1,3-Dihalogenated (F, Cl, Br, I) propanes: torsional potentials, conformational structures and torsional force constants, as obtained by molecular mechanics calculations

Abstract

A series of seven 1,3-dihalogenated propanes, XH 2CCH 2CH 2Y (X = F, Cl, Br, I and Y = F, Cl, Br, I), has been studied by molecular mechanics (MM) calculations. The series contains all the permutations of XH 2CCH 2CH 2Y, with the exceptions of 1,3-difluoropropane, 1,3-dichloropropane and 1,3-dibromopropane, studied earlier. The torsional potentials obtained show five minima (and hence five possible conformations) for the molecules with X = F, Y = Cl, Br, I. The same was found for that with X = Cl, Y = Br. The minima are: AA, AG, GA, GG, GG″. For the other molecules the torsional potentials show four minima: AA, AG, GA, GG. For all molecules except two, GG is the low-energy conformation. The exceptions are (X = F, Y = Br) and (X = F, Y = I), which have GA as the low-energy conformation. Values for energy differences and torsional barrier heights are given. Energy differences between the conformations within each molecule are in the range from 0.1 to 2.5 kcal mol −1. The torsional barrier heights vary from 0.8 to 11.9 kcal mol −1. Structural parameters for all conformations are also given. Torsional force constants are calculated for all conformations, and for the GG conformations the torsional wavenumbers are also given.