Measurement and Assignment of Long-Range C−H Dipolar Couplings in Liquid Crystals by Two-Dimensional NMR Spectroscopy

Abstract

We describe multidimensional NMR techniques to measure and assign 13C−1H dipolar couplings in nematic liquid crystals with high resolution. In particular, dipolar couplings between aromatic and aliphatic sites are extracted, providing valuable information on the structural correlations between these two components of thermotropic liquid crystal molecules. The NMR techniques are demonstrated on 4-pentyl-4‘-biphenylcarbonitrile (5CB), a well-characterized room-temperature nematic liquid crystal. Proton-detected local-field NMR spectroscopy is employed to obtain highly resolved C−H dipolar couplings that are separated according to the chemical shifts of the carbon sites. Each 13C cross section in the 2D spectra exhibits several doublet splittings, with the largest one resulting from the directly bonded C−H coupling. The smaller splittings originate from the long-range C−H dipolar couplings and can be assigned qualitatively by a chemical shift heteronuclear correlation (HETCOR) experiment. The HETCOR experiment incorporates a mixing period for proton spin diffusion to occur, so that maximal polarization transfer can be achieved between the unbonded 13C and 1H nuclei. To assign the long-range C−H couplings quantitatively, we combined these two techniques into a novel reduced-3D experiment, in which the 1H chemical shift-displaced C−H dipolar couplings are correlated with the 13C chemical shifts. The time domain of this experiment involves separate but synchronous incrementation of the evolution periods for the C−H dipolar couplings and the 1H chemical shifts, with a variable ratio of the respective dwell times to optimize the resolution and facilitate resonance assignment in the spectrum.