The conformation of the aromatic rings relative to the alkyl chain in 4-n-pentyl-4'-cyanobiphenyl

A partially-deuteriated sample of the nematogen 4-n-pentyl-4'cyanobiphenyl has been synthesized which contains six protons, four in an aromatic ring and two on the attached methylene group. The proton-deuterium NMR spectrum of a pure sample, and of a mixture with the nematogen I35 have been recorded and analysed to yield a set of inter-proton dipolar couplings. These have been used to derive the potential governing rotation about the ring-C bond, which is found to have a minimum when the bond is in the plane perpendicular to the ring plane. The barrier to rotation is found to be larger (> 22 kJ mol(-1)) than in ethylbenzene (approximate to 3 kJ mol(-1)).