Ab Initio Study of Spin-Vibronic Dynamics in the Ground X̃(2)E and Excited Ã(2)A1 Electronic States of CH3S(•).

A spin-vibronic Hamiltonian including the linear, quadratic, cubic, and quartic Jahn-Teller terms with account for all important anharmonic effects was applied to study electronic and nuclear dynamics in the ground X̃(2)E and first excited Ã(2)A1 electronic states of the CH3S methylthio radical (C3v). The E⊗(3a1+3e) problem of spin-vibronic eigenvalues and eigenfunctions was solved in a basis set of products of electronic, electron spin, and vibrational functions. The Jahn-Teller distortions in X̃(2)E CH3S are totally quenched by the strong spin-orbit coupling. However, Jahn-Teller interaction terms in the spin-vibronic Hamiltonian cannot be neglected for the high precision evaluation of energy levels of CH3S. The results of calculations show the importance of inclusion of at least quadratic vibronic terms into variational treatment. The nonadiabatic (pseudo-Jahn-Teller) coupling of the X̃(2)E and Ã(2)A1 electronic states was found small and safely removable from the spin-vibronic Hamiltonian of CH3S.