M. Hong et al., MEASUREMENT AND ASSIGNMENT OF LONG-RANGE C-H DIPOLAR COUPLINGS IN LIQUID-CRYSTALS BY 2-DIMENSIONAL NMR-SPECTROSCOPY, Journal of physical chemistry, 100(35), 1996, pp. 14815-14822
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.