Rotational isomerism in open-chain polychlorinated hydrocarbons: Problems in predicting conformations from carbon-chlorine stretching frequencies

Normal coordinate analysis calculations have been made to determine the carbon-chlorine stretching (v(C-Cl)) frequencies of model rotational isomers of 1,2-dichloroethane, 1,2-dichloropropane, 1,3-dichloropropane, 1,5-dichloropentane and 1,2,3-trichloropropane. The calculations were based on prior calculations on models of a number of primary and secondary monochlorohydrocarbons. The models were derived from the real isomers by ignoring the hydrogen atoms and considering only the heavy atom skeletal structure. The possible effect on v(C-Cl) frequencies of coupling between C-C1 stretching and adjacent CH₂ rocking modes in some isomers was taken into account where necessary. The results, and in some cases the frequency of the highest ∠CCC deformation mode, are used to establish the conformations of rotational isomers present in these molecules in the vapour, liquid and crystalline states. Conclusions are drawn concerning the applicability of known v(C-Cl) frequency structure correlations for monochlorohydrocarbons to polysubstituted chlorohydrocarbons. v(C-Cl) frequency correlations and assignments have also been made for several chloropropanes containing one CCl₃ group.