Raman optical activity in ring deformations of methyloxirane, methylthiirane and trans-2,3-dimethyloxirane

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Abstract
We have measured polarized and depolarized Raman optical activity in ring deformations of R-(+)-methyloxirane, R-(+)-methylthiirane and (2s,3s)-(-)(2,3)-dimethyloxirane. The observations are consistent with recent assignments of vibrations containing significant contributions from symmetric and antisymmetric combinations of C-O and C-S stretches and with the absolute stereochemical configurations. A detailed analysis is presented of how the ratio of the polarized to the depolarized Raman optical activity varies with the ratio of the magnetic dipole to electric quadrupole contributions. Effects in the oxirane and the thiirane are similar, but the dimethyloxirane shows dramatically different behaviour associated with a dominant electric quadrupole mechanism in the antisymmetric band. It is shown how this might be explained in terms of the selection rules on the first two Rydberg electronic transitions in the dimethyloxirane in conjunction with the C 2 symmetry of the molecule. Our results also indicate that the bond polarizability theory is not realistic for vibrations involving oxygen or sulphur heteroatoms in ring systems.