Jahn−Teller Effect versus Spin−Orbit Coupling in X²E CH₃S:  An ab Initio Study by the Equation-of-Motion Coupled Cluster Method and Multiconfiguration Quasi-Degenerate Second-Order Perturbation Theory

The equation-of-motion coupled cluster method (EOMIP) using a reference Slater determinant based on the CH₃S^--anion ground state with basis sets of triple- and quadruple-ζ quality was applied for numerical optimizations of geometries of stationary points on adiabatic Jahn−Teller surfaces of the X²E ground state of CH₃S crossing at the C_3v symmetry nuclear configuration. Calculations by the multireference configuration interaction method and multiconfiguration quasi-degenerate second-order perturbation (PT2) theory were also performed with use of a complete active space CASSCF reference wave function. The linear and quadratic Jahn−Teller constants were computed for each of three Jahn−Teller active modes. The potential surfaces corresponding to spin−orbit states ²E_3/2 and ²E_1/2 were obtained with eigenenergies of the full Breit−Pauli spin−orbit operator with a PT2−CASSCF reference wave function. The one- and two-electron scalar relativistic effects were included in CASSCF and spin−orbit calculations. The calculated Jahn−Teller stabilization energy, the barrier to pseudorotation, and the spin−orbit splitting are 93, 15, and 358 cm^-1, respectively. The Jahn−Teller distortions are totally quenched by the strong spin−orbit coupling. The recommended values of the geometry parameters of CH₃S are R_e(CS) = 1.794 Å, R_e(CH) = 1.087 Å, and α_e(HCS) = 109.8°.