THE CONFORMATION OF THE AROMATIC RINGS RELATIVE TO THE ALKYL CHAIN IN4-N-PENTYL-4'-CYANOBIPHENYL

A partially-deuteriated sample of the nematogen 4-n-pentyl-4'cyanobiph enyl has been synthesized which contains six protons, four in an aroma tic ring and two on the attached methylene group. The proton-deuterium NMR spectrum of a pure sample, and of a mixture with the nematogen I3 5 have been recorded and analysed to yield a set of inter-proton dipol ar couplings. These have been used to derive the potential governing r otation about the ring-C bond, which is found to have a minimum when t he bond is in the plane perpendicular to the ring plane. The barrier t o rotation is found to be larger (> 22 kJ mol(-1)) than in ethylbenzen e (approximate to 3 kJ mol(-1)).