The proton magnetic resonance determination of the hindered rotation in α,α,α′,α′,3,4,6-heptachloro-o-xylene

Abstract

A high resolution proton magnetic resonance investigation of the hindered rotation of the dichloromethyl groups as a function of temperature in α,α,α′,α′,3,4,6-heptachloro-o-xylene (HCOX) has been conducted using dynamic nuclear magnetic resonance line shape analysis, supplemented with a homonuclear double resonance saturation transfer technique. The rate process in HCOX involves a coupled rotation of the two substituted methyl groups and the nonmutual exchange of the three pairs of protons between two equally populated conformers. The Arrhenius and transition state parameters were determined from the rate constants obtained over a temperature range of −5 °C to 93 °C for a 5 mol% solution of HCOX in methylcyclohexane: ΔG≠ = 17.22 ± 0.07 kcal mol−1 at 298 K, ΔH≠ = 14.6 ± 0.3 kcal mol−1, ΔS≠ = −8.8 ± 0.8 cal mol−1 deg−1, and Ea = 15.3 ± 0.3 kcal mol−1. The results obtained for HCOX are compared with those for other chloro-substituted xylenes and toluenes.