The effects of conformation and solvation on optical rotation: substituted epoxides.

The vapor-phase optical rotation (or circular birefringence) of (S)-1,2-epoxybutane, (S)-epichlorohydrin, and (S)-epifluorohydrin has been measured at the nonresonant excitation wavelengths of 355 nm and 633 nm by means of Cavity Ring-Down Polarimetry (CRDP). Complementary solution-phase studies were performed in a wide variety of dilute solvent media to highlight the pronounced influence of solute-solvent interactions. Density functional theory calculations of optical activity have been enlisted to unravel the structural and electronic provenance of experimental observations. Three stable, low-lying conformers have been identified and characterized for each of the targeted chiral species, with thermal (relative population weighted) averaging of their antagonistic chiroptical properties allowing specific rotation values to be predicted under both isolated and solvated conditions. For (S)-epichlorohydrin and (S)-epifluorohydrin, a self-consistent isodensity polarizable continuum model (SCI-PCM) has been exploited to gain further insight into the underlying nature of solvation effects. (c) 2007 Wiley-Liss, Inc.