@article {427, title = {Measurement and assignment of long-range C-H dipolar couplings in liquid crystals by two-dimensional NMR spectroscopy}, journal = {Journal of Physical Chemistry}, volume = {100}, year = {1996}, note = {J Phys Chem-UsVe373Times Cited:41Cited References Count:22}, month = {Aug 29}, pages = {14815-14822}, abstract = {

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 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\&$\#$39;-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 C-13 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 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 chemical shift-displaced C-H dipolar couplings are correlated with the C-13 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 H-1 chemical shifts, with a variable ratio of the respective dwell times to optimize the resolution and facilitate resonance assignment in the spectrum.

}, keywords = {shape}, isbn = {0022-3654}, doi = {Doi 10.1021/Jp960972m}, url = {://WOS:A1996VE37300036}, author = {Hong, M. and Pines, A. and Caldarelli, S.} } @article {432, title = {Measurement of carbon-proton dipolar couplings in liquid crystals by local dipolar field NMR spectroscopy}, journal = {Journal of Physical Chemistry}, volume = {100}, year = {1996}, note = {J Phys Chem-UsVv354Times Cited:54Cited References Count:31}, month = {Nov 28}, pages = {18696-18701}, abstract = {

The performance of several different two-dimensional NMR methods for the measurement of carbon-proton dipolar couplings in liquid crystalline phases is analyzed. Proton-detected local field spectroscopy allows the measurements of short range C-H couplings, which correspond to directly bond pairs, by direct inspection of the spectra. Off magic angle (OMAS) spinning techniques can be applied to anisotropic phases that can be oriented mechanically at an angle to the magnetic field, such as nematic phases. The consequent scaling of the chemical shift anisotropy and dipolar couplings can be used to resolve otherwise overlapping resonances, Moreover, an estimate of the accuracy of the technique can be achieved by performing a series of OMAS experiments with different sample orientations.

}, keywords = {solids}, isbn = {0022-3654}, doi = {Doi 10.1021/Jp962023z}, url = {://WOS:A1996VV35400008}, author = {Caldarelli, S. and Hong, M. and Emsley, L. and Pines, A.} }