A perturbative description of non-adiabatic effects in methoxy vibrations

Abstract

Non-adiabatic couplings between the quasi-diabatic ground states, used to describe the vibrations of the methoxy (CH3O) radical, are investigated. The vibrations of the state are known to be strongly influenced by Jahn–Teller couplings. These couplings are often modelled in a quasi-diabatic electronic state representation. There is an additional non-adiabatic coupling between these states. This latter coupling is elucidated and its effects are quantified using the Van Vleck perturbation theory. The origins of the non-adiabatic coupling are assumed to arise via interactions of the degenerate ground state with the lowest lying excited electronic state. The resulting three-state electronic Hamiltonian is developed to approximately describe the rocking vibrations of methoxy. It is shown that minor changes to the implementation of the Van Vleck transformations enable one to highlight the role of the non-adiabatic coupling in a straightforward manner. These couplings are found to lead to shifts of the order of a few wavenumbers for the lowest several vibrational states.