The barrier to internal rotation in benzyl fluoride investigated using the N.M.R. spectra of samples dissolved in liquid crystalline solvents

The proton and fluorine spectra of samples of benzyl fluoride dissolved in two nematic solvents (ZLI 1132 and I52) have been analysed to yield sets of dipolar couplings [Dtilde]ij between the interacting nuclei. These were compared with values calculated by a theoretical model which allows for the dependence of orientational order on the angle that the C-F bond makes with the plane of the phenyl ring. The observed and calculated [Dtilde]ij are in excellent agreement, and the potential for rotation about the C-CH2F bond is found to have a minimum when the C-F bond is in the plane at 90° to the phenyl ring, but the barrier height changes from 0·3 ± 0·02 kJ mol-1 in I52 to 1·5 ± 0·08 kJ mol-1 for ZLI 1132 as solvent.