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At temperatures below -40 "C the proton magnetic resonance of a,a,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-CI bonds of the ring and the dichloromethyl groups are eclipsed. Canadian Journal of Chemistry, 48, 1558 (1970) IntroductionExtensive nuclear magnetic resonance (n.m.r.) studies of the rotational isomers of halogenated ethanes are described in the recent literature (1-4). Reliable thermodynamic parameters for the isomers in their ground state as well as rotational barriers are only obtainable if the n.m.r. spectra of the individual isomers can be observed (1, 2). In common with the halogenated ethanes, toluene derivatives present the problem of rotation about an essentially single carbonxarbon bond. In toluene itself the sixfold barrier to rotation of the methyl group lies well below 200 cal/mole in the gaseous and solid states (5, 7) but it has become clear that in toluene derivatives containing halogen atoms both in the methyl groups and in the ortho ring positions (7),3 the barrier to rotation can be large enough for convenient study by dynamic n.m.r. (d.n.m.r.) methods (4).In this laboratory a series of such compounds are under investigation. Previous studies were confined to fluoromethyl toluene derivatives in which the n.m.r. spectra of individual rotamers were not observed (8). In this paper we describe the analytical techniques involved in our studies and report the barriers to rotation about the C-C bond of a,a,2,4,6-pentachlorotoluene (PCT) in toluene-d8 (C,D8) and in methyl cyclohexane (C6HllCH,).

The activation parameters for the rotation of the dichloromethyl groups in the two conformations of a,a,a',af,a",a'',2,4,6-nonachloromesitylene are reported for solutions in toluene-d8 and in methylene chloride. In addition, free energies of activation are given for solutions i n bromochloromethane, trichloroethylene, and in carbon disulfide. The free energies of activation are lower in the toluene solution than in the other solutions. The entropies of activation are near zero, perhaps slightly negative. The symmetrical conformation is more stable than the unsymmetrical one in all the solvents.Les parametres d'activation pour la rotation des groupes dichloromCthyles dans les deux conformations de l'a,a,a',a',a",a",2,4,6-nonachloromesitylene ont t t C rapportks pour des solutions dans le toluene-d8 et le dichloromethane. D e plus, les energies libres d'activation ont 8te presentees pour les solutions dans le bromo chloromCthane, trichloroethylene, et disulfure de carbone. Les energies libres d'activation sont plus basses dans le toluene que pour les autres solvants; les entropies d'activation sont proches de zCro, voire mCme 16gkrement negatives. La conformation symetrique est plus stable que celle qui est non symttrique dans tous les solvants.[Traduit par le journal]Can.

At low temperatures a,a,af,a',2,4,5,6-octachloro-m-xylene exists as three conformers in solution and the proton resonance spectrum can be assigned by comparison to the spectrum of a,a,aJ,a',2,4,6-heptachloro-m-xylene. The temperature dependent spectra of the former in toluene-d8 solution are fitted to computed spectra based on a set of coupled Bloch equations. The activation parameters for the rotation about the spZ-sp3 carbon-carbon bonds are E. = 14.93 + 0.13 kcal/mol, log A = 12.45 i- IntroductionThis paper describes the results of a careful In a series of experiments designed to deter-study the barrier to in MOCX. mine the stable conformations ofthe side-chain and the barrier to rotation about the sp2-sp3 carbon-carbon bond in halotoluene derivatives, the temperature dependence of the ring p.m.r. spectra of u,u,2,4,6-pentachlorotoluene (PCT),

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