Rotational Isomerism in 2,3 Dihalopropenes

Abstract

Previously reported variations in different solvents of the long‐range (1,3) NMR coupling constants in 2,3 dihalopropenes have suggested the existence of rotational isomers in rapid equilibrium. This view is supported by infrared spectra. By assuming an angular dependence of the form J=J₀ cos²φ between the observed coupling constants and the dihedral angle of the methylene C–H bonds as suggested in calculations by Karplus, the identities of the isomers are deduced, the equilibrium constants calculated, and the energy differences expressed as a function of solvent dielectric constant using an Onsager cavity model. The results for 2,3 dichloropropene are compared with published dipole moment measurements in solution and in the vapor at elevated temperatures.