Spin−Orbit Coupling Patterns Induced by Twist and Pyramidalization Modes in C2H4: A Quantitative Study and a Qualitative Analysis

Abstract

A study of the spin−orbit coupling (SOC) mechanisms which couple the triplet ππ* state (T1) to the singlet ground state (S0) in ethylene is carried out at a variety of computational levels and basis sets, using the full Breit−Pauli (BP) SOC Hamiltonian, the one-electron mean-field (MF) operator, and the approximate one-electron operator based on an effective nuclear charge, Z*. The basis set and wave functions requirements needed for good quality SOC calculations are elucidated by studying the SOC interaction using single- and multireference CI as well as MCSCF wavefunctions, with basis sets ranging from the minimal STO-3G all the way to an extended one with quadruple ζ and polarization quality. Two archetype distortion modes of ethylene were considered: a twist mode which changes the symmetry from D2h to D2 and then to D2d and pyramidalization modes which change the ethylene symmetry to C2v (syn-pyramidalization) or C2h (anti-pyramidalization), as well as Cs (i.e., a mono-pyramidalization distortion). I...