MOLECULAR ORBITAL CALCULATIONS OF VIBRATIONAL OPTICAL ACTIVITY IN (+)-(3R)-METHYLCYCLOHEXANONE AND L-ALANINE USING ELECTRIC FIELD PERTRUBATION

Abstract

We have investigated several approaches to using electric field perturbation methods to calculate the vibrational optical activity (VOA) intensities of all the normal modes of a molecule at the same time. In this method, we theoretically evaluate the derivatives of the dipole moment and polarizability with respect to nuclear displacement by calculating the first and second derivatives of the potential energy gradient of a molecule with respect to a finite external electric field. We will present a molecular orbital model of VOA, as well as infrared and Raman intensities, based on these derivatives alone. Our results for the C-H stretching vibrations of (+)-(3R)-methylcyclohexanone and L-alanine, using CNDO-SCF wave functions, will be discussed. The calculated vibrational circular dichroism intensities agree well both with experiment and with the less efficient localized molecular orbital calculations of the individual normal modes using a finite displacement method. Analysis of new experimental and theoretical results for methylcyclohexanone and methylcyclohexanone-$\alpha$ d, enable us to make a more complete assignment of the overlapping bands in the C-H stretching region.