Rotational isomerism. Part XVIII. Rotamer populations in solution of some polychlorinated butanes: a molecular mechanics and nuclear magnetic resonance study

Abstract

The ¹H n.m.r. spectra of meso(1) and racemic (2) 1,1,2,3,4,4-hexachloro- and meso(3) and racemic (4) 1,2,3,4-tetrachloro-butanes are reported and analysed in polar and non-polar solvents. Molecular mechanics calculations and classical solvent theory were used to provide information on the rotamer energies in these molecules and these, together with the observed proton couplings (and in one case dipole moment), allowed a determination of the rotamer populations. Compound (1) exists primarily in one conformation, (2) and (4) as two populated rotamers, but (3) appears to have at least four populated rotamers. The observed rotamer populations and energies are in general in good agreement with the calculated values, and this suggests that this technique could be of value in determining rotamer populations in complex molecules.