1,3-DIHALOGENATED (F, CL, BR, I) PROPANES - TORSIONAL POTENTIALS, CONFORMATIONAL STRUCTURES AND TORSIONAL FORCE-CONSTANTS, AS OBTAINED BY MOLECULAR MECHANICS CALCULATIONS

A series of seven 1,3-dihalogenated propanes, XH2C-CH2-CH2Y (X = F, Cl , Br, I and Y = F, Cl, Br, 1), has been studied by molecular mechanics (MM) calculations. The series contains all the permutations of XH2C-C H2-CH2Y, with the exceptions of 1,3-difluoropropane, 1,3-dichloropropa ne and 1,3-dibromopropane, studied earlier. The torsional potentials o btained show five minima (and hence five possible conformations) for t he molecules with X = F, Y = Cl, Br; I. The same was found for that wi th 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 e xceptions are (X = F, Y - Br) and (X = F, Y = 1), which have GA as the low-energy conformation. Values for energy differences and torsional barrier heights are given. Energy differences between the conformation s within each molecule are in the range from 0.1 to 2.5 kcal mol-1. Th e torsional barrier heights vary from 0.8 to 11.9 kcal mol-1. Structur al parameters for all conformations are also given. Torsional force co nstants are calculated for all conformations, and for the GG conformat ions the torsional wavenumbers are also given.