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Irvine, CA, United States

Freeman F.,University of California at Irvine | Bui A.,University of California at Irvine | Dinh L.,University of California at Irvine | Hehre W.J.,University of California at Irvine | Hehre W.J.,Wavefunction Inc.
Journal of Physical Chemistry A | Year: 2012

Structural features of hydrogen thioperoxide (oxadisulfane, H-S-O-H) and of alkanesulfenic acids (R-S-O-H; R = CH3, CH2CH 3, CH2CH2CH3, CH(CH 3)2, C(CH3)3, CF3, CCl3) and the mechanisms of their dehydrative cyclocondensation to the respective sulfinothioic acid (H-(S=O)-S-H) and alkyl alkanethiosulfinates (R-(S=O)-S-R) have been studied using coupled cluster theory with single and double and perturbative triple excitations [CCSD(T)] and quadratic configuration interaction with single and double and perturbative triple excitations [QCISD(T)] with the cc-pVDZ basis set and also using second-order Møller-Plesset perturbation theory (MP2) and the hybrid density functionals B3LYP, B3PW91, and PBE1PBE with the 6-311+G(d,p) basis set. The concerted cyclodehydration mechanisms include cyclic five-center transition states with relatively long distance sulfur-sulfur bonding interactions. Attractive and repulsive nonbonding interactions are predicted in the sulfenic acids, transition states, and thiosulfinates. In the alkyl alkanethiosulfinates attractive cyclic C-H - - O=S nonbonding interactions are predicted. CCSD(T) and QCISD(T) predict similar values for the relative energies and CCSD(T) predicts the barrier to the cyclocondensation of H-S-O-H to sulfinothioic acid (H-(S=O)-S-H) to be 41.8 kcal/mol, and barriers in the range of 37.5 to 39.6 kcal/mol are predicted for the condensation of alkanesulfenic acids to alkyl alkanethiosulfinates. © 2012 American Chemical Society.

Honmura Y.,Hirosaki University | Takekawa H.,Hirosaki University | Tanaka K.,Hirosaki University | Maeda H.,Hirosaki University | And 3 more authors.
Journal of Natural Products | Year: 2015

The structures of epoxyroussoenone (1) and epoxyroussoedione (3) isolated from a culture broth of Roussoella japanensis KT1651 were determined. Although NMR spectra provided insufficient structural information, computation of the theoretical chemical shifts with DFT EDF2/6-31G∗ enabled us to elucidate not only the planar structure, but also the relative configuration. Their ECD (electric circular dichroism) spectra suggested the absolute configurations, which were confirmed with time-dependent DFT calculations employing BHandHLYP/TZVP. The ECD calculations for other stereoisomers yielded obviously different spectral profiles, thus confirming the relative structures of 1 and 3. (Chemical Equation Presented) © 2015 The American Chemical Society and American Society of Pharmacognosy.

Shao Y.,Q-Chem, Inc. | Gan Z.,Q-Chem, Inc. | Epifanovsky E.,Q-Chem, Inc. | Epifanovsky E.,University of Southern California | And 187 more authors.
Molecular Physics | Year: 2015

A summary of the technical advances that are incorporated in the fourth major release of the Q-Chem quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller-Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr2 dimer, exploring zeolite-catalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube. © 2014 © 2014 Taylor & Francis.

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