The proton N.M.R. spectrum of a sample of 3,5-dibromoethylbenzene dissolved in a liquid-crystalline solvent has been recorded and analysed to yield a set of partially-averaged dipolar couplings, Dij . The Dij are used to test models for the nature of the rotational potential V(ϕ) of the ethyl group about the ring-C bond. It is concluded that V(ϕ) has a minimum at 60°, which contrasts with the predictions of an ab initio molecular orbital calculation for ethyl benzene. The MO calculations find a minimum in V(ϕ) at 90°, which corresponds to the ethyl C-C bond being in a plane orthogonal to the plane of the phenyl ring.
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Celebre et al. (1988) studied this question.
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