AN AB-INITIO THEORY OF VIBRATIONAL CIRCULAR DICHROISM

Author Institution: Department of Chemistry, University of Southern CaliforniaA new, ab initio formalism for the calculation of vibrational rotational strengths is presented. Rotational strengths involve a product of electric and magnetic dipole transition moments. While ab initio electric dipole transition moments are routinely calculated, magnetic dipole transition moments are not because the electronic contribution to the magnetic dipole transition moment of a vibrational transition is zero within the Born-Oppenheimer approximation. This non-physical result can be overcome by using first order perturbation theory to correct the Born-Oppenheimer approximation. The resulting expression for the magnetic dipole transition moment involves a sum over all excited electronic states and is not useful for numerical calculations. We show that a contraction exists reducing the expression to one requiring only ground electronic state wavefunctions. It is necessary to calculate the ground state wave-function as a function of vibrational displacement and as a function of magnetic field at the equilibrium geometry. An ab initio formula for a vibrational magnetic dipole transition moment which is amenable to numerical computation is thus achieved for the first time. The following papers discuss the practical realization of the formalism presented