Applications of Proton Magnetic Resonance to Rotational Isomerism in Halotoluene Derivatives. V. α,α,α′,α′,2,3,5,6-Octachloro-p-xylene, Semiempirical Barrier Calculations

Abstract

In toluene-d8 solution the p.m.r. spectrum of α,α,α′,α′,2,3,5,6-octachloro-p-xylene at temperatures below −25 °C consists of two sharp peaks corresponding to the two conformations in which the methine protons lie cis and trans to each other in the plane of the aromatic ring. The barrier to rotation of the dichloromethyl groups is derived from a line-shape analysis of the rate-dependent spectra using the computer program ABXFIT. The activation parameters are EA = 13.6 ± 0.4 kcal/mol, log A = 11.3 ± 0.3, ΔH‡ = 13.1 ± 0.4 kcal/mol, ΔS‡ = −7.3 ± 1.3 e.u., ΔG‡ = 15.4 kcal/mol at 286 °K. The quoted errors are standard errors from least squares fits. These parameters are compared to the extensive data known for α,α,2,4,6-pentachlorotoluene. A series of barrier calculations, based on modified Buckingham and on van der Waals potential energy functions, are discussed with reference to various halotoluenes.