The nuclear magnetic resonance spectra of some substituted benzenes with D2h‐symmetry, in a nematic solvent

Nuclear magnetic resonance spectra have been measured and interpreted of a number of substituted benzenes with D2h‐symmetry in the nematic phase of 4,4′‐di‐n‐hexyloxyazoxybenzene (1 p‐dichloro‐, 2 p‐dibromo‐, 3 p‐di‐iodo and 4 p‐difluoro‐benzene and 5 symm‐tetrafluorobenzene). The fine structure is caused by the average direct dipolar interaction between the spins and by the indirect couplings1‐3. The direct interactions are distance‐dependent meaning that the spectra are related to the molecular geometry. Thus ratios of proton‐proton distances have been calculated for molecules 1‐4 and molecular models have been designed with which the NMR spectra are in agreement. The liquid crystal NMR spectra allow of comparison of the direct interactions with the indirect interactions. If the sign of the former is known the sign of the latter can be inferred. Thus for symm‐tetrafluorobenzene J = ‐ 17.8 Hz, JFFp = + 11.1 Hz. For p‐difluorobenzene only the sign of the sum (J + JHFm) can be determined. Subject to the restriction that anisotropic contributions to the indirect couplings can be neglected its value is + 12.2 Hz. In symm‐tetrafluorobenzene the indirect coupling between fluorine spins in ortho position contain such anisotropic contributions; the anisotropy of the indirect coupling has, in part, been inferred from the experimental data.