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

Abstract

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 C3v 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 2E3/2 and 2E1/2 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 Re(CS) = 1.794 Å, Re(CH) = 1.087 Å, and αe(HCS) = 109.8°.