Electron Correlation Models for Optical Activity

A two‐system no‐overlap model for rotatory strength is developed for electric‐dipole forbidden as well as allowed transitions. General equations which allow for full utilization of symmetry in the chromophore and in the environment are obtained. The electron correlation terms are developed in full detail for an ${(}^1{A}_2{\leftarrow }^1{A}_1\text{(C2υ)}$ transition of a chromophore interacting with a nonpolar anisotropic perturber. It ensues that perturber anisotropy of polarizability makes substantial contributions even in such zero‐order forbidden transitions. The correlation terms for a strong electric‐dipole allowed transition ${(}^1{A}_1{\leftarrow }^1{A}_1)$ of the same chromophore gives sector rules that are decidedly different. An additive calculational scheme for the $\mathrm{n\hspace{0.17em}-\hspace{0.17em}\pi *}$ transition rotatory strengths of ketones shows the anisotropy effect to substantially modify, indeed sometimes become larger than, the simple octant rule behavior which occurs for isotropic perturbers. Since agreement with experimental rotatory strengths is correct in magnitude and sign and follows closely the observed variations, it appears that electron correlation is an important if not dominant perturbation mechanism for nonpolar substituents. An experimental prototype of the incomplete‐screening‐of‐nuclei perturbation is considered which suggests that such a one‐electron mechanism gives an incorrect sign of rotatory strength for the ketone back octants. The significance of the absolute signs associated with the octant rules of other chromophores are discussed.