Application of proton magnetic resonance to rotational isomerism in halotoluene derivatives. III. α,α-Dichloro-2,4,6-tribromotoluene

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Abstract
At room temperature the proton magnetic resonance spectrum of α,α-dichloro-2,4,6-tribromotoluene is ABX where the methine proton in the sidechain is X and is lying in the plane of the aromatic ring. At higher temperatures the ring proton spectrum, AB, broadens and eventually collapses to yield an A2X spectrum. From an analysis of the ring proton line shapes the barrier to rotation of the dichloromethyl group about the sp2–sp3 carbon–carbon bond is obtained; ΔG* = 17.5 ± 0.1 kcal/mole at 304°K, ΔH* = 15.67 ± 0.08 kcal/mole, ΔS* = −7 e.u., Ea = 16.38 ± 0.08 kcal/mole, log A = 11.78 ± 0.23 where the least squares errors given should probably be multiplied by a factor of from 3 to 5 to take possible systematic errors into account. The barrier is about 2 kcal/mole higher than in α,α,2,4,6-pentachlorotoluene. The barrier to rotation arises from the conformation in which chlorine and bromine atoms are eclipsed.

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