MEASUREMENT AND ASSIGNMENT OF LONG-RANGE C-H DIPOLAR COUPLINGS IN LIQUID-CRYSTALS BY 2-DIMENSIONAL NMR-SPECTROSCOPY

We describe multidimensional NMR techniques to measure and assign C-13 -H-1 dipolar couplings in nematic liquid crystals with high resolution . In particular, dipolar couplings between aromatic and aliphatic site s are extracted, providing valuable information on the structural corr elations between these two components of thermotropic liquid crystal m olecules. The NMR techniques are demonstrated on 4-pentyl-4'-biphenylc arbonitrile (5CB), a well-characterized room-temperature nematic liqui d crystal. Proton-detected local-field NMR spectroscopy is employed to obtain highly resolved C-H dipolar couplings that are separated accor ding to the chemical shifts of the carbon sites. Each C-13 cross secti on in the 2D spectra exhibits several doublet splittings, with the lar gest 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 correlati on (HETCOR) experiment. The HETCOR experiment incorporates a mixing pe riod for proton spin diffusion to occur, so that maximal polarization transfer can be achieved between the unbonded C-13 and H-1 nuclei. To assign the long-range C-H couplings quantitatively, we combined these two techniques into a novel reduced-3D experiment, in which the H-1 ch emical shift-displaced C-H dipolar couplings are correlated with the C -13 chemical shifts. The time domain of this experiment involves separ ate but synchronous incrementation of the evolution periods for the C- H dipolar couplings and the H-1 chemical shifts, with a variable ratio of the respective dwell times to optimize the resolution and facilita te resonance assignment in the spectrum.