Molecular Spectroscopy beyond the Born−Oppenheimer Approximation:  A Computational Study of the CF₃O and CF₃S Radicals^†

This paper addresses some advances in the theoretical description of molecular spectroscopy beyond the Born−Oppenheimer adiabatic approximation. A solution of the nuclear dynamics problem complicated by the EE Jahn−Teller effect and spin−orbit coupling is considered for the case of the CF₃O and CF₃S radicals, all the model parameters being obtained solely from ab initio calculations without any adjustment to experimental numbers. Vibrational and vibronic model parameters were calculated at the equation-of-motion coupled cluster level of theory with basis sets of triple-ζ quality. The spin−orbit coupling in ²E CF₃O and CF₃S was parametrized by means of a perturbative solution of the full Breit−Pauli spin−orbit operator. Spin-vibronic eigenvalues and eigenfunctions were computed in a basis set of products of electronic, electron spin, and vibrational functions. Results demonstrate the importance of explicit inclusion of the spin−orbit coupling and at least cubic Jahn−Teller terms in the model Hamiltonian for the high precision evaluation of spin-vibronic energy levels of CF₃O and CF₃S. The theoretical results support and complement the spectroscopic data observed for these species.