Vibrational optical activity in perturbed degenerate modes: Concepts and model calculations in 1-substituted haloethanes

Abstract

Vibrational optical activity arising from perturbed degenerate modes (PDM) is defined in terms of the chiral perturbation of symmetric chemical groups containing degenerate pairs of modes. This approach predicts that each such degenerate pair of vibrations will give rise to a couplet signal having equal and opposite VOA intensity for the two modes. The PDM formalism is developed explicitly for the case of a chemical group of C3v symmetry in vibrational circular dichroism (VCD), which complements earlier descriptions of degeneracy in Raman optical activity (ROA). Calculations using the fixed partial charge (FPC) model for VCD and the atom dipole interaction (ADI) model of ROA were carried out for the molecules (R)-1-bromo-1-chloro-1-fluoroethane and (S)-1-chlor-1-fluoroethane-1-d1 (and d0) as simple examples of molecules with a methyl group in a chiral environment. To further analyze the nature of the chiral perturbation of the methyl group, VOA calculations for bromochlorofluoroethane were carried out for perturbations of charge or polarizability, mass, geometry and potential energy, applied individually and in combination to the symmetric parent molecule 1,1,1-trichloroethane. In most cases degenerate mode pairs gave calculated VOA couplets of nearly equal and opposite intensity. The signs and magnitudes of these couplets are discussed in relation to the perturbations applied. It is found that different types of degenerate vibrations behave differently thus reducing the scope of generalizations regarding the effect of the chiral perturbations. However, a more consistent picture emerges if the net sign of the combined VOA from each degenerate mode pair is used and a strong correlation of these signs to the sign of the methyl torsion mode is apparent. The significance of these results to actual VOA observation and interpretation is discussed.