Ab Initio Study of Spin-Vibronic Dynamics in the Ground ²E and Excited òA₁ 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 ²E and first excited òA₁ electronic states of the CH₃S methylthio radical (C_3v). 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 ²E 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 ²E and òA₁ electronic states was found small and safely removable from the spin-vibronic Hamiltonian of CH₃S.