Kenji Morihashi

Calculation of contributions of one- and two-electron spin-orbit coupling terms to the parity-violating energy shifts for amino acids and helical alkanes

Rigorous calculations were performed for spin-orbit coupling terms of the Breit–Pauli Hamiltonian in the calculation of the parity-violating energy shifts Epν using the sum-over-state perturbation theory, and the role of one-electron and two-electron spin-orbit coupling terms was examined. Calculations on amino acids and helical n-alkanes have suggested that inclusion of one-center terms both of one- and two-electron spin couplings is sufficient to estimate the Epν values quantitatively.

An MNDO-effective charge model study of the solvent effect: The internal rotation about partial double bonds and the nitrogen inversion in amine

Abstract The effective charge model, which introduces the solvent effect into MO calculation, was incorporated with the MNDO—MO method. One-center solvation energy associated with the atom having the fractional charge was evaluated empirically by Borns formula, while the two-center values by Ohno—Klopman like equation. The atomic radius was varied as the function of the atomic charge. The method reproduced well the observed solvent effect on the barrier to internal rotation of formamide, 4-aminopyrimidine, furfuraldehyde and the amidinium ion, and the barrier to nitrogen inversion in amine.

Ab initio GB study of the acid-catalyzed cis–trans isomerization of methyl yellow and methyl orange in aqueous solution

Abstract Ab initio GB calculation that includes solvent effects by the generalized Born formula (GB) was applied to methyl yellow (4-dimethylaminophenylazobenzene, MY) and methyl orange (4-[4′-dimethylaminophenylazo]-benzenesulfonate, MO) and their protonated species, and the acid-catalyzed cis–trans isomerization of MY and MO was analyzed. The HF/3-21G(∗)/GB calculations determined the structures and relative energies of MY and MO, their ammonium and azonium ions, and the transitions state for the cis–trans isomerization of the azonium ions in aqueous solution. The calculations indicated that (i) the energy difference between the cis and trans forms of the azonium ion is much smaller than those of the ammonium ion and the unprotonated species, (ii) the cis-azonium ion is much more stable than the cis-ammonium ion, and (iii) the cis–trans isomerization occurs easily in the azonium ion which is produced by the protonation of the azo compound or by the acid-catalyzed proton transfer from the cis-ammonium ion.

Ab initio calculations of g values of free radicals by finite perturbation theory

Abstract A theoretical method for evaluating the g tensor of free radicals was developed on the basis of the finite perturbation theory (FPT) and the open-shell restricted Hartree-Fock wavefunction, and ab initio calculations were performed for several free radicals. Calculations were also carried out by using the sum-over-state perturbation theory (SOS-PT); both FPT and SOS-PT methods gave similar accuracy for g values. The spin-other-orbit coupling term was important in the ab initio FPT calculation.

Ab initio GB study of solvent effect on the cis–trans isomerization of 4-dimethylamino-4′-nitroazobenzene

Abstract Ab initio RHF and ROHF methods including solvent effects based on the generalized Born formula (GB) were applied to a typical push–pull azobenzene, 4-(dimethylamino)-4′-nitroazobenzene (DMANAB), and the solvent effect on the inversion and rotation mechanisms of thermal cis – trans isomerization of DMANAB was examined. The pull-group inversion path was more favorable than the push-group one in the gas phase and in solution, and the calculated activation energies were compared well with experimental values. Analysis of two-dimensional potential energy map and electron configurations of the transition states indicated that the transition state of the rotation path, if it exists, resembles closely to that of the inversion path, and it is unlikely that both transition states are found separately in the potential energy surface of the closed-shell electronic state. Although large stabilization by solvent was expected for the rotation transition state, searching for such a transition state on the closed-shell electronic state gave the inversion transition state even in polar solvent. The transition state for the rotation path in the diradical electronic state was calculated to be comparable in terms of energy to the transition state for the pull-group inversion path in the closed-shell electronic state.

Theoretical study of the thermal interconversion mechanism between the norbornadiene and quadricyclane radical cations

Abstract Geometry optimizations at the UHF/6-31G* and UMP2/6-31G* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS + · 1 and TS + · 2 , were obtained which have C1 and C2 symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS + · 1 is the true transition state connecting N+· and Q+·, while TS + · 2 is a second order saddle point.

An ab initio MO study of allene episulfide, cyclopropanethione and thioxyallyl

Abstract The electronic structures of allene episulfide, cyclopropanethione and thioxyallyl were examined by ab initio MO calculations and were compared with those of the corresponding oxygen compounds, allene oxide, cyclopropanone and oxyallyl. The difference in reactivities of allene episulfide and allene oxide was also speculatively estimated from the calculated electronic structures. The lowest singlet state of thioxyallyl was predicted to be the B2 state, which corresponds to the σ, π-diradical. A small activation energy is required for the cyclization of the B2 state to give allene episulfide. The A1 singlet state lies 11 kcal mol−1 higher than the B2 singlet state and undergoes the disrotatory rotation of methylene groups to give cyclopropanethione with no activation energy.

Ab initio study of 1,4-dithiin and its cation radical and dication

Abstract The structures of 1,4-dithiin in the neutral, radical cation, and dication states have been investigated by ab initio RHF and UHF calculations with the MIDI-4 basis set. The MIDI-4 calculations predict a folded form for the neutral state, and planar forms for the cation and dication states. These structural changes caused by one- or two-electron oxidation processes are influenced by two electronic factors: one is the weak π conjugation between the sulfur and carbon atoms and the other is the σ interaction between two sulfur atoms.

Expansion of hartree-fock atomic orbitals by a small number of gaussian functions

Abstract Analytical Hartree-Fock-Roothaan atomic orbitals expressed by Slater-type functions were approximated by using a small number of Gaussian-type functions for the hydrogen atom, the first-row atoms, and some of the second-row atoms. The HFO-NG basis sets thus obtained for N = 3 to 6 gave better behaviour near the atomic nucleus than the other basis sets of comparable size.

Density functional theory calculation of hyperfine coupling constants of small radicals using Becke97-type exchange-correlation functionals

Abstract Hyperfine coupling constants (hfccs) of H2O+, CH3, NH2, allyl, NO2, HCO, CH2CH, H2CN and H2CO+ radicals were calculated with Density functional theory using Becke97-type exchange correlation functionals, B97, B97-1 and HCTH. These functionals gave the accuracy comparable with the conventional B3LYP functional for hfccs of these small π and σ radicals. It was found that the calculated hfccs of the heavy atoms depend largely on the parameters in the HCTH functional which were determined by referring the molecular properties selected in the parameter-fitting process.