A unified approach to the determination of infrared and Raman vibrational optical activity intensities using localized molecular orbitals

Abstract

The localized molecular orbital (LMO) model of infrared vibrational circular dichroism (VCD) is extended to vibrational Raman optical activity (ROA). Using the equations arising from the finite perturbation approach to ordinary infrared and Raman intensities we show that VCD and ROA intensities can be determined in the same manner provided that localized molecular orbitals are used to describe the motion and response of electronic charge density in the molecule. In this approach a small perturbing electric field is used to obtain the first and second derivatives of the potential energy gradient of the molecule with respect to the electric field and these derivatives are in turn related to infrared and Raman intensities, respectively. Since only a few SCF calculations are required by this method to obtain both VCD and ROA intensities for all the vibrational modes of a molecule, the way is now open for the efficient and complete calculation of vibrational optical activity intensities using molecular orbital methods.