NMR spectra of 1,2-difluoroethane in nematic solvents

In 1,2-difluoroethane, the gauche conformation predominates strongly. This circumstance allows the determination of its relative geometry by NMR in liquid crystals, though certain assumptions have still to be made, e.g. neglect of intermediate conformations. The principal result of the structure determination is the value of 69–70° for the dihedral angle ∠FCCF, which is quite insensitive to variations of the other internal coordinates. The value is, within the limits of error, equal to that obtained by microwave spectroscopy, but deviates from results of electron diffraction measurements.It is impossible to describe the average orientation of 1,2-difluoroethane using less than three orientation parameters. This indicates that the interconversion of the two gauche rotamers is slow compared with the reorientational motion.In an attempt to explain the lower energy of the gauche conformation relative to the trans conformation, and the high value of the dihedral angle, ∠FCCF, some explorative empirical calculations were carried out. These could not, however, reproduce the experimental data.