Application of the Bergson model to the optical properties of chiral disulfides

Abstract

Bergson developed a simple model to explain the observed red shift of the long-wavelength transition of disulfides as the dihedral angle is reduced from 90° to smaller values. This model is extended here to a calculation of the oscillator strengths and rotational strengths of the n → σ * transitions of chiral disulfides. Explicit expressions for the gradient and angular momentum matrix elements for the n → σ * transitions as functions of the dihedral angle are presented. Transition monopoles suitable for the calculation of the coupling of disulfide n → σ * transitions with other chromophoric groups in complex disulfides are also presented. It is shown that this extension of the Bergson model can account quite well for the observed intensities and rotational strengths of chiral disulfides. The quadrant behavior of the long-wavelength rotational strength derived by Linderberg and Michl and verified experimentally by Ludescher and Schwyzer is observed in our results as well. The observed transitions in the far ultraviolet spectrum of disulfides are assigned to an n → σ * transition at 205 nm and a σ → σ * transition at 190 nm.