Intrinsic rotation and molecular structure.

Abstract

The intrinsic rotation, limiting value of specific rotation at zero concentration, of (R)-(-)-epichlorohydrin was measured in four different solvents, CH(3)OH, CH(2)Cl(2), CHCl(3) and CCl(4). It was found that the sign of rotation in CH(3)OH, and CH(2)Cl(2) solvents is opposite to that in CCl(4). The intrinsic rotation in CHCl(3) is close to zero. This observed pattern was explained using density functional calculations of specific rotation using very large basis sets. It was found that the g-I and g-II conformations of epichlorohydrin have nearly the same magnitude of specific rotation but with opposite sign. When these two conformations have equal populations, as in CHCl(3) solvent, the net rotation is close to zero. When g-II conformation dominates, as in CCl(4) solvent, the observed sign of rotation will be opposite to that when g-I conformation dominates, as in CH(2)Cl(2) and CH(3)OH solvents. A combination of intrinsic rotation measurement with density functional prediction of specific rotation is demonstrated to be a practical method for determining the structures of molecules.