Bifurcation analysis in terms of second-order Jahn—Teller effect

Abstract

It is known that the reaction path for the dissociation reaction H 2CS → H 2 + CS has C s symmetry while the reaction path for the isomerization reaction H 2CS → HCSH has C 1 symmetry at the Hartree-Fock (HF) level. In addition, the former reaction path is known to have a bifurcating region. In order to clarify the bifurcation mechanism in terms of the second-order Jahn-Teller (SOJT) effect, the intrinsic reaction coordinates (IRCs) and energies of the first-excited state (A″) along the IRC were calculated for both reactions under the constraint of C s symmetry with 6–31G** basis sets at the HF level and at the complete active space SCF (CASSCF) level. At the HF level, bifurcation was proven to occur in both dissociation and isomerization reactions due to a vibronic interaction through the SOJT effect. At the CASSCF level, however, the energy separations between the ground and the A″ excited states in both reactions are large enough to avoid state intermixing due to the SOJT effect, and the bifurcation disappears from each IRC. For the purpose of comparison, the IRCs of H 2CO for the dissociation to H 2 + CO and for the isomerization to HCOH were also calculated with 6–31G** basis sets at the HF level. As expected from the SOJT theory for the calculated large energy separation, bifurcation does not occur in both dissociation and isomerization reactions of H 2CO.