Wesley D. Allen

Tunneling Isomerizations on the Potential Energy Surfaces of Formaldehyde and Methanol Radical Cations

Co-reporter:J. Philipp Wagner, Marcus A. Bartlett, Wesley D. Allen, and Michael A. Duncan

ACS Earth and Space Chemistry - New in 2017 August 17, 2017 Volume 1(Issue 6) pp:361-361

Publication Date(Web):July 13, 2017

DOI:10.1021/acsearthspacechem.7b00068

Formaldehyde (H2CO+) and methanol (H3COH+) radical cations, well-known in mass spectrometry, potentially form from radiative ionization or ion–molecule reactions in the interstellar medium. For both ions, other tautomeric forms exist that are accessible via [1,2]hydrogen shifts involving reaction barriers in excess of 25 kcal mol–1. Here, we compute the tunneling rates of the isomerization processes connecting the hydroxymethylene radical cation (HCOH+) to its more stable formaldehyde isomer (H2CO+) and the methanol radical cation (H3COH+) to its methylene oxonium isomer (H2COH2+) using the Wentzel–Kramers–Brillouin method at the CCSD(T)/cc-pVQZ//B3LYP/cc-pVTZ level of theory. While the hydroxymethylene radical cation features a half-life of over 3500 years and thus represents a potentially observable molecule, the methanol radical cation is predicted to decay with a half-life of about 4 days and is thus not likely to be present in appreciable quantities in space. We discuss the potential relevance of the hydroxymethylene and methylene oxonium cations for interstellar carbohydrate formation because both species represent potentially reactive, cationic, carbon-centered radicals.Keywords: formaldehyde; isomers; methanol; radical cations; tunneling;

Ring-Walking of Zerovalent Nickel on Aryl Halides

Co-reporter:Jenna A. Bilbrey, Andrea N. Bootsma, Marcus A. Bartlett, Jason Locklin, Steven E. Wheeler, and Wesley D. Allen

Journal of Chemical Theory and Computation April 11, 2017 Volume 13(Issue 4) pp:1706-1706

Publication Date(Web):March 9, 2017

DOI:10.1021/acs.jctc.6b01143

While ring-walking is a critical step in transition metal catalyzed cross-coupling reactions, the associated metastable intermediates are often difficult to isolate and characterize. In this work, theoretical structures and energetics for ring-walking and oxidative addition of zerovalent nickel with 1-bromo-2-methylbenzene, 2-bromopyridine, 2-bromo-3-methyl-thiophene, and 2-bromopyrrole were computed at the B3LYP-D3/TZ2P-LANL2TZ(f)-LANL08d level. The mechanisms vary qualitatively with substrate ring size and type—the catalyst weaves along the edges of the benzene and pyridine rings, cuts through the interior of the thiophene ring, and arcs along the bond opposite the nitrogen atom in the pyrrole ring. Analogous computations on the ring-walking and oxidative addition of zerovalent palladium with 1-bromo-2-methylbenzene reveal an energetic profile similar to that of Ni but with much weaker overall binding to the arene. In all cases, dispersion corrections are found to be very important for computing accurate metal–substrate binding energies.

Tunnelling in carbonic acid

Co-reporter:J. Philipp Wagner, Hans Peter Reisenauer, Viivi Hirvonen, Chia-Hua Wu, Joseph L. Tyberg, Wesley D. Allen and Peter R. Schreiner

Chemical Communications 2016 vol. 52(Issue 50) pp:7858-7861

Publication Date(Web):24 May 2016

DOI:10.1039/C6CC01756H

The cis,trans-conformer of carbonic acid (H2CO3), generated by near-infrared radiation, undergoes an unreported quantum mechanical tunnelling rotamerization with half-lives in cryogenic matrices of 4–20 h, depending on temperature and host material. First-principles quantum chemistry at high levels of theory gives a tunnelling half-life of about 1 h, quite near those measured for the fastest rotamerizations.

Conformers of Gaseous Serine

Co-reporter:Kedan He and Wesley D. Allen

Journal of Chemical Theory and Computation 2016 Volume 12(Issue 8) pp:3571-3582

Publication Date(Web):June 13, 2016

DOI:10.1021/acs.jctc.6b00314

The myriad conformers of the neutral form of natural amino acid serine (Ser) have been investigated by systematic computations with reliable electronic wave function methods. A total of 85 unique conformers were located using the MP2/cc-pVTZ level of theory. The 12 lowest-energy conformers of serine fall within a 8 kJ mol–1 window, and for these species, geometric structures, precise relative energies, equilibrium and vibrationally averaged rotational constants, anharmonic vibrational frequencies, infrared intensities, quartic and sextic centrifugal distortion constants, dipole moments, and 14N nuclear quadrupole coupling constants were computed. The relative energies were refined through composite focal-point analyses employing basis sets as large as aug-cc-pV5Z and correlation treatments through CCSD(T). The rotational constants for seven conformers measured by Fourier-transform microwave spectroscopy are in good agreement with the vibrationally averaged rotational constants computed in this study. Our anharmonic vibrational frequencies are compared to the large number of experimental vibrational absorptions attributable to at least six conformers.

Polytriangulane

Co-reporter:Wesley D. Allen, Henrik Quanz, and Peter R. Schreiner

Journal of Chemical Theory and Computation 2016 Volume 12(Issue 9) pp:4707-4716

Publication Date(Web):August 1, 2016

DOI:10.1021/acs.jctc.6b00669

The infinite spiro-annelation of cyclopropanes in a nonbranched form will produce a σ-helicene called polytriangulane, which is an unknown hydrocarbon with the formula CnHn comprised exclusively of formal C(sp3) atoms. The structure of polytriangulane is elucidated here via a rigorous mathematical analysis of a C85H88 prototype optimized by M06-2X/6-31G(d) density functional theory and an idealized polymer composed of equilateral cyclopropane units. The spiro carbons in polytriangulane form an exact, nonrepeating helix with a steep rise angle near 35°, a radius of only 0.41 Å, and irrational periodicity parameter very close to . A focal point analysis of the ring opening of cyclopropane to propene employing basis sets as large as cc-pCV5Z and correlation treatments as extensive as CCSDT(Q) yields ΔfH0°(cyclopropane) = 17.2 ± 0.1 kcal mol–1. Subsequent application of CCSD(T)/CBS theory to a homodesmotic equation for ring aggregation predicts that ΔfH0° = +16.1 kcal (mol CH)−1 for polytriangulane; hence, this compound is much more stable thermodynamically than acetylene. Similar computations on another hypothetical homodesmotic transformation indicate that the total strain energy in polytriangulane is 42.7 kcal per mole of cyclopropane units.

σ Bond activation through tunneling: formation of the boron hydride cations BHn+ (n = 2, 4, 6)

Co-reporter:Yudong Qiu, Chia-Hua Wu, Henry F. Schaefer III, Wesley D. Allen and Jay Agarwal

Physical Chemistry Chemical Physics 2016 vol. 18(Issue 5) pp:4063-4070

Publication Date(Web):07 Jan 2016

DOI:10.1039/C5CP05505A

The network of H2 additions to B+ and subsequent insertion reactions serve as a tractable model for hydrogen storage in elementary boron-containing compounds. Here, they are investigated using state-of-the-art ab initio methods (up to CCSDTQ and cc-pCV6Z basis sets). The binding energies of H2 to HBH+ (14.9 kcal mol−1) and HBH(H2)+ (18.1 kcal mol−1) are determined to be much higher than those for B(H2)+ (3.8 kcal mol−1), B(H2)2+ (3.0 kcal mol−1), and B(H2)3+ (2.5 kcal mol−1) at the CCSDTQ/CBS level of theory. These predictions are in agreement with the experiments of Kemper, Bushnell, Weis, and Bowers (J. Am. Chem. Soc., 1998, 120, 7577). Molecular orbital analyses show that the enhanced binding in HBH(H2)m+ complexes originates from the strong interaction between the 1σu HOMO of HBH+ and the 1σu LUMO of H2. For the insertion reactions B(H2)n+ → HBH(H2)n−1+, activation barriers are determined to be 58.3 kcal mol−1 [Mk-MRCCSD(T)/CBS], 12.2 kcal mol−1 (CCSDTQ/CBS) and 4.6 kcal mol−1 (CCSDTQ/CBS) for n = 1, 2, and 3, respectively. After using theoretical results to remove tunneling effects from the experimental rate constants, new Arrhenius fits yield activation barriers of 4.6(3) kcal mol−1 and 3.8(1) kcal mol−1 for the BH6+ and BD6+ insertion reactions, respectively, which are in near perfect agreement with converged theoretical values (4.6 kcal mol−1 and 3.9 kcal mol−1). These findings demonstrate that earlier Arrhenius fits considerably underestimate these barriers, and that quantum tunneling dominates the σ bond activation mechanism witnessed in previous experiments involving BH6+.

Domino Tunneling

Co-reporter:Peter R. Schreiner; J. Philipp Wagner; Hans Peter Reisenauer; Dennis Gerbig; David Ley; János Sarka; Attila G. Császár; Alexander Vaughn

Journal of the American Chemical Society 2015 Volume 137(Issue 24) pp:7828-7834

Publication Date(Web):June 1, 2015

DOI:10.1021/jacs.5b03322

Matrix-isolation experiments near 3 K and state-of-the-art quantum chemical computations demonstrate that oxalic acid [1, (COOH)2] exhibits a sequential quantum mechanical tunneling phenomenon not previously observed. Intensities of numerous infrared (IR) bands were used to monitor the temporal evolution of the lowest-energy O–H rotamers (1cTc, 1cTt, 1tTt) of oxalic acid for up to 19 days following near-infrared irradiation of the matrix. The relative energies of these rotamers are 0.0 (1cTc), 2.6 (1cTt), and 4.0 (1tTt) kcal mol–1. A 1tTt → 1cTt → 1cTc isomerization cascade was observed with half-lives (t1/2) in different matrix sites ranging from 30 to 360 h, even though the sequential barriers of 9.7 and 10.4 kcal mol–1 are much too high to be surmounted thermally under cryogenic conditions. A general mathematical model was developed for the complex kinetics of a reaction cascade with species in distinct matrix sites. With this model, a precise, global nonlinear least-squares fit was achieved simultaneously on the temporal profiles of nine IR bands of the 1cTc, 1cTt, and 1tTt rotamers. Classes of both fast (t1/2 = 30–50 h) and slow (t1/2 > 250 h) matrix sites were revealed, with the decay rate of the former in close agreement with first-principles computations for the conformational tunneling rates of the corresponding isolated molecules. Rigorous kinetic and theoretical analyses thus show that a “domino” tunneling mechanism is at work in these oxalic acid transformations.

Bond Angles around a Tetravalent Atom

Co-reporter:Robert K. Bohn and Wesley D. Allen

The Journal of Physical Chemistry A 2015 Volume 119(Issue 9) pp:1534-1538

Publication Date(Web):October 7, 2014

DOI:10.1021/jp5073999

Relationships among the six bond angles about a central tetravalent atom depend on symmetry, ranging from the most symmetrical Td point group to the least symmetrical C1 point group having only the identity element. Exact relationships are derived here in two ways: (1) a purely algebraic treatment of the general mathematical conditions among the bond angles, followed by factorizations that arise from various symmetry constraints and (2) a reverse approach based on geometric analysis, starting with the most symmetrical Td case and relaxing constraints stepwise to lower point groups. The mathematical formulas show systematically how the degrees of freedom among the bond angles increase from zero to a maximum of five as the symmetry is relaxed from the Td symmetry.

Do π-Conjugative Effects Facilitate SN2 Reactions?

Co-reporter:Chia-Hua Wu ; Boris Galabov ; Judy I-Chia Wu ; Sonia Ilieva ; Paul von R. Schleyer

Journal of the American Chemical Society 2014 Volume 136(Issue 8) pp:3118-3126

Publication Date(Web):January 22, 2014

DOI:10.1021/ja4111946

Rigorous quantum chemical investigations of the SN2 identity exchange reactions of methyl, ethyl, propyl, allyl, benzyl, propargyl, and acetonitrile halides (X = F–, Cl–) refute the traditional view that the acceleration of SN2 reactions for substrates with a multiple bond at Cβ (carbon adjacent to the reacting Cα center) is primarily due to π-conjugation in the SN2 transition state (TS). Instead, substrate–nucleophile electrostatic interactions dictate SN2 reaction rate trends. Regardless of the presence or absence of a Cβ multiple bond in the SN2 reactant in a series of analogues, attractive Cβ(δ+)···X(δ–) interactions in the SN2 TS lower net activation barriers (Eb) and enhance reaction rates, whereas repulsive Cβ(δ–)···X(δ–) interactions increase Eb barriers and retard SN2 rates. Block-localized wave function (BLW) computations confirm that π-conjugation lowers the net activation barriers of SN2 allyl (1t, coplanar), benzyl, propargyl, and acetonitrile halide identity exchange reactions, but does so to nearly the same extent. Therefore, such orbital interactions cannot account for the large range of Eb values in these systems.

π-Complexation in Nickel-Catalyzed Cross-Coupling Reactions

Co-reporter:S. Kyle Sontag, Jenna A. Bilbrey, N. Eric Huddleston, Gareth R. Sheppard, Wesley D. Allen, and Jason Locklin

The Journal of Organic Chemistry 2014 Volume 79(Issue 4) pp:1836-1841

Publication Date(Web):February 3, 2014

DOI:10.1021/jo402259z

The kinetic isotope effect (KIE) is used to experimentally elucidate the first irreversible step in oxidative addition reactions of a zerovalent nickel catalyst to a set of haloarene substrates. Halogenated o-methylbenzene, dimethoxybenzene, and thiophene derivatives undergo intramolecular oxidative addition through irreversible π-complexation. Density functional theory computations at the B3LYP-D3/TZ2P-LANL2TZ(f)-LANL08d level predict η2-bound π-complexes are generally stable relative to a solvated catalyst plus free substrate and that ring-walking of the Ni(0) catalyst and intramolecular oxidative addition are facile in these intermediates.

Polytwistane

Co-reporter:Shiblee R. Barua;Henrik Quanz;M.Sc. Martin Olbrich;Dr. Peter R. Schreiner;Dr. Dirk Trauner;Dr. Wesley D. Allen

Chemistry - A European Journal 2014 Volume 20( Issue 6) pp:1638-1645

Publication Date(Web):

DOI:10.1002/chem.201303081

Abstract

Twistane, C₁₀H₁₆, is a classic D₂-symmetric chiral hydrocarbon that has been studied for decades due to its fascinating stereochemical and thermodynamic properties. Here we propose and analyze in detail the contiguous linear extension of twistane with ethano (ethane-1,2-diyl) bridges to create a new chiral, C₂-symmetric hydrocarbon nanotube called polytwistane. Polytwistane, (CH)_n, has the same molecular formula as polyacetylene but is composed purely of C(sp³)H units, all of which are chemically equivalent. The polytwistane nanotube has the smallest inner diameter (2.6 Å) of hydrocarbons considered to date. A rigorous topological analysis of idealized polytwistane and a C₂₃₆H₂₄₂ prototype optimized by B3LYP density functional theory reveals that the polymer has a nonrepeating, alternating σ-helix, with an irrational periodicity parameter and an instantaneous rise (or lead) angle near 15 °. A theoretical analysis utilizing homodesmotic equations and explicit computations as high as CCSD(T)/cc-pVQZ yields the enthalpies of formation (twistane)=−1.7 kcal mol⁻¹ and (polytwistane)= +1.28 kcal (mol CH)⁻¹, demonstrating that the hypothetical formation of polytwistane from acetylene is highly exothermic. Hence, polytwistane is synthetically viable both on thermodynamic grounds and also because no obvious pathways exist for its rearrangement to lower-lying isomers. The present analysis should facilitate the preparation and characterization of this new chiral hydrocarbon nanotube.

Free Cyclooctatetraene Dianion: Planarity, Aromaticity, and Theoretical Challenges

Co-reporter:Alexander Yu. Sokolov, D. Brandon Magers, Judy I. Wu, Wesley D. Allen, Paul v. R. Schleyer, and Henry F. Schaefer III

Journal of Chemical Theory and Computation 2013 Volume 9(Issue 10) pp:4436-4443

Publication Date(Web):August 29, 2013

DOI:10.1021/ct400642y

The planarity and 10 π-electron aromaticity of the free cyclooctatetraene dianion (C8H82–, COT2–) have been questioned recently on the basis of conflicting density functional and second-order Møller–Plesset perturbation computations. Rigorous coupled-cluster methods are employed here to establish the structure and properties of COT2–. Like many multiply charged anions, COT2– exists in isolation only as a short-lived resonance state lying above neutral COT. Wave function stability analysis demonstrates that predictions of nonplanar COT2– rings are artifacts of using overly diffuse basis sets. The resulting broken-symmetry wave functions are not characteristic of COT2– but mainly describe COT in a continuum of free electrons. All-electron coupled cluster theory extended through triple excitations [AE-CCSD(T)] yields a planar D8h symmetry COT2– structure. Final focal point analyses place the COT2– resonance state 61.6 kcal mol–1 above neutral COT. Nonetheless, COT2– exhibits structural, magnetic, and energetic properties characteristic of aromatic compounds. Comparison with all-trans octatetraene indicates that COT2– has a substantial aromatic stabilization energy (25 kcal mol–1) approaching that of benzene (33 kcal mol–1), but this favorable influence is swamped by Coulomb repulsion. Charge-compensating complexation of COT2– with two sodium cations results in a thermodynamically stable Na2COT compound (D8h symmetry), for which high-level structures are also presented.

Exact Ligand Solid Angles

Co-reporter:Jenna A. Bilbrey, Arianna H. Kazez, Jason Locklin, and Wesley D. Allen

Journal of Chemical Theory and Computation 2013 Volume 9(Issue 12) pp:5734-5744

Publication Date(Web):October 14, 2013

DOI:10.1021/ct400426e

Steric demands of a ligand can be quantified by the area occluded by the ligand on the surface of an encompassing sphere centered at the metal atom. When viewed as solid spheres illuminated by the metal center, the ligand atoms generally cast a very complicated collective shadow onto the encompassing sphere, causing mathematical difficulties in computing the subtended solid angle. Herein, an exact, analytic solution to the ligand solid angle integration problem is presented based on a line integral around the multisegmented perimeter of the ligand shadow. The solution, which is valid for any ligand bound to any metal center, provides an excellent method for analyzing geometric structures from quantum chemical computations or X-ray crystallography. Over 275 structures of various metals bound to diverse mono- and multidentate ligands were optimized using B3LYP density functional theory to exhibit exact solid angle (Ω°) computations. Among the intriguing Ω° solutions, Pd(xantphos) and ferrocene exhibit holes in their ligand shadows, and Fe(EDTA)2– has a surprisingly simple shadow defined by only four arcs, despite having a multitude of overlaps among individual shadow cones.

Nearly Degenerate Isomers of C(BH)2: Cumulene, Carbene, or Carbone?

Co-reporter:Shiblee R. Barua; Wesley D. Allen; Elfi Kraka;Paul Jerabek;Rebecca Sure; Gernot Frenking

Chemistry - A European Journal 2013 Volume 19( Issue 47) pp:15941-15954

Publication Date(Web):

DOI:10.1002/chem.201302181

Abstract

The ground electronic state of C(BH)₂ exhibits both a linear minimum and a peculiar angle-deformation isomer with a central B-C-B angle near 90°. Definitive computations on these species and the intervening transition state have been executed by means of coupled-cluster theory including single and double excitations (CCSD), perturbative triples (CCSD(T)), and full triples with perturbative quadruples (CCSDT(Q)), in concert with series of correlation-consistent basis sets (cc-pVXZ, X=D, T, Q, 5, 6; cc-pCVXZ, X=T, Q). Final energies were pinpointed by focal-point analyses (FPA) targeting the complete basis-set limit of CCSDT(Q) theory with auxiliary core correlation, relativistic, and non-Born–Oppenheimer corrections. Isomerization of the linear species to the bent form has a minuscule FPA reaction energy of 0.02 kcal mol⁻¹ and a corresponding barrier of only 1.89 kcal mol⁻¹. Quantum tunneling computations reveal interconversion of the two isomers on a timescale much less than 1 s even at 0 K. Highly accurate CCSD(T)/cc-pVTZ and composite c∼CCSDT(Q)/cc-pCVQZ anharmonic vibrational frequencies confirm matrix-isolation infrared bands previously assigned to linear C(BH)₂ and provide excellent predictions for the heretofore unobserved bent isomer. Chemical bonding in the C(BH)₂ species was exhaustively investigated by the atoms-in-molecules (AIM) approach, molecular orbital plots, various population analyses, local mode vibrations and force constants, unified reaction valley analysis (URVA), and other methods. Linear C(BH)₂ is a cumulene, whereas bent C(BH)₂ is best characterized as a carbene with little carbone character. Weak B–B attraction is clearly present in the unusual bent isomer, but its strength is insufficient to form a CB₂ ring with a genuine boron–boron bond and attendant AIM bond path.

On the Role of Disproportionation Energy in Kumada Catalyst-Transfer Polycondensation

Co-reporter:Jenna A. Bilbrey, S. Kyle Sontag, N. Eric Huddleston, Wesley D. Allen, and Jason Locklin

ACS Macro Letters 2012 Volume 1(Issue 8) pp:995

Publication Date(Web):July 20, 2012

DOI:10.1021/mz3002929

Kumada catalyst-transfer polycondensation (KCTP) is an effective method for the controlled polymerization of conjugated polymers. Nevertheless, side reactions leading to early termination and unwanted chain coupling cause deviations from the target molecular weight, along with increasing polydispersity and end group variation. The departure from the KCTP cycle stems from a disproportionation reaction that leads to experimentally observed side products. The disproportionation energies for a series of nickel-based initiators containing bidentate phosphino attendant ligands were computed using density functional theory at the B3LYP/DZP level. The initiator was found to be less favorable toward disproportionation by 0.5 kcal mol–1 when ligated by 1,3-bis(diphenylphosphino)propane (dppp) rather than 1,2-bis(diphenylphosphino)ethane (dppe). Trends in disproportionation energy (Edisp) with a variety of bidentate phosphine ligands match experimental observations of decreased polymerization control. Theoretical Edisp values can thus be used to predict the likelihood of disproportionation in cross-coupling reactions and, therefore, aid in catalyst design.

Methylhydroxycarbene: Tunneling Control of a Chemical Reaction

Co-reporter:Peter R. Schreiner;Hans Peter Reisenauer;David Ley;Dennis Gerbig;Chia-Hua Wu

Science 2011 Vol 332(6035) pp:1300-1303

Publication Date(Web):10 Jun 2011

DOI:10.1126/science.1203761

Quantum tunneling induces the opposite outcome expected from traditional kinetic factors in a chemical rearrangement.

Phenylhydroxycarbene

Co-reporter:Dennis Gerbig ; Hans Peter Reisenauer ; Chia-Hua Wu ; David Ley ; Wesley D. Allen ;Peter R. Schreiner

Journal of the American Chemical Society 2010 Volume 132(Issue 21) pp:7273-7275

Publication Date(Web):May 12, 2010

DOI:10.1021/ja9107885

Phenylhydroxycarbene (Ph−C−OH, 1), the parent of all arylhydroxycarbenes, was generated by high-vacuum flash pyrolysis of phenylglyoxylic acid at 600 °C and spectroscopically characterized (IR, UV−vis) via immediate matrix isolation in solid Ar at 11 K. The identity of 1 was unequivocally confirmed by the precise agreement between the observed IR bands and (unscaled) anharmonic vibrational frequencies computed from a CCSD(T)/cc-pVDZ quartic force field. The UV−vis spectrum of 1 displays a broad band with maximum absorption at 500 ± 25 nm (2.5 ± 0.1 eV) that extends to ∼640 nm (1.9 eV), in full accord with combined CCSD(T)/cc-pVQZ and EOM-CCSD/cc-pVTZ computations that yield a gas-phase vertical (adiabatic) excitation energy of 2.7 (1.9) eV. Unlike singlet phenylchlorocarbene, 1 does not undergo photochemical ring expansion. Instead, 1 exhibits quantum-mechanical hydrogen tunneling to benzaldehyde underneath a formidable barrier of 28.8 kcal mol−1, even at cryogenic temperatures. The remarkable hydrogen tunneling mechanism is supported by the temperature insensitivity of the observed half-life (2.5 h) and substantiated by a comparable theoretical half-life (3.3 h) determined from high-level barrier penetration integrals computed along the intrinsic reaction path. As expected, deuteration turns off the tunneling mechanism, so d-1 is stable under otherwise identical conditions.

Lowest-Lying Conformers of Alanine: Pushing Theory to Ascertain Precise Energetics and Semiexperimental Re Structures

Co-reporter:Heather M. Jaeger, Henry F. Schaefer III, Jean Demaison, Attila G. Császár, and Wesley D. Allen

Journal of Chemical Theory and Computation 2010 Volume 6(Issue 10) pp:3066-3078

Publication Date(Web):September 23, 2010

DOI:10.1021/ct1000236

The two lowest-energy gas-phase conformers, Ala-I and Ala-IIA, of the natural amino acid l-alanine (Ala) have been investigated by means of rigorous ab initio computations. Born−Oppenheimer (BO) equilibrium structures (reBO) were fully optimized at the coupled-cluster [CCSD(T)/cc-pVTZ] level of electronic structure theory. Corresponding semiexperimental (SE) equilibrium structures (reSE) of each conformer were determined for the first time by least-squares refinement of 11−15 structural parameters on modified, experimental rotational constant data from 10 isotopologues. The SE equilibrium rotational constants were obtained by, first, refitting Fourier transform microwave spectra using the method of predicate observations and, second, correcting the resulting effective rotational constants with theoretical vibration−rotation interaction constants (αi). Careful analysis is made of the procedures to account for vibrational distortion, which proves essential to defining precise structures in flexible molecules such as Ala. Because Ala possesses no symmetry, has several large-amplitude nuclear motions, and exhibits conformers with different hydrogen bonding patterns, it is one of the most difficult cases where reliable equilibrium structures have now been determined. The relative energy of the alanine conformers was pinpointed using first-principles composite focal point analyses (FPA), which employed extrapolations using basis sets as large as aug-cc-pV5Z and electron correlation treatments as extensive as CCSD(T). The FPA computations place the Ala-IIA equilibrium structure higher in energy than that of Ala-I by a mere 0.45 kJ mol−1 (38 cm−1), showing that the two lowest-lying conformers of alanine are nearly isoenergetic; inclusion of zero-point vibrational energy increases the relative energy to 2.11 kJ mol−1 (176 cm−1). The yet unobserved Ala-IIB conformer is found to be separated from Ala-IIA by a vibrationally adiabatic isomerization barrier of only 16 cm−1.

Conformers of Gaseous Cysteine

Co-reporter:Jeremiah J. Wilke, Maria C. Lind, Henry F. Schaefer III, Attila G. Császár and Wesley D. Allen

Journal of Chemical Theory and Computation 2009 Volume 5(Issue 6) pp:1511-1523

Publication Date(Web):May 15, 2009

DOI:10.1021/ct900005c

Structures, accurate relative energies, equilibrium and vibrationally averaged rotational constants, quartic and sextic centrifugal distortion constants, dipole moments, 14N nuclear quadrupole coupling constants, anharmonic vibrational frequencies, and double-harmonic infrared intensities have been determined from ab initio electronic structure computations for conformers of the neutral form of the natural amino acid l-cysteine (Cys). A systematic scan located 71 unique conformers of Cys using the MP2(FC)/cc-pVTZ method. The large number of structurally diverse low-energy conformers of Cys necessitates the highest possible levels of electronic structure theory to determine their relative energies with some certainty. For this reason, we determined the relative energies of the lowest-energy eleven conformers, accurate within a standard error (1σ) of about 0.3 kJ mol−1, through first-principles composite focal-point analyses (FPA), which employed extrapolations using basis sets as large as aug-cc-pV(5+d)Z and correlation treatments as extensive as CCSD(T). Three and eleven conformers of l-cysteine fall within a relative energy of 6 and 10 kJ mol−1, respectively. The vibrationally averaged rotational constants computed in this study agree well with Fourier-transform microwave spectroscopy results. The effects determining the relative energies of the low-energy conformers of cysteine are analyzed in detail on the basis of hydrogen bond additivity schemes and natural bond orbital analysis.

Infrared signatures of the NCCO radical

Co-reporter:Peter R. Schreiner, Hans Peter Reisenauer, Edit Mátyus, Attila G. Császár, Ali Siddiqi, Andrew C. Simmonett and Wesley D. Allen

Physical Chemistry Chemical Physics 2009 vol. 11(Issue 44) pp:10385-10390

Publication Date(Web):28 Sep 2009

DOI:10.1039/B912803D

The first definitive infrared signatures of the elusive NCCO radical have been measured using a microwave discharge technique combined with low-temperature matrix-isolation spectroscopy, resulting in a consistent set of vibrational assignments for six isotopologues. The infrared spectra of these NCCO isotopologues were concomitantly established by rigorous variational nuclear-motion computations based on a high-level coupled-cluster quartic vibrational force field [ROCCSD(T)/cc-pCVQZ] and cubic dipole field [ROCCSD/cc-pCVTZ]. Our experimental and theoretical results for NCCO overturn the vibrational assignments in a NIST-JANAF compilation and those from a recent two-dimensional cross-spectral correlation analysis. For the parent isotopologue at 11 K in a nitrogen matrix, we find the signature bands ν2(CO str.) = 1889.2 cm−1 and ν3(CC str.) = 782.0 cm−1. Our variational vibrational computations reveal strong mixing of the ν3 stretching fundamental and the ν4 + ν5 bending combination level for all isotopologues. These Fermi resonances manifest a clear breakdown of the simple normal-mode picture of molecular vibrations at low energies.

A companion perturbation theory for state-specific multireference coupled cluster methods

Co-reporter:Francesco A. Evangelista, Andrew C. Simmonett, Henry F. Schaefer III, Debashis Mukherjee and Wesley D. Allen

Physical Chemistry Chemical Physics 2009 vol. 11(Issue 23) pp:4728-4741

Publication Date(Web):07 Apr 2009

DOI:10.1039/B822910D

A partitioning scheme is applied to the state-specific Mukherjee multireference coupled cluster method to derive a companion perturbation theory (Mk-MRPT2). A production-level implementation of Mk-MRPT2 is reported. The effectiveness of the Mk-MRPT2 method is demonstrated by application to the classic F2 dissociation problem and the lowest-lying electronic states of meta-benzyne, including computations with up to 766 atomic orbitals. We show that Mk-MRPT2 theory is particularly useful in multireference focal point extrapolations to determine ab initio limits.

Barrier To Linearity and Anharmonic Force Field of the Ketenyl Radical†

Co-reporter:Andrew C. Simmonett, Nathan J. Stibrich, Brian N. Papas, Henry F. Schaefer III and Wesley D. Allen

The Journal of Physical Chemistry A 2009 Volume 113(Issue 43) pp:11643-11650

Publication Date(Web):September 17, 2009

DOI:10.1021/jp9024365

The troublesome barrier to linearity of the ketenyl radical (HCCO) is precisely determined using state-of-the-art computations within the focal point approach, by combining complete basis set extrapolation, utilizing the aug-cc-pVXZ (X = D, T, Q, 5, 6) family of basis sets, with electron correlation treatments as extensive as coupled cluster theory with single, double, triple, and perturbative quadruple excitations [CCSDT(Q)]. Auxiliary terms such as diagonal Born−Oppenheimer corrections (DBOCs) and relativistic contributions are included. To gain a definitive theoretical treatment and to assess the effect of higher-order correlation on the structure of HCCO, we employ a composite approximation (c∼) to all-electron (AE) CCSDT(Q) theory at the complete basis set (CBS) limit for geometry optimizations. A final classical barrier to linearity of 630 ± 30 cm−1 is obtained for reaching the 2Π Renner−Teller configuration of HCCO from the 2A′′ ground state. Additionally, we compute fundamental vibrational frequencies and other spectroscopic constants by application of second-order vibrational perturbation theory (VPT2) to the full quartic force field at the AE-CCSD(T)/aug-cc-pCVQZ level. The resulting (ν1, ν2, ν5) fundamental frequencies of (3212, 2025, 483) cm−1 agree satisfactorily with the experimental values (3232, 2023, 494) cm−1.

Anchoring the Absolute Proton Affinity Scale

Co-reporter:Gábor Czakó, Edit Mátyus, Andrew C. Simmonett, Attila G. Császár, Henry F. Schaefer III and Wesley D. Allen

Journal of Chemical Theory and Computation 2008 Volume 4(Issue 8) pp:1220-1229

Publication Date(Web):July 2, 2008

DOI:10.1021/ct800082r

Converged first-principles proton affinities (PA) of ammonia and carbon monoxide have been determined by the focal-point analysis (FPA) approach, thus fixing the high and low ends of the molecular proton affinity scale. The electronic structure computations employed the all-electron (AE) coupled-cluster (CC) method up to single, double, triple, quadruple, and pentuple excitations. Aug-cc-pCVXZ [X = 2(D), 3(T), 4(Q), 5, and 6] correlation-consistent (cc) Gaussian basis sets for C, N, and O were used in conjunction with the corresponding aug-cc-pVXZ (X = 2−6) sets for H. Our FPA study supersedes previous computational work by accounting for (a) electron correlation beyond the “gold standard” CCSD(T) level; (b) the nonadditivity of core electron correlation effects; (c) scalar relativity; (d) diagonal Born−Oppenheimer corrections (DBOC); (e) anharmonicity of zero-point vibrational energies, based on accurate AE-CCSD(T)/cc-pCVQZ internal coordinate quartic force fields and fully variational vibrational computations; and (f) thermal corrections to enthalpies by direct summation over rovibrational energy levels. Our final proton affinities at 298.15(0.0) K are ΔpaH°(NH3) = 852.6(846.4) ± 0.3 kJ mol−1 and ΔpaH°(CO) = 592.4(586.5) ± 0.2 kJ mol−1. These values have better accuracy and considerably lower uncertainty than the best previous recommendations and thus anchor the proton affinity scale of molecules for future use.

Capture of hydroxymethylene and its fast disappearance through tunnelling

Co-reporter:Peter R. Schreiner, Hans Peter Reisenauer, Frank C. Pickard IV, Andrew C. Simmonett, Wesley D. Allen, Edit Mátyus & Attila G. Császár

Nature 2008 453(7197) pp:906

Publication Date(Web):2008-06-12

DOI:10.1038/nature07010

Singlet carbenes exhibit a divalent carbon atom whose valence shell contains only six electrons, four involved in bonding to two other atoms and the remaining two forming a non-bonding electron pair. These features render singlet carbenes so reactive that they were long considered too short-lived for isolation and direct characterization. This view changed when it was found that attaching the divalent carbon atom to substituents that are bulky and/or able to donate electrons produces carbenes that can be isolated and stored1. N-heterocyclic carbenes are such compounds now in wide use, for example as ligands in metathesis catalysis2. In contrast, oxygen-donor-substituted carbenes are inherently less stable and have been less studied. The pre-eminent case is hydroxymethylene, H–C–OH; although it is the key intermediate in the high-energy chemistry of its tautomer formaldehyde3, 4, 5, 6, 7, has been implicated since 1921 in the photocatalytic formation of carbohydrates8, and is the parent of alkoxycarbenes that lie at the heart of transition-metal carbene chemistry, all attempts to observe this species or other alkoxycarbenes have failed9. However, theoretical considerations indicate that hydroxymethylene should be isolatable10. Here we report the synthesis of hydroxymethylene and its capture by matrix isolation. We unexpectedly find that H–C–OH rearranges to formaldehyde with a half-life of only 2 h at 11 K by pure hydrogen tunnelling through a large energy barrier in excess of 30 kcal mol–1.

Tunnelling in carbonic acid

Co-reporter:J. Philipp Wagner, Hans Peter Reisenauer, Viivi Hirvonen, Chia-Hua Wu, Joseph L. Tyberg, Wesley D. Allen and Peter R. Schreiner

Chemical Communications 2016 - vol. 52(Issue 50) pp:NaN7861-7861

Publication Date(Web):2016/05/24

DOI:10.1039/C6CC01756H

The cis,trans-conformer of carbonic acid (H2CO3), generated by near-infrared radiation, undergoes an unreported quantum mechanical tunnelling rotamerization with half-lives in cryogenic matrices of 4–20 h, depending on temperature and host material. First-principles quantum chemistry at high levels of theory gives a tunnelling half-life of about 1 h, quite near those measured for the fastest rotamerizations.

Infrared signatures of the NCCO radical

Co-reporter:Peter R. Schreiner, Hans Peter Reisenauer, Edit Mátyus, Attila G. Császár, Ali Siddiqi, Andrew C. Simmonett and Wesley D. Allen

Physical Chemistry Chemical Physics 2009 - vol. 11(Issue 44) pp:NaN10390-10390

Publication Date(Web):2009/09/28

DOI:10.1039/B912803D

The first definitive infrared signatures of the elusive NCCO radical have been measured using a microwave discharge technique combined with low-temperature matrix-isolation spectroscopy, resulting in a consistent set of vibrational assignments for six isotopologues. The infrared spectra of these NCCO isotopologues were concomitantly established by rigorous variational nuclear-motion computations based on a high-level coupled-cluster quartic vibrational force field [ROCCSD(T)/cc-pCVQZ] and cubic dipole field [ROCCSD/cc-pCVTZ]. Our experimental and theoretical results for NCCO overturn the vibrational assignments in a NIST-JANAF compilation and those from a recent two-dimensional cross-spectral correlation analysis. For the parent isotopologue at 11 K in a nitrogen matrix, we find the signature bands ν2(CO str.) = 1889.2 cm−1 and ν3(CC str.) = 782.0 cm−1. Our variational vibrational computations reveal strong mixing of the ν3 stretching fundamental and the ν4 + ν5 bending combination level for all isotopologues. These Fermi resonances manifest a clear breakdown of the simple normal-mode picture of molecular vibrations at low energies.

A companion perturbation theory for state-specific multireference coupled cluster methods

Co-reporter:Francesco A. Evangelista, Andrew C. Simmonett, Henry F. Schaefer III, Debashis Mukherjee and Wesley D. Allen

Physical Chemistry Chemical Physics 2009 - vol. 11(Issue 23) pp:NaN4741-4741

Publication Date(Web):2009/04/07

DOI:10.1039/B822910D

A partitioning scheme is applied to the state-specific Mukherjee multireference coupled cluster method to derive a companion perturbation theory (Mk-MRPT2). A production-level implementation of Mk-MRPT2 is reported. The effectiveness of the Mk-MRPT2 method is demonstrated by application to the classic F2 dissociation problem and the lowest-lying electronic states of meta-benzyne, including computations with up to 766 atomic orbitals. We show that Mk-MRPT2 theory is particularly useful in multireference focal point extrapolations to determine ab initio limits.

σ Bond activation through tunneling: formation of the boron hydride cations BHn+ (n = 2, 4, 6)

Co-reporter:Yudong Qiu, Chia-Hua Wu, Henry F. Schaefer III, Wesley D. Allen and Jay Agarwal

Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 5) pp:NaN4070-4070

Publication Date(Web):2016/01/07

DOI:10.1039/C5CP05505A

The network of H2 additions to B+ and subsequent insertion reactions serve as a tractable model for hydrogen storage in elementary boron-containing compounds. Here, they are investigated using state-of-the-art ab initio methods (up to CCSDTQ and cc-pCV6Z basis sets). The binding energies of H2 to HBH+ (14.9 kcal mol−1) and HBH(H2)+ (18.1 kcal mol−1) are determined to be much higher than those for B(H2)+ (3.8 kcal mol−1), B(H2)2+ (3.0 kcal mol−1), and B(H2)3+ (2.5 kcal mol−1) at the CCSDTQ/CBS level of theory. These predictions are in agreement with the experiments of Kemper, Bushnell, Weis, and Bowers (J. Am. Chem. Soc., 1998, 120, 7577). Molecular orbital analyses show that the enhanced binding in HBH(H2)m+ complexes originates from the strong interaction between the 1σu HOMO of HBH+ and the 1σu LUMO of H2. For the insertion reactions B(H2)n+ → HBH(H2)n−1+, activation barriers are determined to be 58.3 kcal mol−1 [Mk-MRCCSD(T)/CBS], 12.2 kcal mol−1 (CCSDTQ/CBS) and 4.6 kcal mol−1 (CCSDTQ/CBS) for n = 1, 2, and 3, respectively. After using theoretical results to remove tunneling effects from the experimental rate constants, new Arrhenius fits yield activation barriers of 4.6(3) kcal mol−1 and 3.8(1) kcal mol−1 for the BH6+ and BD6+ insertion reactions, respectively, which are in near perfect agreement with converged theoretical values (4.6 kcal mol−1 and 3.9 kcal mol−1). These findings demonstrate that earlier Arrhenius fits considerably underestimate these barriers, and that quantum tunneling dominates the σ bond activation mechanism witnessed in previous experiments involving BH6+.

TOPICS

Abstract

Abstract