Vibrational Structuring in Optical Activity. II. ``Forbidden'' Character in Circular Dichroism

Publisher Website
Google Scholar
Abstract
The influence of vibration on rotatory strength in the adiabatic approximation is considered and vibronic band rotatory strengths are developed. At a level of approximation which gives ``forbidden'' character and mixed polarization in ordinary absorption, circular dichroism with mixed sign can be expected. At this level a nonvanishing contribution to integrated rotatory strength obtains from vibrational perturbation, even in the low‐temperature limit. Under such circumstances the sign of rotatory strength is not simply related to molecular conformation but depends: (1) on symmetry of the perturbing vibration(s), (2) the sign of ``borrowed'' rotatory strength, and (3) the degree of electronic—vibrational coupling. An upper limit to coupled rotatory strength is estimated as 10−2 Debye—Bohr magneton units, corresponding to molar ellipticity values for reasonable uv bands of 103 deg·cm2 dmole−1. The classic case of β‐octyl nitrite as well as the many recent cases of organic carbonyl compound dichroisms of mixed sign are considered as examples of the phenomena.

This publication has 23 references indexed in Scilit: