Knowledge Bank

Theoretical expressions for infrared absorption due to vibrational transitions and vibrational circular dichroism will be presented in terms of vibronic coupling $theory.1$ The position electric dipole representation of the interaction of radiation with matter for vibrational transitions is expressed by Herzberg-Teller vibronic coupling and reduces to the usual expression for infrared intensity as the derivative of the electric dipole moment with respect to nuclear displacement. Expressions for the momentum form of the interaction of radiation with matter and the magnetic dipole interaction are expressed by Born-Oppenheimer vibronic coupling and are formally beyond the Born-Oppenheimer approximation. It is shown, however, that these latter expressions are still adiabatic in form and describe oscillating electronic current density that is induced by the momenta, rather than the displacements, of the nuclei during vibrational motion. The significance of electronic current density for vibrational circular dichroism intensity will be discussed.