Vibronic coupling theory of infrared vibrational transitions

Abstract

The theory of vibronic coupling is developed for infrared vibrational transitions. It is shown that the lowest order nonadiabatic Born–Oppenheimer correction terms contain an important adiabatic component which may be used to describe infrared transition intensity for imaginary Hermitian operators, such as the momentum and angular momentum operators. This previously unrecognized source of adiabatic infrared intensity forms a complement to the traditional Herzberg–Teller vibronic coupling expressions, which are active for the position operator, and resolves the paradox of vanishing electronic intensity for momentum operators in the Born–Oppenheimer approximation. Expressions for infrared absorption and vibrational circular dichroism are derived that utilize only ground electronic statewave functions; LCAO wave functions are used in these expressions to provide a more detailed description of these new momentum intensity contributions.