aims were to determine the preferred adsorption sites and how the microhydration shell influences the adsorption process. The preferred adsorption sites were characterized in terms of structural parameters and energetic stability. Stability analysis of the M+–pyrene complexes revealed that the binding strength and the barrier to transitions between neighboring sites generally decreased with increasing cation size from Li+ to Cs+. Such transitions were practically barrierless (<<1 kcal/mol) for the large Rb+ and Cs+ ions. Further, the influence of the first hydration shell on the adsorption behavior was investigated for Li+ and K+ as representatives of small and large (alkali metal) cations, respectively. While the isolated complexes possessed only one minimum, two minima—corresponding to an inner and an outer complex—were observed for microhydrated complexes. The small Li+ ion formed a stable hydration shell and preferentially interacted with water rather than pyrene. In contrast, K+ favored cation–π over cation–water interactions. It was found that the mechanism for complex formation depends on the balance between cation–π interactions, cation–water complexation, and the hydrogen bonding of water to the π-system.
Co-reporter:Max Pinheiro, Jr;Luiz F. A. Ferrão;Fernanda Bettanin;Adélia J. A. Aquino;Francisco B. C. Machado
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 29) pp:19225-19233
Publication Date(Web):2017/07/26
DOI:10.1039/C7CP03198J
Acenes are fascinating polyaromatic compounds that combine impressive semiconductor properties with an open-shell character by varying their molecular sizes. However, the increasing chemical instabilities related to their biradicaloid structures pose a great challenge for synthetic chemistry. Modifying the π-bond topology through chemical doping allows modulation of the electronic properties of graphene-related materials. In spite of the practical importance of these techniques, remarkably little is known about the basic question – the extent of the radical character created or quenched thereby. In this work, we report a high-level computational study on two acene oligomers doubly-doped with boron and nitrogen, respectively. These calculations demonstrate precisely which the chemical route is in order to either quench or enhance the radical character. Moving the dopants from the terminal rings to the central ones leads to a remarkable variation in the biradicaloid character (and thereby also in the chemical stability). This effect is related to a π-charge transfer involving the dopants and the radical carbon centers at the zigzag edges. This study also provides specific guidelines for a rational design of large polyaromatic compounds with enhanced chemical stability.
Co-reporter:Nadeesha J. Silva, Francisco B. C. Machado, Hans Lischka and Adelia J. A. Aquino
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 32) pp:22300-22310
Publication Date(Web):19 Jul 2016
DOI:10.1039/C6CP03749F
High level ab initio calculations ranging from coupled cluster methods including explicitly correlated approaches to standard second order Møller–Plesset theory using spin scaling (SOS-MP2) have been performed on sandwich and slipped parallel dimer structures of a series of quasi one-dimensional acenes and on two-dimensional sheets containing the pyrene to coronene series encircled with two layers of benzene rings. Sandwich (graphitic AA type) and slipped parallel (AB type) structures were considered and, within the given symmetry restrictions, full geometry optimizations were performed. Basis set superposition effects have been considered. The computed geometries show a significant biconcave deviation of the two-dimensional sheets from planarity with the central intersheet C⋯C distances considerably smaller that van der Waals distances. The computed intersheet binding energy per carbon atom extrapolated for N → ∞ of −74.3 meV (1.713 kcal mol−1) per atom agrees quite well with an experimental defoliation energy of −52 meV (1.199 kcal mol−1) per atom (−67 meV (1.545 kcal mol−1) per carbon atom without corrections for H binding contributions) for polyaromatic hydrocarbons (PAHs) from graphite. A limited investigation of density functional theory (DFT) calculations using empirical dispersion contributions has been performed also showing a significant underbinding character of the D3 method. For most of the DFT variants investigated the graphene sheet models retain a quasi-planar structure in strong contrast to the aforementioned SOS-MP2 results.
Co-reporter:Itamar Borges Jr., Elmar Uhl, Lucas Modesto-Costa, Adélia J. A. Aquino, and Hans Lischka
The Journal of Physical Chemistry C 2016 Volume 120(Issue 38) pp:21818-21826
Publication Date(Web):August 31, 2016
DOI:10.1021/acs.jpcc.6b07689
Structural and electronic properties of the ground and lowest excited states of the electron-donor conjugated copolymer poly(thieno[3,4-b]thiophene benzodithiophene) (PTB1) are reported based on high-level theoretical investigations. PTB1 combined with phenyl-C61-butyric acid methyl ester (PCBM) results in a bulk heterojunction blend with promising properties for use in organic solar cells. The ab initio algebraic diagrammatic construction method to second order, ADC(2), was used to obtain benchmark data for excited state energies, oscillator strengths, and bond length alternation (BLA) analysis. Time dependent density functional theory (TDDFT) calculations using the exchange-correlation functionals PBE, B3LYP, BHandHLYP, CAM-B3LYP, and LC-wPBE were also performed and compared with ADC(2) calculations. It was shown that a minimum of 20% Hartree–Fock exchange in the functional is necessary to reproduce the major features of the ADC(2) results. Analysis of the BLA results indicates the possibility of exciton trapping by geometry relaxation occurring in the middle of the polymer chain. The corresponding exciton binding energy is about 0.4 eV. Charge distributions in the ground and lowest excited singlet state were analyzed as well. The natural population analysis (NPA) confirms the electronegative character of the benozodithiophene group and the corresponding positive one of the thienothiophene moiety leading to an alternant chain polarization of positive and negative charges. Electron density differences between the S0 and S1 states show a transfer of electron density from double bond regions to areas of single bonds, a feature which parallels nicely the lengthening of double bonds and shortening of single bonds in the S1 state.
Co-reporter:Anita Das, Thomas Müller, Felix Plasser, and Hans Lischka
The Journal of Physical Chemistry A 2016 Volume 120(Issue 9) pp:1625-1636
Publication Date(Web):February 9, 2016
DOI:10.1021/acs.jpca.5b12393
In this work, two different classes of polyaromatic hydrocarbon (PAH) systems have been investigated in order to characterize the amount of polyradical character and to localize the specific regions of chemical reactivity: (a) the non-Kekulé triangular structures phenalenyl, triangulene and a π-extended triangulene system with high-spin ground state and (b) PAHs based on zethrenes, p-quinodimethane-linked bisphenalenyl, and the Clar goblet containing varying polyradical character in their singlet ground state. The first class of structures already have open-shell character because of their high-spin ground state, which follows from the bonding pattern, whereas for the second class the open-shell character is generated either because of the competition between the closed-shell quinoid Kekulé and the open-shell singlet biradical resonance structures or the topology of the π-electron arrangement of the non-Kekulé form. High-level ab initio calculations based on multireference theory have been carried out to compute singlet–triplet splitting for the above-listed compounds and to provide insight into their chemical reactivity based on the polyradical character by means of unpaired densities. Unrestricted density functional theory and Hartree–Fock calculations have been performed for comparison also in order to obtain better insight into their applicability to these types of complicated radical systems.
Co-reporter:Mario Barbatti and Hans Lischka
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 23) pp:15452-15459
Publication Date(Web):12 May 2015
DOI:10.1039/C5CP01151E
2-Aminopurine (2AP) is often chosen as a fluorescent replacement for purine bases and used as a probe in nucleic acid research. The luminescence of this molecule is strongly dependent on the environment. Through computational simulations of isolated 2AP and a series of 2AP–water clusters, we show that the experimentally-observed dependence of the excited-state lifetime of 2AP on the number and location of water molecules is controlled by a barrier for internal conversion between the S1 minimum and a conical intersection. Other possible competing pathways (proton transfer, intersystem crossing, and internal conversion at other intersections) were also investigated but discarded. The tuning of the luminescence of 2AP by water is related to the order of the nπ* and ππ* states. When a water molecule interacts with the amino group, the pathway from the S1 minimum to the conical intersection requires a nonadiabatic change, thus increasing the energy barrier for internal conversion. As a consequence, a single water molecule hydrogen-bonded to the amino group is sufficient to make 2AP fluorescent.
Co-reporter:Francisco B. C. Machado, Adélia J. A. Aquino and Hans Lischka
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 19) pp:12778-12785
Publication Date(Web):13 Apr 2015
DOI:10.1039/C4CP05751A
The electronic states occurring in a double vacancy defect for graphene nanoribbons have been calculated in detail based on a pyrene model. Extended ab initio calculations using the MR configuration interaction (MRCI) method have been performed to describe in a balanced way the manifold of electronic states derived from the dangling bonds created by initial removal of two neighboring carbon atoms from the graphene network. In total, this study took into account the characterization of 16 electronic states (eight singlets and eight triplets) considering unrelaxed and relaxed defect structures. The ground state was found to be of 1Ag character with around 50% closed shell character. The geometry optimization process leads to the formation of two five-membered rings in a pentagon–octagon–pentagon (5–8–5) structure. The closed shell character increases thereby to ∼70%; the analysis of unpaired density shows only small contributions confirming the chemical stability of that entity. For the unrelaxed structure the first five excited states (3B3g, 3B2u, 3B1u, 3Au and 1Au) are separated from the ground state by less than 2.5 eV. For comparison, unrestricted density functional theory (DFT) calculations using several types of functionals have been performed within different symmetry subspaces defined by the open shell orbitals. Comparison with the MRCI results gave good agreement in terms of finding the 1Ag state as a ground state and in assigning the lowest excited states. Linear interpolation curves between the unrelaxed and relaxed defect structures also showed good agreement between the two classes of methods opening up the possibilities of using extended nanoflakes for multistate investigations at the DFT level.
Co-reporter:Rene F. K. Spada, Luiz F. A. Ferrão, Roberta J. Rocha, Koshun Iha, José A. F. F. Rocco, Orlando Roberto-Neto, Hans Lischka, and Francisco B. C. Machado
The Journal of Physical Chemistry A 2015 Volume 119(Issue 9) pp:1628-1635
Publication Date(Web):September 8, 2014
DOI:10.1021/jp507784n
Thermochemical and kinetics properties of the hydrogen abstraction from the hydrazine molecule (N2H4) by an oxygen atom were computed using high-level ab initio methods and the M06-2X DFT functional with aug-cc-pVXZ (X = T, Q) and maug-cc-pVTZ basis sets, respectively. The properties along the reaction path were obtained using the dual-level methodology to build the minimum energy path with the potential energy surface obtained with the M06-2X method and thermochemical properties corrected with the CCSD(T)/CBS//M06-2X/maug-cc-pVTZ results. The thermal rate constants were calculated in the framework of variational transition-state theory. Wells on both sides of the reaction (reactants and products) were found and considered in the chemical kinetics calculations. Additionally, the product yields were investigated by means of a study of the triplet and singlet surfaces of the N2H4 + O → N2H2 + H2O reaction.
Co-reporter:Thiago M. Cardozo, Adélia J. A. Aquino, Mario Barbatti, Itamar Borges Jr., and Hans Lischka
The Journal of Physical Chemistry A 2015 Volume 119(Issue 9) pp:1787-1795
Publication Date(Web):November 21, 2014
DOI:10.1021/jp508512s
The absorption and fluorescence spectra of poly(p-phenylenevinylene) (PPV) oligomers with up to seven repeat units were theoretically investigated using the algebraic diagrammatic construction method to second order, ADC(2), combined with the resolution-of-the-identity (RI) approach. The ground and first excited state geometries of the oligomers were fully optimized. Vertical excitation energies and oscillator strengths of the first four transitions were computed. The vibrational broadening of the absorption and fluorescence spectra was studied using a semiclassical nuclear ensemble method. After correcting for basis set and solvent effects, we achieved a balanced description of the absorption and fluorescence spectra by means of the ADC(2) approach. This fact is documented by the computed Stokes shift along the PPV series, which is in good agreement with the experimental values. The experimentally observed band width of the UV absorption and fluorescence spectra is well reproduced by the present simulations showing that the nuclear ensemble generated should be well suitable for consecutive surface hopping dynamics simulations.
Co-reporter:Rene F. K. Spada, Luiz F. A. Ferrão, Orlando Roberto-Neto, Hans Lischka, and Francisco B. C. Machado
The Journal of Physical Chemistry A 2015 Volume 119(Issue 51) pp:12607-12614
Publication Date(Web):November 23, 2015
DOI:10.1021/acs.jpca.5b09655
The kinetics of the reaction of N2H4 with oxygen depends sensitively on the initial conditions used. In oxygen-rich systems, the rate constant shows a conventional positive temperature dependence, while in hydrazine-rich setups the dependence is negative in certain temperature ranges. In this study, a theoretical model is presented that adequately reproduces the experimental results trend and values for hydrazine-rich environment, consisting of the hydrogen abstraction from the hydrazine (N2H4) dimer by an oxygen atom. The thermochemical properties of the reaction were computed using two quantum chemical approaches, the coupled cluster theory with single, double, and noniterative triple excitations (CCSD(T)) and the M06-2X DFT approach with the aug-cc-pVTZ and the maug-cc-pVTZ basis sets, respectively. The kinetic data were calculated with the improved canonical variational theory (ICVT) using a dual-level methodology to build the reaction path. The tunneling effects were considered by means of the small curvature tunneling (SCT) approximation. Potential wells on both sides of the reaction ((N2H4)2 + O → N2H4···N2H3 + OH) were determined. A reaction path with a negative activation energy was found leading, in the temperature range of 250–423 K, to a negative dependence of the rate constant on the temperature, which is in good agreement with the experimental measurements. Therefore, the consideration of the hydrazine dimer model provides significantly improved agreement with the experimental data and should be included in the mechanism of the global N2H4 combustion process, as it can be particularly important in hydrazine-rich systems.
Co-reporter:Ivelina Georgieva, Adélia J. A. Aquino, Felix Plasser, Natasha Trendafilova, Andreas Köhn, and Hans Lischka
The Journal of Physical Chemistry A 2015 Volume 119(Issue 24) pp:6232-6243
Publication Date(Web):May 19, 2015
DOI:10.1021/acs.jpca.5b03282
The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and in acetonitrile solvent were investigated using a combination of multireference configuration interaction (MRCI) and the second-order algebraic diagrammatic construction (ADC(2)) methods. Solvent effects were included on the basis of the conductor-like screening model. The MRCI method was used for computing the nonadiabatic interaction between the two lowest excited ππ* states (S2(La, CT) and S1(Lb, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S2 to S1. Polar solvent effects are not found to have a significant influence on this situation. Applying Cs symmetry restrictions, the ADC(2) calculations show that CCN bending leads to a strong stabilization and to significant charge transfer (CT). Nevertheless, this structure is not a minimum but converts to the local excitation (LE) structure on releasing the symmetry constraint. These findings suggest that the main role in the dynamics is played by the nonadiabatic interaction of the LE and CT states and that the main source for the dual fluorescence is the twisted internal charge-transfer state in addition to the LE state.
Co-reporter:Hao Li, Reed Nieman, Adélia J. A. Aquino, Hans Lischka, and Sergei Tretiak
Journal of Chemical Theory and Computation 2014 Volume 10(Issue 8) pp:3280-3289
Publication Date(Web):May 29, 2014
DOI:10.1021/ct500072f
Long-range corrected time-dependent density functional theory (LC-TDDFT) has been applied to compute singlet vertical electronic excitations of oligothiophene molecules and their dimers and compared with the algebraic diagrammatic construction method to second order [ADC(2)], a wave function-based polarization propagator method. The excitation energies obtained from both methods agree to each other excellently. In particular, energetics of charge transfer states is concertedly reproduced. The linear response (LR) and the state specific (SS) approaches have been evaluated to appraise solvent effect on excited states. Benchmarked by the reference wave function method, the necessity of the SS treatment is justified in the prediction of charge transfer (CT) states under the TDDFT framework.
Co-reporter:Adélia A. J. Aquino, Itamar Borges, Reed Nieman, Andreas Köhn and Hans Lischka
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 38) pp:20586-20597
Publication Date(Web):05 Aug 2014
DOI:10.1039/C4CP02900C
A comprehensive theoretical study of the electronically excited states in complexes between tetracyanoethylene (TCNE) and three aromatic electron donors, benzene, naphthalene and anthracene, was performed with a focus on charge transfer (CT) transitions. The results show that the algebraic diagrammatic construction method to second order (ADC(2)) provides excellent possibilities for reliable calculations of CT states. Significant improvements in the accuracy of the computed transition energies are obtained by using the scaled opposite-spin (SOS) variant of ADC(2). Solvent effects were examined on the basis of the conductor-like screening model (COSMO) which has been implemented recently in the ADC(2) method. The dielectric constant and the refractive index of dichloromethane have been chosen in the COSMO calculations to compare with experimental solvatochromic effects. The computation of optimized ground state geometries and enthalpies of formation has been performed at the second-order Møller–Plesset perturbation theory (MP2) level. By comparison with experimental data and with high-level coupled-cluster methods including explicitly correlated (F12) wave functions, the importance of the SOS approach is demonstrated for the ground state as well. In the benzene–TCNE complex, the two lowest electronic excitations are of CT character whereas in the naphthalene and anthracene TCNE complexes three low-lying CT states are observed. As expected, they are strongly stabilized by the solvent. Geometry optimization in the lowest excited state allowed the calculation of fluorescence transitions. Solvent effects lead to a zero gap between S1 and S0 for the anthracene–TCNE complex. Therefore, in the series of benzene–TCNE to anthracene a change from a radiative to a nonradiative decay mechanism to the ground state is to be expected.
Co-reporter:Itamar Borges Jr., Adélia J. A. Aquino, and Hans Lischka
The Journal of Physical Chemistry A 2014 Volume 118(Issue 51) pp:12011-12020
Publication Date(Web):August 26, 2014
DOI:10.1021/jp507396e
Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H2C═CHNO2) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S0) and the first five valence excited electronic states (S1–S5) were investigated. In the first step, vertical excitations and potential energy curves for CH2 and NO2 torsions and CH2 out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH2 moiety, different distortions of the NO2 group and CC, CN, and NO bond stretchings. In these conical intersections, the NO2 group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π–π* excitation to the bright S5 state is proposed. In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.
Co-reporter: Francisco B. C. Machado; Adélia J. A. Aquino; Hans Lischka
ChemPhysChem 2014 Volume 15( Issue 15) pp:3334-3341
Publication Date(Web):
DOI:10.1002/cphc.201402304
Abstract
Detailed calculations have been performed on the electronic states occurring in a single vacancy defect model based on pyrene from which one of the central carbon atoms has been removed. Complete active space self-consistent field and multireference configuration interaction with singles and doubles calculations have been performed using the 6-31G and 6-31G* basis sets. Two types of defect geometries have been defined: 1) The unrelaxed defect structure based on pyrene and 2) a relaxed structure. In total 12 electronic states have been computed for the unrelaxed structure at C2v symmetry, comprising four singlets, triplets and quintets each. The lowest six states are formed from singlet and triplet states and appear in a rather narrow gap of ∼0.6 eV. The lowest quintet state is found 1.43 eV above the 3B1 ground state. As predicted from Jahn–Teller distortions, a CC bond is formed between dangling carbon bonds in the 1, 3B1 states, leading to the formation of a five-membered ring. The 1, 3A2 states show initial repulsive behavior along the bond formation coordinate until an avoided crossing is reached by which these states are furnished with CC bonding character so that finally also in these cases a CC bond is established. Linear interpolation curves between the initial unrelaxed defect structure and the final optimized structure are used to give an overview of the evolution of electronic states and the occurrence of avoided crossings. Out-of-plane structures are investigated with special emphasis on the carbon atom containing a dangling bond in the relaxed structure. Unpaired electron densities are used to characterize the electronic structure of the different states.
Co-reporter:Aditya N. Panda, Felix Plasser, Adelia J. A. Aquino, Irene Burghardt, and Hans Lischka
The Journal of Physical Chemistry A 2013 Volume 117(Issue 10) pp:2181-2189
Publication Date(Web):February 21, 2013
DOI:10.1021/jp400372t
Ab initio second-order algebraic diagrammatic construction (ADC(2)) calculations using the resolution of the identity (RI) method have been performed on poly-(p-phenylenevinylene) (PPV) oligomers with chain lengths up to eight phenyl rings. Vertical excitation energies for the four lowest π–π* excitations and geometry relaxation effects for the lowest excited state (S1) are reported. Extrapolation to infinite chain length shows good agreement with analogous data derived from experiment. Analysis of the bond length alternation (BLA) based on the optimized S1 geometry provides conclusive evidence for the localization of the defect in the center of the oligomer chain. Torsional potentials have been computed for the four excited states investigated and the transition densities divided into fragment contributions have been used to identify excitonic interactions. The present investigation provides benchmark results, which can be used (i) as reference for lower level methods and (ii) give the possibility to parametrize an effective Frenkel exciton Hamiltonian for quantum dynamical simulations of ultrafast exciton transfer dynamics in PPV type systems.
Co-reporter:Matthias Ruckenbauer, Mario Barbatti, Thomas Müller, and Hans Lischka
The Journal of Physical Chemistry A 2013 Volume 117(Issue 13) pp:2790-2799
Publication Date(Web):March 7, 2013
DOI:10.1021/jp400401f
The nonadiabatic photodynamics of the all-trans-2,4-pentadiene-iminium cation (protonated Schiff base 3, PSB3) and the all-trans-3-methyl-2,4-pentadiene-iminium cation (MePSB3) were investigated in the gas phase and in polar (aqueous) and nonpolar (n-hexane) solutions by means of surface hopping using a multireference configuration-interaction (MRCI) quantum mechanical/molecular mechanics (QM/MM) level. Spectra, lifetimes for radiationless deactivation to the ground state, and structural and electronic parameters are compared. A strong influence of the polar solvent on the location of the crossing seam, in particular in the bond length alternation (BLA) coordinate, is found. Additionally, inclusion of the polar solvent changes the orientation of the intersection cone from sloped in the gas phase to peaked, thus enhancing considerably its efficiency for deactivation of the molecular system to the ground state. These factors cause, especially for MePSB3, a substantial decrease in the lifetime of the excited state despite the steric inhibition by the solvent.
Co-reporter:Dr. Felix Plasser;Dr. Hasan Pa&x161;ali&x107;; Martin H. Gerzabek;Dr. Florian Libisch;Rafael Reiter; Joachim Burgdörfer;Dr. Thomas Müller;Dr. Ron Shepard; Hans Lischka
Angewandte Chemie International Edition 2013 Volume 52( Issue 9) pp:2581-2584
Publication Date(Web):
DOI:10.1002/anie.201207671
Co-reporter:Dr. Felix Plasser;Dr. Hasan Pa&x161;ali&x107;; Martin H. Gerzabek;Dr. Florian Libisch;Rafael Reiter; Joachim Burgdörfer;Dr. Thomas Müller;Dr. Ron Shepard; Hans Lischka
Angewandte Chemie 2013 Volume 125( Issue 9) pp:2641-2644
Publication Date(Web):
DOI:10.1002/ange.201207671
Co-reporter:Ivana Antol, Mirjana Eckert-Maksić, Mario Vazdar, Matthias Ruckenbauer and Hans Lischka
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 38) pp:13262-13272
Publication Date(Web):23 Aug 2012
DOI:10.1039/C2CP41830D
Non-adiabatic on-the-fly dynamics simulations of the photodynamics of formamide in water and n-hexane were performed using a QM/MM approach. It was shown that steric restrictions imposed by the solvent cage do not have an influence on the initial motion which leads to the lowest energy conical intersection seam. The initial deactivation in water is faster than in n-hexane and in the gas phase. However, most of the formamide molecules in water do not reach the ground state. The reason for the deactivation inefficiency in water is traced back to a decrease of close CO⋯HOH and NH⋯OH2 contacts which fall in the range of hydrogen bonds. The energy deposition into H-bond breaking events leaves molecules with less energy for surmounting the CN dissociation barrier. In both solvents, after hopping to the ground state, the solvent cage keeps the HCO and NH2 fragments or CO and NH3 products in close proximity. Consequently, the number of trajectories where fast recombination happens is augmented with delayed recombinations that start when the dissociation fragments hit the cage wall and return back. The hot ground state formamide is formed in an internal conversion process identical to the path leading to CN photodissociation. In the case of aqueous formamide, good agreement with experimental results is achieved by combining dynamics simulations starting from the S1 and the S2 excited states collecting high and low energy trajectories, respectively.
Co-reporter:Hasan Pašalić, Daniel Tunega, Adélia J. A. Aquino, Georg Haberhauer, Martin H. Gerzabek and Hans Lischka
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 12) pp:4162-4170
Publication Date(Web):21 Feb 2012
DOI:10.1039/C2CP23015A
The thermodynamic stability of the acetic acid dimer conformers in microhydrated environments and in aqueous solution was studied by means of molecular dynamics simulations using the density functional based tight binding (DFTB) method. To confirm the reliability of this method for the system studied, density functional theory (DFT) and second order Møller–Plesset perturbation theory (MP2) calculations were performed for comparison. Classical optimized potentials for liquid simulations (OPLS) force field dynamics was used as well. One focus of this work was laid on the study of the capabilities of water molecules to break the hydrogen bonds of the acetic acid dimer. The barrier for insertion of one water molecule into the most stable cyclic dimer is found to lie between 3.25 and 4.8 kcal mol−1 for the quantum mechanical methods, but only at 1.2 kcal mol−1 for OPLS. Starting from different acetic acid dimer structures optimized in gas phase, DFTB dynamics simulations give a different picture of the stability in the microhydrated environment (4 to 12 water molecules) as compared to aqueous solution. In the former case all conformers are converted to the hydrated cyclic dimer, which remains stable over the entire simulation time of 1 ns. These results demonstrate that the considered microhydrated environment is not sufficient to dissociate the acetic acid dimer. In aqueous solution, however, the DFTB dynamics shows dissociation of all dimer structures (or processes leading thereto) starting after about 50 ps, demonstrating the capability of the water environment to break up the relatively strong hydrogen bridges. The OPLS dynamics in the aqueous environment shows—in contrast to the DFTB results—immediate dissociation, but a similar long-term behavior.
Co-reporter:Hasan Pašalić, Martina Roeselová, and Hans Lischka
The Journal of Physical Chemistry B 2011 Volume 115(Issue 8) pp:1807-1816
Publication Date(Web):February 4, 2011
DOI:10.1021/jp107989k
The solvation properties of methyl and pentyl chloride were studied in a microhydrated environment with up to 10 explicit water molecules and at the liquid water−vapor interface. Geometry optimizations were performed in the former case using the density functional based tight binding (DFTB), DFTB-D, and Møller−Plesset perturbation theory (MP2) levels of theory. The microhydrated alkyl chloride complexes were characterized in terms of hydrogen bonding and energetic stability. The DFTB and DFTB-D results were verified by comparison with those obtained by MP2. Good agreement between the MP2 and DFTB-D results is found. Complexes where the alkyl chloride molecule is attached to an edge of the water cluster are found to be most stable. Pronounced stability is also observed for cubic arrangements of the alkyl chloride−water complexes. Molecular dynamics simulations based on the DFTB and DFTB-D methods were used to study the adsorption process of the alkyl chloride molecules to a water surface. The dynamics simulations show that the methyl chloride molecule is located at the water surface preferentially with the methyl group oriented toward the water surface, while for pentyl chloride, owing to the longer nonpolar hydrocarbon chain, a parallel alignment at the water surface is found with the hydrocarbon chain pointing slightly to the gas phase. Despite some quantitative differences, the present work provides confirmation of the somewhat surprising preferential orientation of the methyl chloride molecule at the water−vapor interface predicted in a recent study using molecular dynamics simulations based on an empirical force field (Harper et al., J. Phys. Chem. A2009, 113, 2015−2024). The observed difference in preferred alignment at the aqueous surface between the methyl chloride and the longer-chain alkyl chloride is likely to have consequences for the chemistry of alkyl halides adsorbed on the surface of aqueous and ice particles in the atmosphere.
Co-reporter:Dana Nachtigallová, Adélia J. A. Aquino, Jaroslaw J. Szymczak, Mario Barbatti, Pavel Hobza, and Hans Lischka
The Journal of Physical Chemistry A 2011 Volume 115(Issue 21) pp:5247-5255
Publication Date(Web):May 6, 2011
DOI:10.1021/jp201327w
Nonadiabatic dynamics simulations performed at the state-averaged CASSCF method are reported for uracil. Supporting calculations on stationary points and minima on the crossing seams have been performed at the MR-CISD and CASPT2 levels. The dominant mechanism is characterized by relaxation into the S2 minimum of ππ* character followed by the relaxation to the S1 minimum of nπ* character. This mechanism contributes to the slower relaxation with a decay constant larger than 1.5 ps, in good agreement with the long time constants experimentally observed. A minor fraction of trajectories decay to the ground state with a time constant of about 0.7 ps, which should be compared to the experimentally observed short constant. The major part of trajectories decaying with this time constant follows the ππ* channel and hops to the ground state via an ethylenic conical intersection. A contribution of the relaxation proceeding via a ring-opening conical intersection was also observed. The existence of these two latter channels together with a reduced long time constant is responsible for a significantly shorter lifetime of uracil compared to that of thymine.
Co-reporter:Marek Pederzoli and Jiří Pittner, Mario Barbatti , Hans Lischka
The Journal of Physical Chemistry A 2011 Volume 115(Issue 41) pp:11136-11143
Publication Date(Web):June 20, 2011
DOI:10.1021/jp2013094
Ab initio nonadiabatic dynamics simulations of cis-to-trans isomerization of azobenzene upon S1 (n–π*) excitation are carried out employing the fewest-switches surface hopping method. Azobenzene photoisomerization occurs purely as a rotational motion of the central CNNC moiety. Two nonequivalent rotational pathways corresponding to clockwise or counterclockwise rotation are available. The course of the rotational motion is strongly dependent on the initial conditions. The internal conversion occurs via an S0/S1 crossing seam located near the midpoint of both of these rotational pathways. Based on statistical analysis, it is shown that the occurrence of one or other pathway can be completely controlled by selecting adequate initial conditions.
Co-reporter:Dana Nachtigallová ; Tomáš Zelený ; Matthias Ruckenbauer ; Thomas Müller ; Mario Barbatti ; Pavel Hobza
Journal of the American Chemical Society 2010 Volume 132(Issue 24) pp:8261-8263
Publication Date(Web):June 1, 2010
DOI:10.1021/ja1029705
Nonadiabatic photodynamical simulations of 4-aminopyrimidine (4-APy) used as a model for adenine were performed by embedding it between two stacking methyl-guanine (mGua) molecules to determine the effect of spatial restrictions on the ultrafast photodeactivation mechanism of this nucleobase. A hybrid multiconfigurational ab initio/molecular mechanical approach in combination with surface hopping was used. During the dynamics the formation of a significant fraction of intrastrand hydrogen bonding from 4-APy to mGua above and below is observed. These findings show that this type of hydrogen bond may play an important role for the photodynamics within one DNA strand and that it should be of interest even in irregular segments of double stranded nucleic acids structures. The relaxation mechanism of internal conversion to the ground state is dominated by ring puckering, and an overall elongation of the lifetime of the embedded system by ∼20% as compared to the isolated 4-APy is computed.
Co-reporter:Ivelina Georgieva ; Adélia J. A. Aquino ; Natasha Trendafilova ; Paulo S. Santos
Inorganic Chemistry 2010 Volume 49(Issue 4) pp:1634-1646
Publication Date(Web):January 7, 2010
DOI:10.1021/ic9020299
Solvatochromic and ionochromic effects of the iron(II)bis(1,10-phenanthroline)dicyano (Fe(phen)2(CN)2) complex were investigated by means of combined DFT/TDDFT calculations using the PBE and B3LYP functionals. Extended solvation models of Fe(phen)2(CN)2 in acetonitrile and aqueous solution, as well as including interaction with Mg2+, were constructed. The calculated vertical excitation energies reproduce well the observed solvatochromism in acetonitrile and aqueous solutions, the ionochromism in acetonitrile in the presence of Mg2+, and the absence of ionochromic effect in aqueous solution. The vertical excitation energies and the nature of the transitions were reliably predicted after inclusion of geometry relaxation upon aqueous micro- and global solvation and solvent polarization effect in the TDDFT calculations. The two intense UV−vis absorption bands occurring for all systems studied are interpreted as transitions from a hybrid FeII(d)/cyano N(p) orbital to a phenanthroline π* orbital rather than a pure metal-to-ligand-charge transfer (MLCT). The solvatochromic and ionochromic blue band shifts of Fe(phen)2(CN)2 were explained with preferential stabilization of the highest occupied FeII(d)/cyano N(p) orbitals as a result of specific interactions with water solvent molecules or Mg2+ ions in solution. Such interactions occur through the CN− groups in the complex, and they have a decisive role for the observed blue shifts of UV−vis absorption bands.
Co-reporter:Mario Barbatti, Adelia J. A. Aquino and Hans Lischka
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 19) pp:4959-4967
Publication Date(Web):29 Mar 2010
DOI:10.1039/B924956G
Semi-classical simulations of the UV-photoabsorption cross sections of adenine, guanine, cytosine, thymine, and uracil in gas phase were performed at the resolution-of-identity coupled cluster to the second-order (RI-CC2) level. With the exception of cytosine, the spectra of the other four nucleobases show a two band pattern separated by a low intensity region. The spectrum of cytosine is shaped by a sequence of three bands of increasing intensity. The first band of guanine is composed by two ππ* transitions of similar intensities. The analysis of individual contributions to the spectra allows a detailed assignment of bands. It is shown that the semi-classical simulations are able to predict general features of the experimental spectra, including their absolute intensities.
Co-reporter:Jaroslaw J. Szymczak, Thomas Müller, Hans Lischka
Chemical Physics 2010 Volume 375(Issue 1) pp:110-117
Publication Date(Web):14 September 2010
DOI:10.1016/j.chemphys.2010.07.034
Abstract
The influence of water on the photo-deactivation process of 4-aminopyrimidine has been investigated by means of microsolvation and continuum solvation models. Two- and four-water models were used for the former purpose. Vertical excitations, stationary points on the first excited singlet surface, conical intersections (minima on the crossing seam) and reaction paths have been investigated at the state-averaged complete active space self-consistent field (CASSCF) and multistate CAS perturbation theory to second order (MS-CASPT2) levels of theory. A destabilizing effect of 0.2–0.3 eV has been observed for nπ∗ states while the ππ∗ state is almost unaffected. The two-water model gives already a good representation of hydration effects, especially when combined with the continuum model. A small enhancement of energy barriers (∼0.1 eV) is observed leading to the conclusion that the photodynamics of 4-aminopyrimidine should be affected only little by these solvent effects.
Co-reporter:Mario Barbatti, Jiří Pittner, Marek Pederzoli, Ute Werner, Roland Mitrić, Vlasta Bonačić-Koutecký, Hans Lischka
Chemical Physics 2010 Volume 375(Issue 1) pp:26-34
Publication Date(Web):14 September 2010
DOI:10.1016/j.chemphys.2010.07.014
Abstract
Non-adiabatic dynamics simulations were performed for pyrrole at time-dependent density functional theory level using the trajectory surface hopping approach. Initial conditions were prepared based on the UV-absorption spectrum so as to simulate monochromatic absorption in three distinct spectral regions. The results showed predominance of the NH-stretch mechanism for excited-state relaxation. With increasing initial energy, however, other mechanisms are activated as well, even though they still occurred for a minor fraction of the trajectories. Dynamics starting at the origin of the absorption spectrum exhibited internal conversion to the ground state with a time constant of 20 fs. In contrast, dynamics starting at higher energies gave rise to much longer time constants for internal conversion near 200 fs.
Co-reporter:Mario Barbatti;Adélia J. A. Aquino;Jaroslaw J. Szymczak;Dana Nachtigallová;Pavel Hobza
PNAS 2010 Volume 107 (Issue 50 ) pp:21453-21458
Publication Date(Web):2010-12-14
DOI:10.1073/pnas.1014982107
A comprehensive effort in photodynamical ab initio simulations of the ultrafast deactivation pathways for all five nucleobases
adenine, guanine, cytosine, thymine, and uracil is reported. These simulations are based on a complete nonadiabatic surface-hopping
approach using extended multiconfigurational wave functions. Even though all five nucleobases share the basic internal conversion
mechanisms, the calculations show a distinct grouping into purine and pyrimidine bases as concerns the complexity of the photodynamics.
The purine bases adenine and guanine represent the most simple photodeactivation mechanism with the dynamics leading along
a diabatic ππ* path directly and without barrier to the conical intersection seam with the ground state. In the case of the
pyrimidine bases, the dynamics starts off in much flatter regions of the ππ* energy surface due to coupling of several states.
This fact prohibits a clear formation of a single reaction path. Thus, the photodynamics of the pyrimidine bases is much richer
and includes also nπ* states with varying importance, depending on the actual nucleobase considered. Trapping in local minima
may occur and, therefore, the deactivation time to the ground state is also much longer in these cases. Implications of these
findings are discussed (i) for identifying structural possibilities where singlet/triplet transitions can occur because of sufficient retention time
during the singlet dynamics and (ii) concerning the flexibility of finding other deactivation pathways in substituted pyrimidines serving as candidates for alternative
nucleobases.
Co-reporter:Matthias Ruckenbauer, Mario Barbatti, Thomas Müller and Hans Lischka
The Journal of Physical Chemistry A 2010 Volume 114(Issue 25) pp:6757-6765
Publication Date(Web):June 2, 2010
DOI:10.1021/jp103101t
A new implementation of nonadiabatic excited-state dynamics using hybrid methods is presented. The current approach is aimed at the simulation of photoexcited molecules in solution. The chromophore is treated at the ab initio level, and its interaction with the solvent is approximated by point charges within the electrostatic embedding approach and by a Lennard-Jones potential for the nonbonded interactions. Multireference configuration interaction (MRCI) and multiconfiguration self-consistent field (MCSCF) methods can be used. The program implementation has been performed on the basis of the Columbus and Newton-X program systems. For example, the dynamics of penta-2,4-dien-1-iminium (PSB3) and 4-methyl-penta-2,4-dien-1-iminium cations (MePSB3) was investigated in gas phase and in n-hexane solution. The excited-state (S1) lifetime and temporal evolution of geometrical parameters were computed. In the case of PSB3 the n-hexane results resemble closely the gas phase data. MePSB3, however, shows a distinct extension of lifetime due to steric hindering of the torsion around the central bond because of solute−solvent interactions.
Co-reporter:Matthias Ruckenbauer, Mario Barbatti, Bernhard Sellner, Thomas Muller, and Hans Lischka
The Journal of Physical Chemistry A 2010 Volume 114(Issue 48) pp:12585-12590
Publication Date(Web):November 11, 2010
DOI:10.1021/jp108844g
The nonadiabatic deactivation of trans-azomethane starting from the nπ* state has been investigated in gas phase, water, and n-hexane using an on-the-fly surface-hopping method. A quantum mechanical/molecular mechanics (QM/MM) approach was used employing a flexible quantum chemical level for the description of electronically excited states and bond dissociation (generalized valence bond perfect-pairing complete active space). The solvent effect on the lifetime and structural parameters of azomethane was investigated in detail. The calculations show that the nonadiabatic deactivation is characterized by a CNNC torsion, mainly impeded by mechanic interaction with the solvent molecules. The similar characteristics of the dynamics in polar and nonpolar solvent indicate that solvent effects based on electrostatic interactions do not play a major role. Lifetimes increase by about 20 fs for both solvents with respect to the 113 fs found for the gas phase. The present subpicosecond dynamics also nicely show an example of the suppression of C−N dissociation by the solvent cage.
Co-reporter:Mario Barbatti, Adélia J. A. Aquino, Hans Lischka, Christian Schriever, Stefan Lochbrunner and Eberhard Riedle
Physical Chemistry Chemical Physics 2009 vol. 11(Issue 9) pp:1406-1415
Publication Date(Web):13 Jan 2009
DOI:10.1039/B814255F
We study the ultrafast electronic relaxation of the proton transfer compound 2-(2′-hydroxyphenyl)benzothiazole (HBT) in a joint approach of femtosecond pump–probe experiments and dynamics simulations. The measurements show a lifetime of 2.6 ps for the isolated molecule in the gas phase in contrast to ∼100 ps for cyclohexane solution. This unexpected decrease by a factor of 40 for the gas phase is explained by ultrafast internal conversion to the ground state promoted by an inter-ring torsional mode. The quantum chemical calculations based on multireference configuration interaction clearly demonstrate that a S0/S1 conical intersection at a 90° twisted structure exists and is responsible for the ultrafast decay. The reaction path leading from the keto form of HBT to this intersection is practically barrierless on the S1 surface. The on-the-fly dynamics simulations using time-dependent density functional theory show that after electronic excitation to the S1 state and after fast excited-state proton transfer (30–50 fs), HBT reaches the region of the S1/S0 crossing within about 500 fs, which will lead to the observed 2.6 ps deactivation to the ground state. After the internal conversion, HBT branches in two populations, one that rapidly closes the proton transfer cycle and another (trans-keto) that takes ∼100 ps for that step.
Co-reporter:Adélia J. A. Aquino, Daniel Tunega, Gabriele E. Schaumann, Georg Haberhauer, Martin H. Gerzabek and Hans Lischka
The Journal of Physical Chemistry C 2009 Volume 113(Issue 37) pp:16468-16475
Publication Date(Web):August 19, 2009
DOI:10.1021/jp9054796
Molecular simulations using density functional theory (DFT/PBE and DFT/tight-binding (DFTB)) have been performed to study wetting processes of model nanopore segments in humic substances (HS). A complex of two poly(acrylic acid) trimers (trimer complex, TC) arranged in parallel alignment was used to provide the structural example for supramolecular contact of two HS chains by means of hydrogen bonds. Their interaction with a local network of water molecules represented the influence of wet spots. Displaced TC structures were constructed by horizontal motion of the chains relative to each other in order to study the capacity of the water cluster to hold the two chains together even though their distance is too far for direct hydrogen bonding between the carboxyl groups. Geometry optimizations and molecular dynamics simulations were used to investigate the hydrogen-bonded structures formed and to compute their energetic stabilities. At shorter distances between the two oligomer chains an outer solvation was most stable. However, with increasing distance of the two polyacrylic trimers the water molecules penetrated into the inside of the created free space, keeping the two chains together by means of a hydrogen-bonded network. Significant stabilization effects of 10−20 kcal/mol were observed by this intrusion of water molecules at trimer distances of ∼13 Å. The present model, therefore, strongly supports the hypothesized bridging function of water molecules in humic substances provided a local distribution of appropriate functional groups is available in the HS matrix.
Co-reporter:Jaroslaw J. Szymczak, Mario Barbatti, Jason T. Soo Hoo, Jaclyn A. Adkins, Theresa L. Windus, Dana Nachtigallová and Hans Lischka
The Journal of Physical Chemistry A 2009 Volume 113(Issue 45) pp:12686-12693
Publication Date(Web):August 19, 2009
DOI:10.1021/jp905085x
Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S2 → S1 deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S1 and S2 minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR-CISD methods. The photodynamical process starts with a fast (<100 fs) planar relaxation from the S2 ππ* state into the πOπ* minimum of the S2 state. The calculations demonstrate that two π-excited states (denoted ππ* and πOπ*) are actually involved in this stage. The time in reaching the S2/S1 intersections, through which thymine can deactivate to S1, is delayed by both the change in character between the states as well as the flatness of the S2 surface. This deactivation occurs in an average time of 2.6 ps at the lowest-energy region of the crossing seam. After that, thymine relaxes to the nπ* minimum of the S1 state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the πOπ* S2 trapping but also the trapping in the nπ* S1 minimum contribute to the elongation of the excited-state lifetime of thymine.
Co-reporter:Mirjana Eckert-Maksić, Hans Lischka, Zvonimir B. Maksić and Mario Vazdar
The Journal of Physical Chemistry A 2009 Volume 113(Issue 29) pp:8351-8358
Publication Date(Web):July 1, 2009
DOI:10.1021/jp9015273
The energy profiles of the isomerization of mono, di-, and tetracyano-substituted cyclobutadienes (CBDs) are computed at the multireference average quadratic coupled cluster/complete active space self-consistent field level of theory. It was found that the energy barrier heights for the automerization reaction are 2.6 (tetracyano-CBD), 5.1 (1,3-dicyano-CBD), and 6.4 (cyano-CBD) kcal mol−1, implying that they are lowered relative to that in the parent CBD (6.4 kcal mol−1), the monosubstituted derivative being an exception. Since the free CBD shuttles between two equivalent structures even at low temperature of 10 K, it follows that bond-stretch isomerism does not take place in cyanocyclobutadienes. Instead, these compounds exhibit rapid fluxional interconversion at room temperature between two bond-stretch isomers by the double bond flipping mechanism. The reason behind the decrease in the barrier heights is identified as a slightly enhanced resonance effect at the saddle points separating two (equivalent) bond-stretch isomers, compared to that in the equilibrium structures, predominantly due to the diradical character of the former. It is also shown that the energy gap between the singlet ground state saddle point structure and the first triplet equilibrium geometry decreases upon multiple substitution by the cyano groups. The splitting of the S and T energy is small being within the range of 6.5−8.2 kcal mol−1.
Co-reporter:Vladimír Lukeš, Roland Šolc, Hans Lischka and Harald Friedrich Kauffmann
The Journal of Physical Chemistry A 2009 Volume 113(Issue 51) pp:14141-14149
Publication Date(Web):November 23, 2009
DOI:10.1021/jp902658u
A systematic study of fluorenone and model oligofluorenes (trimer, pentamer, and heptamer) with a central keto defect was performed at ab initio Hartree−Fock (HF), density functional theory (DFT), configuration interaction singles (CIS), and time-dependent density functional theory (TD-DFT) levels. The main aim of this work was the investigation of the direct influence of the central keto defect on the optimal geometry, torsional potentials, and photophysical properties. From the structural point of view, the optimal all-trans electronic ground state geometries of studied oligomers exhibit a uniform torsion of ca. 44−45° (HF) or 37−38° (DFT). The optical excitation leads to the planarization of the fluorenone and fluorene fragments in the central part of the molecule (∼34° for CIS and ∼29° for TD-DFT). The computed excitation and fluorescence energies show a good agreement with the experiment. These presented theoretical results can be useful in designing novel fluorene−fluorenone optical materials as well as understanding of excitation−relaxation phenomena which may occur in various time-dependent optical experiments.
Co-reporter:Jaroslaw J. Szymczak, Mario Barbatti and Hans Lischka
The Journal of Physical Chemistry A 2009 Volume 113(Issue 43) pp:11907-11918
Publication Date(Web):August 4, 2009
DOI:10.1021/jp903329j
Nonadiabatic photodynamical simulations are presented for the all-trans and 5-cis isomers of the hepta-3,5,7-trieniminium cation (PSB4) with the goal of characterizing the types of torsional modes occurring in the cis-trans isomerization processes in retinal protonated Schiff base (RPSB), the rhodopsin and bacteriorhodopsin chropomhore. Steric hindrance of these processes due to environmental effects have been modeled by imposing different sets of mechanical restrictions on PSB4 and studying its response in the photodynamics. Both the mechanism toward the conical intersection and the initial phase of the hot ground state dynamics has been studied in detail. A total of 600 trajectories have been computed using a complete active space self-consistent field wave function. Careful comparison with higher level methods has been made in order to verify the accuracy of the results. The most important mechanism driving restricted PSB4 isomerization in the excited state is characterized by two concerted twist motions (bipedal and closely related to it nonrigid bipedal) from which only one torsion tends to be continued during the relaxation into the ground state. The one-bond-flip is found to be important for the trans isomer as well. The main isomerization trend is a torsion around C5C6 (equivalent to C11C12 in RPSB) in the case of the cis isomer and around C3C4 (C13C14 in RPSB) in the case of the trans isomer. The simulations show an initial 70 fs relaxation into twisted regions and give an average internal conversion time of 130−140 fs, timings that are fully compatible with the general picture described by femtosecond transient absorption spectroscopic studies.
Co-reporter:Felix Plasser, Mario Barbatti, Adélia J. A. Aquino and Hans Lischka
The Journal of Physical Chemistry A 2009 Volume 113(Issue 30) pp:8490-8499
Publication Date(Web):July 2, 2009
DOI:10.1021/jp9032172
The excited-state mono- and diproton transfer has been investigated in the S1 state of [2,2′-bipyridyl]-3,3′-diol using the quantum mechanical resolution-of-identity second-order approximate coupled-cluster (RI-CC2) and time-dependent density functional theory (TDDFT) methods. Static investigation of stationary points and scans of the ππ* and nπ* energy surfaces have been performed. These calculations show that the concerted diproton transfer in S1 proceeds along a ridge thus making this process highly unlikely since it will stabilize toward the unsymmetrical monoproton transfer. A small energy barrier of about 0.11 eV (RI-CC2 result) between the mono- and diketo structures is obtained allowing rapid continuation of the proton transfer to the diketo form. On-the-fly dynamics simulations performed at the RI-CC2 level confirm this picture. The first proton transfer step is so fast (7 fs) that it probably cannot be resolved by experimental techniques. Important participation of the nπ* state is predicted. The present results shed a completely new light on the interpretation of the experimental results. The simulations clearly show that what has been experimentally determined as concerted transfer is in fact a combination of two sequential proton transfers separated by a small delay below the present experimental resolution. Concerning the second step of the sequential proton transfer the dynamics calculations indicate the existence of a highly dynamic system. Both the forward and reverse reactions of a monoketo/diketo equilibrium were found within the 300 fs period of the simulation. Environmental effects will certainly lead to a substantial cooling of the initially hot molecule and a concomitant decrease in the monoketo/diketo conversion rates, which will result in the experimentally observed overall time scale of 10 ps for the second proton transfer step.
Co-reporter:Jaroslaw J. Szymczak, Mario Barbatti and Hans Lischka
Journal of Chemical Theory and Computation 2008 Volume 4(Issue 8) pp:1189-1199
Publication Date(Web):July 26, 2008
DOI:10.1021/ct800148n
Ab initio surface-hopping dynamics simulations for the trans-penta-3,5-dieniminium cation (PSB3) are presented imposing different sets of mechanical restrictions in order to investigate the response of the molecular system to certain environmental degrees of hindrance. A general scheme for classification of photoisomerization mechanisms in conjugated chains based on the analysis of torsional angles is proposed allowing direct characterization of the different isomerization mechanisms proposed previously. On the basis of a statistical analysis of 300 trajectories a new photoisomerization mechanism−the Folding Table−was found. This mechanism and the One-Bond-Flip are almost entirely responsible for the photoisomerization process in PSB3.
Co-reporter:Mario Barbatti, Matthias Ruckenbauer, Jaroslaw J. Szymczak, Adélia J. A. Aquino and Hans Lischka
Physical Chemistry Chemical Physics 2008 vol. 10(Issue 4) pp:482-494
Publication Date(Web):12 Sep 2007
DOI:10.1039/B709315M
Multireference ab initio dynamics simulations have become available as a tool for the investigation of photochemical processes, mainly for those related to nonadiabatic phenomena taking place in the sub-picosecond time scale. For organic molecules, these phenomena are in many cases deeply dependent on the relaxation of the photoexcited π-system. We review the latest contributions of our group to this subject and report new results for systems studied previously, grouping them in single π bonds, chains and aromatic rings. The dynamics of ethylene and substituted ethylenes is discussed mainly in connection to the competition between the two available relaxation paths in the excited states and their relation to the conical intersections in large systems. The trans–cis and the cis–trans dynamics of the pentadieniminium cation is investigated as well. Finally, we discuss the photodynamics of aminopyrimidine starting in the S1 and S2 states and the conclusions, which can be drawn from this for the interpretation of the adenine dynamics.
Co-reporter:Horst Köppel, Bernd Schubert, Hans Lischka
Chemical Physics 2008 Volume 343(2–3) pp:319-328
Publication Date(Web):29 January 2008
DOI:10.1016/j.chemphys.2007.06.017
Abstract
The conically intersecting potential energy surfaces of the S1 and S2 excited states of acetylene and the resulting strong nonadiabatic couplings are investigated theoretically. The adiabatic potential energy surfaces are obtained from high-level MRCI calculations. They are diabatized using the concept of regularized diabatic states and then used as a basis for the subsequent wave-packet dynamical treatment of the nuclear motion. All three angular degrees of freedom are included in the present study, while the bond lengths are kept frozen. The importance of the nonadiabatic interactions for the fine structure of the VUV spectrum of acetylene in the 6.5–8 eV excitation energy range is established. The electronic populations display an S2 → S1 internal conversion process on the order of 50 fs, which is, however, incomplete owing to the relatively small S2–S1 energy gap and the present reduced-dimensionality treatment. Snapshots of the wave-packet as well as angular probability densities are analyzed and reveal, for the first time, an incipient excited-state cis–trans isomerization in this system.
Co-reporter:Christian Schriever, Mario Barbatti, Kai Stock, Adélia J.A. Aquino, Daniel Tunega, Stefan Lochbrunner, Eberhard Riedle, Regina de Vivie-Riedle, Hans Lischka
Chemical Physics 2008 Volume 347(1–3) pp:446-461
Publication Date(Web):23 May 2008
DOI:10.1016/j.chemphys.2007.10.021
Abstract
The excited-state intramolecular proton transfer in the aromatic polycycle 10-hydroxybenzo[h]quinoline is investigated by means of transient absorption experiments with 30 fs time resolution, classical dynamics and wavepacket dynamics. The experiments establish the ultrafast transfer after UV excitation and show signatures of coherent vibrational motion in the keto product. To elucidate details of the proton transfer mechanism, the classical dynamics is also performed for 2-(2′-hydroxyphenyl)benzothiazole and the results are compared. For both systems the proton transfer takes place on the ultrafast scale of 30–40 fs, with good agreement between the theoretical investigations and the measurements. The dynamics simulations show that for both molecules the proton is handed over by means of skeletal deformation of the molecule. Due to the more rigid structure of 10-hydroxybenzo[h]quinoline the hydrogen migration mode participates more actively than in 2-(2′-hydroxyphenyl)benzothiazole.
Co-reporter:Ivana Antol;Mario Barbatti
Monatshefte für Chemie - Chemical Monthly 2008 Volume 139( Issue 4) pp:319-328
Publication Date(Web):2008 April
DOI:10.1007/s00706-007-0803-2
The properties of formamide, its protonated form and interaction complexes with lithium and sodium cations were studied in electronically excited singlet states by means of high-level multireference ab initio methods. The vertical excitation energies show a marked influence on protonation with particular large effects found for the O-protonated form as compared to neutral formamide. Complexation with Li+ and Na+ leads to a pronounced shift of the nO–π* state to higher energies while the π–π* state moves in opposite direction. Geometry optimizations in the lowest excited singlet show strong geometrical effects leading to pyramidalization at the N and C atoms. The photodynamical simulations performed for formamide in the first excited singlet state show that the main primary deactivation path is CN dissociation with a lifetime of about 420 fs.
Co-reporter:Gunther Zechmann, Mario Barbatti, Hans Lischka, Jiří Pittner, Vlasta Bonačić-Koutecký
Chemical Physics Letters 2006 Volume 418(4–6) pp:377-382
Publication Date(Web):6 February 2006
DOI:10.1016/j.cplett.2005.11.015
Abstract
Surface-hopping dynamics carried out on-the-fly was performed by means of quantum chemical multireference configuration interaction methods in order to investigate the photodynamics of silaethylene (SiCH4). The evolution of the S0 and S1 states were investigated during the first 100 fs after photoexcitation. In contrast to expectations based on previous static calculations, two mechanisms were found, corresponding to two minimum energy paths, which show characteristically different lifetimes. The bipyramidalization vs. torsion and stretching modes were identified to be responsible for this behavior driving the molecule into one of the two pathways.
Co-reporter:Mario Barbatti Dr.;Adélia J. A. Aquino Dr. Dr.
ChemPhysChem 2006 Volume 7(Issue 10) pp:2089-2096
Publication Date(Web):29 AUG 2006
DOI:10.1002/cphc.200600199
Complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), density functional theory (DFT), time dependent DFT (TDDFT) and the singles and doubles coupled-cluster (CC2) methodologies have been used to study the ground state and excited states of protonated and neutral Schiff bases (PSB and SB) as models for the retinal chromophore. Systems with two to four conjugated double bonds are investigated. Geometry relaxation effects are studied in the excited ππ* state using the aforementioned methods. Taking the MRCI results as reference we find that CASSCF results are quite reliable even though overshooting of geometry changes is observed. TDDFT does not reproduce bond alternation well in the ππ* state. CC2 takes an intermediate position. Environmental effects due to solvent or protein surroundings have been studied in the excited states of the PSBs and SBs using a water molecule and solvated formate as model cases. Particular emphasis is given to the proton transfer process from the PSB to its solvent partner in the excited state. It is found that its feasibility is significantly enhanced in the excited state as compared to the ground state, which means that a proton transfer could be initiated already at an early step in the photodynamics of PSBs.
Co-reporter:M. Barbatti, G. Granucci, M. Persico, H. Lischka
Chemical Physics Letters 2005 Volume 401(1–3) pp:276-281
Publication Date(Web):1 January 2005
DOI:10.1016/j.cplett.2004.11.069
Abstract
Semiempirical molecular dynamics with surface hopping was employed to investigate the lifetime of excited states of ethylene. Based on previous ab initio multireference configuration interaction results, a complete reparametrization of the AM1 semiempirical parameters was performed. Depending on the initial vertical excitation energy, lifetimes from 105 to 139 fs were found for the V-state decay. Comparison to the pump–probe experiments was performed in order to explain the large differences between the theoretically and experimentally obtained lifetimes. The results show that probe energies of at least 7.4 eV should be employed to ionize the system for geometries close to the conical intersections.
Co-reporter:Ivana Antol, Mirjana Eckert-Maksić, Thomas Müller, Michal Dallos, Hans Lischka
Chemical Physics Letters 2003 Volume 374(5–6) pp:587-593
Publication Date(Web):18 June 2003
DOI:10.1016/S0009-2614(03)00770-X
MR-CISD and MR-CISD + Q calculations have been performed for the vertical excitations of protonated formaldehyde in comparison to formaldehyde. Singlet and triplet states have been investigated. It is shown that the protonation causes the Rydberg states to be shifted to higher energies by several eV. This finding is discussed by means of the Rydberg formula in terms of quantum defects for the two lowest vertical ionization energies. For protonated formaldehyde the π–π* valence state is energetically the second lowest state at 9.80 eV, about 1.50 eV below the first Rydberg n–3s state. This finding is in strong contrast to the case of formaldehyde where the π–π* state is embedded within a series of Rydberg states.
Co-reporter:Adélia J. A. Aquino, Daniel Tunega, Georg Haberhauer, Martin H. Gerzabek and Hans Lischka
Physical Chemistry Chemical Physics 2001 vol. 3(Issue 11) pp:1979-1985
Publication Date(Web):03 May 2001
DOI:10.1039/B008987G
Density-functional
calculations have been performed on tridentate, hydrated aluminium–citrate complexes using
fully (quadruply) and triply deprotonated citric acid ligands. Water molecules in the inner solvation sphere
have been included explicitly in the quantum-chemical calculation, whereas the remaining solvent effects have
been computed using the polarized continuum model (PCM). As is to be expected, solvation effects play an important
role for the calculation of formation energies of the complexes. Optimized geometries are in good agreement
with X-ray data. Reaction enthalpies and Gibbs reaction energies have been computed for the substitution
of water molecules of the aluminium–hexaaquo complex by citrate molecules. Formation of the tridentate complexes
is strongly favored by entropy effects in analogy to previous findings for bidentate acetate and
oxalate complexes. Comparison of the stability
of acetate, oxalate
and citrate complexes
shows a pronounced preference for the latter.
Co-reporter:Adélia J. A. Aquino, Daniel Tunega, Georg Haberhauer, Martin Gerzabek and Hans Lischka
Physical Chemistry Chemical Physics 2000 vol. 2(Issue 13) pp:2845-2850
Publication Date(Web):07 Jun 2000
DOI:10.1039/B002495N
Density
functional theory calculations using split-valence polarized basis sets augmented by diffuse s and
p functions on the carbon and oxygen atoms were carried out for the complexes of hydrated Al3+ with oxalic acid,
and the oxalate mono- and dianions. Monodentate and bidentate structures with up to three ligands have been computed. The polarized continuum model was used to study the solvent effect on the structures
and stabilities of the complexes. Reaction energies for the replacement of water molecules in
the aluminum–hexaaquo complex by oxalate ligands have been computed. Based on a detailed thermodynamical analysis,
characteristic differences in the formation of mono- and bidentate structures were found. In the former
case the entropy contributions to ΔG are rather small, whereas in the latter case they are substantial.
Thus, the formation reactions for the monodentate complexes are energy-driven whereas those for the
bidentate complexes are entropy-driven. In agreement with experiment, the most dominant complex in solution is [AlOx3]3−. From the complexes with oxalic acid and the partially deprotonated
form HOx−,
only the latter should
give stable bidentate complexes in solution.
Co-reporter:Reed Nieman, Anita Das, Adélia J.A. Aquino, Rodrigo G. Amorim, Francisco B.C. Machado, Hans Lischka
Chemical Physics (12 January 2017) Volume 482() pp:
Publication Date(Web):12 January 2017
DOI:10.1016/j.chemphys.2016.08.007
Graphene is regarded as one of the most promising materials for nanoelectronics applications. Defects play an important role in modulating its electronic properties and also enhance its chemical reactivity. In this work the reactivity of single vacancies (SV) and double vacancies (DV) in reaction with a hydrogen atom Hr is studied. Because of the complicated open shell electronic structures of these defects due to dangling bonds, multireference configuration interaction (MRCI) methods are being used in combination with a previously developed defect model based on pyrene. Comparison of the stability of products derived from CHr bond formation with different carbon atoms of the different polyaromatic hydrocarbons is made. In the single vacancy case the most stable structure is the one where the incoming hydrogen is bound to the carbon atom carrying the dangling bond. However, stable CHr bonded structures are also observed in the five-membered ring of the single vacancy. In the double vacancy, most stable bonding of the reactant Hr atom is found in the five-membered rings. In total, CHr bonds, corresponding to local energy minimum structures, are formed with all carbon atoms in the different defect systems and the pyrene itself. Reaction profiles for the four lowest electronic states show in the case of a single vacancy a complex picture of curve crossings and avoided crossings which will give rise to a complex nonadiabatic reaction dynamics involving several electronic states.
Co-reporter:Adelia J.A. Aquino, Daniel Tunega, Georg Haberhauer, Martin H. Gerzabek, Hans Lischka
Geochimica et Cosmochimica Acta (1 August 2008) Volume 72(Issue 15) pp:3587-3602
Publication Date(Web):1 August 2008
DOI:10.1016/j.gca.2008.04.037
Density functional theory is used to compute the effect of protonation, deprotonation, and dehydroxylation of different reactive sites of a goethite surface modeled as a cluster containing six iron atoms constructed from a slab model of the (1 1 0) goethite surface. Solvent effects were treated at two different levels: (i) by inclusion of up to six water molecules explicitly into the quantum chemical calculation and (ii) by using additionally a continuum solvation model for the long-range interactions. Systematic studies were made in order to test the limit of the fully hydrated cluster surfaces by a monomolecular water layer. The main finding is that from the three different types of surface hydroxyl groups (hydroxo, μ-hydroxo, and μ3-hydroxo), the hydroxo group is most active for protonation whereas μ- and μ3-hydroxo sites undergo deprotonation more easily. Proton affinity constants (pKa values) were computed from appropriate protonation/deprotonation reactions for all sites investigated and compared to results obtained from the multisite complexation model (MUSIC). The approach used was validated for the consecutive deprotonation reactions of the [Fe(H2O)6]3+ complex in solution and good agreement between calculated and experimental pKa values was found. The computed pKa for all sites of the modeled goethite surface were used in the prediction of the pristine point of zero charge, pHPPZN. The obtained value of 9.1 fits well with published experimental values of 7.0–9.5.
Co-reporter:Adélia A. J. Aquino, Itamar Borges, Reed Nieman, Andreas Köhn and Hans Lischka
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 38) pp:NaN20597-20597
Publication Date(Web):2014/08/05
DOI:10.1039/C4CP02900C
A comprehensive theoretical study of the electronically excited states in complexes between tetracyanoethylene (TCNE) and three aromatic electron donors, benzene, naphthalene and anthracene, was performed with a focus on charge transfer (CT) transitions. The results show that the algebraic diagrammatic construction method to second order (ADC(2)) provides excellent possibilities for reliable calculations of CT states. Significant improvements in the accuracy of the computed transition energies are obtained by using the scaled opposite-spin (SOS) variant of ADC(2). Solvent effects were examined on the basis of the conductor-like screening model (COSMO) which has been implemented recently in the ADC(2) method. The dielectric constant and the refractive index of dichloromethane have been chosen in the COSMO calculations to compare with experimental solvatochromic effects. The computation of optimized ground state geometries and enthalpies of formation has been performed at the second-order Møller–Plesset perturbation theory (MP2) level. By comparison with experimental data and with high-level coupled-cluster methods including explicitly correlated (F12) wave functions, the importance of the SOS approach is demonstrated for the ground state as well. In the benzene–TCNE complex, the two lowest electronic excitations are of CT character whereas in the naphthalene and anthracene TCNE complexes three low-lying CT states are observed. As expected, they are strongly stabilized by the solvent. Geometry optimization in the lowest excited state allowed the calculation of fluorescence transitions. Solvent effects lead to a zero gap between S1 and S0 for the anthracene–TCNE complex. Therefore, in the series of benzene–TCNE to anthracene a change from a radiative to a nonradiative decay mechanism to the ground state is to be expected.
Co-reporter:Max Pinheiro, Luiz F. A. Ferrão, Fernanda Bettanin, Adélia J. A. Aquino, Francisco B. C. Machado and Hans Lischka
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 29) pp:NaN19233-19233
Publication Date(Web):2017/06/27
DOI:10.1039/C7CP03198J
Acenes are fascinating polyaromatic compounds that combine impressive semiconductor properties with an open-shell character by varying their molecular sizes. However, the increasing chemical instabilities related to their biradicaloid structures pose a great challenge for synthetic chemistry. Modifying the π-bond topology through chemical doping allows modulation of the electronic properties of graphene-related materials. In spite of the practical importance of these techniques, remarkably little is known about the basic question – the extent of the radical character created or quenched thereby. In this work, we report a high-level computational study on two acene oligomers doubly-doped with boron and nitrogen, respectively. These calculations demonstrate precisely which the chemical route is in order to either quench or enhance the radical character. Moving the dopants from the terminal rings to the central ones leads to a remarkable variation in the biradicaloid character (and thereby also in the chemical stability). This effect is related to a π-charge transfer involving the dopants and the radical carbon centers at the zigzag edges. This study also provides specific guidelines for a rational design of large polyaromatic compounds with enhanced chemical stability.
Co-reporter:Mario Barbatti, Adélia J. A. Aquino, Hans Lischka, Christian Schriever, Stefan Lochbrunner and Eberhard Riedle
Physical Chemistry Chemical Physics 2009 - vol. 11(Issue 9) pp:NaN1415-1415
Publication Date(Web):2009/01/13
DOI:10.1039/B814255F
We study the ultrafast electronic relaxation of the proton transfer compound 2-(2′-hydroxyphenyl)benzothiazole (HBT) in a joint approach of femtosecond pump–probe experiments and dynamics simulations. The measurements show a lifetime of 2.6 ps for the isolated molecule in the gas phase in contrast to ∼100 ps for cyclohexane solution. This unexpected decrease by a factor of 40 for the gas phase is explained by ultrafast internal conversion to the ground state promoted by an inter-ring torsional mode. The quantum chemical calculations based on multireference configuration interaction clearly demonstrate that a S0/S1 conical intersection at a 90° twisted structure exists and is responsible for the ultrafast decay. The reaction path leading from the keto form of HBT to this intersection is practically barrierless on the S1 surface. The on-the-fly dynamics simulations using time-dependent density functional theory show that after electronic excitation to the S1 state and after fast excited-state proton transfer (30–50 fs), HBT reaches the region of the S1/S0 crossing within about 500 fs, which will lead to the observed 2.6 ps deactivation to the ground state. After the internal conversion, HBT branches in two populations, one that rapidly closes the proton transfer cycle and another (trans-keto) that takes ∼100 ps for that step.
Co-reporter:Mario Barbatti, Matthias Ruckenbauer, Jaroslaw J. Szymczak, Adélia J. A. Aquino and Hans Lischka
Physical Chemistry Chemical Physics 2008 - vol. 10(Issue 4) pp:NaN494-494
Publication Date(Web):2007/09/12
DOI:10.1039/B709315M
Multireference ab initio dynamics simulations have become available as a tool for the investigation of photochemical processes, mainly for those related to nonadiabatic phenomena taking place in the sub-picosecond time scale. For organic molecules, these phenomena are in many cases deeply dependent on the relaxation of the photoexcited π-system. We review the latest contributions of our group to this subject and report new results for systems studied previously, grouping them in single π bonds, chains and aromatic rings. The dynamics of ethylene and substituted ethylenes is discussed mainly in connection to the competition between the two available relaxation paths in the excited states and their relation to the conical intersections in large systems. The trans–cis and the cis–trans dynamics of the pentadieniminium cation is investigated as well. Finally, we discuss the photodynamics of aminopyrimidine starting in the S1 and S2 states and the conclusions, which can be drawn from this for the interpretation of the adenine dynamics.
Co-reporter:Francisco B. C. Machado, Adélia J. A. Aquino and Hans Lischka
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 19) pp:NaN12785-12785
Publication Date(Web):2015/04/13
DOI:10.1039/C4CP05751A
The electronic states occurring in a double vacancy defect for graphene nanoribbons have been calculated in detail based on a pyrene model. Extended ab initio calculations using the MR configuration interaction (MRCI) method have been performed to describe in a balanced way the manifold of electronic states derived from the dangling bonds created by initial removal of two neighboring carbon atoms from the graphene network. In total, this study took into account the characterization of 16 electronic states (eight singlets and eight triplets) considering unrelaxed and relaxed defect structures. The ground state was found to be of 1Ag character with around 50% closed shell character. The geometry optimization process leads to the formation of two five-membered rings in a pentagon–octagon–pentagon (5–8–5) structure. The closed shell character increases thereby to ∼70%; the analysis of unpaired density shows only small contributions confirming the chemical stability of that entity. For the unrelaxed structure the first five excited states (3B3g, 3B2u, 3B1u, 3Au and 1Au) are separated from the ground state by less than 2.5 eV. For comparison, unrestricted density functional theory (DFT) calculations using several types of functionals have been performed within different symmetry subspaces defined by the open shell orbitals. Comparison with the MRCI results gave good agreement in terms of finding the 1Ag state as a ground state and in assigning the lowest excited states. Linear interpolation curves between the unrelaxed and relaxed defect structures also showed good agreement between the two classes of methods opening up the possibilities of using extended nanoflakes for multistate investigations at the DFT level.
Co-reporter:Nadeesha J. Silva, Francisco B. C. Machado, Hans Lischka and Adelia J. A. Aquino
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 32) pp:NaN22310-22310
Publication Date(Web):2016/07/19
DOI:10.1039/C6CP03749F
High level ab initio calculations ranging from coupled cluster methods including explicitly correlated approaches to standard second order Møller–Plesset theory using spin scaling (SOS-MP2) have been performed on sandwich and slipped parallel dimer structures of a series of quasi one-dimensional acenes and on two-dimensional sheets containing the pyrene to coronene series encircled with two layers of benzene rings. Sandwich (graphitic AA type) and slipped parallel (AB type) structures were considered and, within the given symmetry restrictions, full geometry optimizations were performed. Basis set superposition effects have been considered. The computed geometries show a significant biconcave deviation of the two-dimensional sheets from planarity with the central intersheet C⋯C distances considerably smaller that van der Waals distances. The computed intersheet binding energy per carbon atom extrapolated for N → ∞ of −74.3 meV (1.713 kcal mol−1) per atom agrees quite well with an experimental defoliation energy of −52 meV (1.199 kcal mol−1) per atom (−67 meV (1.545 kcal mol−1) per carbon atom without corrections for H binding contributions) for polyaromatic hydrocarbons (PAHs) from graphite. A limited investigation of density functional theory (DFT) calculations using empirical dispersion contributions has been performed also showing a significant underbinding character of the D3 method. For most of the DFT variants investigated the graphene sheet models retain a quasi-planar structure in strong contrast to the aforementioned SOS-MP2 results.
Co-reporter:Ivana Antol, Mirjana Eckert-Maksić, Mario Vazdar, Matthias Ruckenbauer and Hans Lischka
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 38) pp:NaN13272-13272
Publication Date(Web):2012/08/23
DOI:10.1039/C2CP41830D
Non-adiabatic on-the-fly dynamics simulations of the photodynamics of formamide in water and n-hexane were performed using a QM/MM approach. It was shown that steric restrictions imposed by the solvent cage do not have an influence on the initial motion which leads to the lowest energy conical intersection seam. The initial deactivation in water is faster than in n-hexane and in the gas phase. However, most of the formamide molecules in water do not reach the ground state. The reason for the deactivation inefficiency in water is traced back to a decrease of close CO⋯HOH and NH⋯OH2 contacts which fall in the range of hydrogen bonds. The energy deposition into H-bond breaking events leaves molecules with less energy for surmounting the CN dissociation barrier. In both solvents, after hopping to the ground state, the solvent cage keeps the HCO and NH2 fragments or CO and NH3 products in close proximity. Consequently, the number of trajectories where fast recombination happens is augmented with delayed recombinations that start when the dissociation fragments hit the cage wall and return back. The hot ground state formamide is formed in an internal conversion process identical to the path leading to CN photodissociation. In the case of aqueous formamide, good agreement with experimental results is achieved by combining dynamics simulations starting from the S1 and the S2 excited states collecting high and low energy trajectories, respectively.
Co-reporter:Mario Barbatti, Adelia J. A. Aquino and Hans Lischka
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 19) pp:NaN4967-4967
Publication Date(Web):2010/03/29
DOI:10.1039/B924956G
Semi-classical simulations of the UV-photoabsorption cross sections of adenine, guanine, cytosine, thymine, and uracil in gas phase were performed at the resolution-of-identity coupled cluster to the second-order (RI-CC2) level. With the exception of cytosine, the spectra of the other four nucleobases show a two band pattern separated by a low intensity region. The spectrum of cytosine is shaped by a sequence of three bands of increasing intensity. The first band of guanine is composed by two ππ* transitions of similar intensities. The analysis of individual contributions to the spectra allows a detailed assignment of bands. It is shown that the semi-classical simulations are able to predict general features of the experimental spectra, including their absolute intensities.
Co-reporter:Mario Barbatti and Hans Lischka
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 23) pp:NaN15459-15459
Publication Date(Web):2015/05/12
DOI:10.1039/C5CP01151E
2-Aminopurine (2AP) is often chosen as a fluorescent replacement for purine bases and used as a probe in nucleic acid research. The luminescence of this molecule is strongly dependent on the environment. Through computational simulations of isolated 2AP and a series of 2AP–water clusters, we show that the experimentally-observed dependence of the excited-state lifetime of 2AP on the number and location of water molecules is controlled by a barrier for internal conversion between the S1 minimum and a conical intersection. Other possible competing pathways (proton transfer, intersystem crossing, and internal conversion at other intersections) were also investigated but discarded. The tuning of the luminescence of 2AP by water is related to the order of the nπ* and ππ* states. When a water molecule interacts with the amino group, the pathway from the S1 minimum to the conical intersection requires a nonadiabatic change, thus increasing the energy barrier for internal conversion. As a consequence, a single water molecule hydrogen-bonded to the amino group is sufficient to make 2AP fluorescent.
Co-reporter:Hasan Pašalić, Daniel Tunega, Adélia J. A. Aquino, Georg Haberhauer, Martin H. Gerzabek and Hans Lischka
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 12) pp:NaN4170-4170
Publication Date(Web):2012/02/21
DOI:10.1039/C2CP23015A
The thermodynamic stability of the acetic acid dimer conformers in microhydrated environments and in aqueous solution was studied by means of molecular dynamics simulations using the density functional based tight binding (DFTB) method. To confirm the reliability of this method for the system studied, density functional theory (DFT) and second order Møller–Plesset perturbation theory (MP2) calculations were performed for comparison. Classical optimized potentials for liquid simulations (OPLS) force field dynamics was used as well. One focus of this work was laid on the study of the capabilities of water molecules to break the hydrogen bonds of the acetic acid dimer. The barrier for insertion of one water molecule into the most stable cyclic dimer is found to lie between 3.25 and 4.8 kcal mol−1 for the quantum mechanical methods, but only at 1.2 kcal mol−1 for OPLS. Starting from different acetic acid dimer structures optimized in gas phase, DFTB dynamics simulations give a different picture of the stability in the microhydrated environment (4 to 12 water molecules) as compared to aqueous solution. In the former case all conformers are converted to the hydrated cyclic dimer, which remains stable over the entire simulation time of 1 ns. These results demonstrate that the considered microhydrated environment is not sufficient to dissociate the acetic acid dimer. In aqueous solution, however, the DFTB dynamics shows dissociation of all dimer structures (or processes leading thereto) starting after about 50 ps, demonstrating the capability of the water environment to break up the relatively strong hydrogen bridges. The OPLS dynamics in the aqueous environment shows—in contrast to the DFTB results—immediate dissociation, but a similar long-term behavior.
![Dibenzo[cd,lm]perylene](/data/chemimg/1089200/188-96-5.png)
![Dibenzo[cd,lm]perylene](/data/chemimg/1089200/188-96-5_b.png)
