Application of proton magnetic resonance to rotational isomerism in halotoluene derivatives. II. α,α,2,4,6-Pentachlorotoluene

Abstract

At temperatures below −40 °C the proton magnetic resonance of α,α,2,4,6-pentachlorotoluene in toluene-d8 or methylcyclohexane is an ABX spectrum where X is the methine proton. The C—H bond of the dichloromethyl group lies in the plane of the ring and its proton couples to one ring proton only. As the temperature increases the mean lifetime before interconversion of the "mirror image" isomers decreases and at 70 °C the proton spectrum is A2X. A density matrix formalism is used to calculate the line shape as a function of the lifetime before exchange. The free energy barrier to rotation of the dichloromethyl group is 15.0 ± 0.1 kcal/mole in both solvents. In toluene-d8 the entropy of activation is near zero. The high energy transition state is very likely one in which the C—Cl bonds of the ring and the dichloromethyl groups are eclipsed.