Intrinsic rotation and molecular structure

The intrinsic rotation, limiting value of specific rotation at zero concentration, of (R)-(-)-epichlorohydrin was measured in four different solvents, CH3OH, CH2Cl2, CHCl3 and CCl4. It was found that the sign of rotation in CH3OH, and CH2Cl2 solvents is opposite to that in CCl4. The intrinsic rotation in CHCl3 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 CHCl3 solvent, the net rotation is close to zero. When g-II conformation dominates, as in CCl4 solvent, the observed sign of rotation will be opposite to that when g-I conformation dominates, as in CH2Cl2 and CH3OH 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. (C) 2003 Wiley-Liss, Inc.