Multiple-quantum N.M.R. study of molecular structure and ordering in a liquid crystal

Abstract

Proton multiple-quantum N.M.R. spectroscopy was used to investigate the anisotropic ordering in a magnetic field and molecular structure for the nematic phase of 4-cyano-4′-n-pentyl-d₁₁-biphenyl (5CB-d₁₁). The multiple-quantum spectra exhibit a greater resolution of line splittings than the conventional single-quantum Fourier transform N.M.R. spectrum of 5CB-d₁₁. This greatly simplifies a spectral analysis for the proton dipole-dipole coupling constants. Splittings among the five-, six-, and seven-quantum spectra are used to obtain the biphenyl proton couplings for 5CB-d₁₁. Two models for the biphenyl symmetry are considered in the analysis. In one model (D ₄ symmetry), the two phenyl rings are assumed to be equivalent. In the other model (D ₂ symmetry), the rings are inequivalent by virtue of structural or motional differences between them. Both models produce acceptable fits to the splittings assigned from the experimental spectra. However, we conclude that the spectra cannot be used to make a choice between D ₂ and D ₄ symmetry for the biphenyl group of 5CB-d₁₁. The proton dipolar couplings obtained in both cases are used to derive values for internuclear distances and the biphenyl order parameters. The D ₄ symmetry dipolar couplings produce the most reasonable structural description of the biphenyl unit in which the phenyl ring geometry is close to that of benzene. The angle of twist between the ring planes is determined to be 30 ± 2°. The order parameters from the D ₄ symmetry analysis are in good agreement with results reported by others for the same liquid crystal but with a higher degree of deuterium substitution. Possible causes for experimentally observed seven-quantum splittings that are not predicted by the symmetry model are discussed.