Understanding Molecular Optical Activity

Abstract

An expression for the rotatory strength of an electronic transition isomorphic with Kirkwood’s theory of optical rotatory power is obtained by reformulation of the rotatory intensity expression given by the Random Phase Approximation utilizing a basis of localized molecular orbitals. All terms in the resulting expression are manifestly origin-independent, and this formulation therefore allows the extraction of physically meaningful values for the contributions from the three characteristic mechanisms, namely the intrinsic bond contributions, the electric dipole — magnetic dipole (μ - m) terms and the polarizability (μ - μ) terms. At the same time it provides a structurally useful decomposition into bond terms and bond-bond coupling terms. The method is applied to the molecules (-)transcyclooctene, 3-methylcyclopentene and (β-methyladamantanone, and the results show that the dominating mechanism and the nature of the interaction between the respective chromo-phores and their molecular surroundings are quite different for these three compounds. The results also provide insight into the validity of the assumptions behind the various sector rules for optical activity.