INTERNAL-ROTATION IN SOME SYMMETRICALLY HALOGENATED PROPANES, OBTAINED BY MOLECULAR MECHANICS CALCULATIONS

A series of sixteen symmetrically halogenated propanes has been studie d by molecular mechanics calculations. The series can be represented b y the formula XH2C-CY2-CH2X, where X and Y = F, Cl, Br, I. For all com pounds except X = 1, Y = F and X = Br, Y = F the torsion potentials ob tained show four torsion minima: AA, AG, GG'' and GG. The compounds X = I. Y = F and X = Br, Y = F do not possess the minimum GG''. Values f or the energy differences and torsion barrier heights between the conf ormations are given, as well as the structural parameters for all conf ormations. Energy differences between the conformations are in the ran ge 0.4 to 9.7 kcal mol 1. Values for the torsion barrier heights lie b etween 1.0 and 13.7 kcal mol-1. Torsion force constants are calculated for all conformations. Values are in the range 0.06 to 0.42 mdyn angs trom rad-2.