An investigation of the conformation of 4-chloroethylbenzene as a solute in a nematic liquid-crystalline solvent

The conformation of the ethyl group relative to the phenyl ring plane in 4-chroroethylbenzene has been investigated by recording and analysing the proton NMR spectrum of a sample dissolved in a nematic liquid crystalline solvent. The dipolar couplings obtained are compared with values calculated by the additive potential (AP) and maximum entropy (ME) molecular mean field theoretical models. In both cases good agreement between observed and calculated couplings could be obtained only by adopting a model for rotation about the ring-C bond which allows for geometry relaxation. The ME method produces a conformational distribution with an absolute maximum at 90 degrees and a secondary maximum at 0, which is also the shape found to give the best fit to the data by the AP method. However, constraining the shape of the conformational distribution to that found by the molecular orbital calculations and using the AP method yields an acceptable, if not the best, fit to the dipolar couplings, and so is the preferred solution. It is concluded that the inclusion of geometry relaxation on rotation in this molecule is particularly important when using dipolar couplings to determine the potential for rotation about the ring-C bond.