Infrared absorption and the Born–Oppenheimer approximation. II. Vibrational circular dichroism

A general set of equations is derived for VCD intensities. In order to overcome a vanishing electronic contribution to the rotational strength in the Born–Oppenheimer approximation, these equations include correction terms which provide the necessary correlation of the velocities of the electrons and the nuclei. The summation over all the excited electronic states of the molecule, normally encountered in such correction terms, is formally removed by the introduction of a vibronic gauge function which satisfies a known differential equation. As a special case, this equation is solved for a molecule having an electronic wavefunction comprised of a product of molecular orbitals which are Gaussian and ellipsoidal in form. The resulting VCD expressions for fundamental transitions are identical to those previously derived in the localized molecular orbital model of VCD intensities.