Conformational Stability of (+)-Epichlorohydrin

Abstract

Vibrational absorption and circular dichroism spectra of (+)-epichlorohydrin have been recorded for the neat liquid and in organic solvents CCl4, CS2, CHCl3, CH2Cl2, and CH3I. These spectra are compared with the ab initio predictions of absorption and vibrational circular dichroism (VCD) spectra obtained with density functional theory using the B3LYP/6-311G(2d,2p) basis set for three different conformers of (S)-epichlorohydrin. The Boltzmann populations, obtained from Gibbs free energies, indicate that the populations of gauche-II, gauche-I, and cis conformers for isolated molecule are 65, 32, and 3%, respectively. The analysis of experimental and theoretical absorption data indicates that the population ratios of conformers gauche-II:gauche I:cis are: ∼35.7%:∼54.6%:9.7% (neat liquid); ∼58.6%:∼34.0%:7.4% (CCl4); ∼51.0%:∼40.6%:8.4% (CS2); ∼46.1%:∼44.8%:9.1% (CHCl3); ∼42.2%:∼48.5%:9.3% (CH3I); ∼37.2%:∼53.3%:9.5% (CH2Cl2). These compositions are consistent with the experimental VCD spectra. This information clearly indicates that as the polarity of the solvent (as measured by its dipole moment) is increased, the percent composition of the gauche I form increases and that of the gauche II form decreases, but that of cis form is nearly the same in all cases. Vibrational assignments are suggested for the observed bands in the 1500−400-cm-1 region, with the fundamentals assigned mainly to the gauche-II and gauche-I conformers.

Intrinsic rotation and molecular structure

“…There are three different conformations 21 for epichlorohydrin which differ in the dihedral angle of Cl-C-C*-C segment (where C* is the chiral carbon atom). These conformations (see Fig.…”

“…Among these three conformers, the cis conformer is of higher energy and its population for isolated molecules is very small. 21 Nevertheless, the geometries of all three conformations were optimized at three different levels of large basis sets, B3LYP/aug-cc-pVDZ, B3LYP/6-31 1++G(2d,2p), and B3LYP/aug-cc-pVTZ. Then specific rotation was computed for all three conformers, using the same level of theory as that used for geometry optimization.…”

“…This observation suggests that, to interpret the observed intrinsic rotations, it is necessary to have a reliable knowledge of the populations of individual conformers of epichlorohydrin in different solvents. These conformer populations were obtained recently 21 in our laboratory for epichlorohydrin by analyzing the experimental infrared absorption spectra with the aid of B3LYP/6-311G(2d,2p) prediction of infrared spectrum. We have repeated this analysis with a higher-level B3LYP/aug-ccpVTZ prediction of infrared spectrum.…”

“…Since ECD and optical rotation Determined from the analysis of experimental infrared absorption intensities using B3LYP/aug-cc-pVTZ predictions of infrared spectra as described in Ref. 21. The values in parentheses were obtained in Ref.…”

“…The values in parentheses were obtained in Ref. 21 when experimental infrared absorption intensities were analyzed using B3LYP/6-311G(2d,2p) predictions. The two sets of populations do not vary more than 3%.…”