MEASUREMENT OF CARBON-PROTON DIPOLAR COUPLINGS IN LIQUID-CRYSTALS BY LOCAL DIPOLAR FIELD NMR-SPECTROSCOPY

The performance of several different two-dimensional NMR methods for t he measurement of carbon-proton dipolar couplings in liquid crystallin e phases is analyzed. Proton-detected local field spectroscopy allows the measurements of short range C-H couplings, which correspond to dir ectly 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 a s nematic phases. The consequent scaling of the chemical shift anisotr opy and dipolar couplings can be used to resolve otherwise overlapping resonances, Moreover, an estimate of the accuracy of the technique ca n be achieved by performing a series of OMAS experiments with differen t sample orientations.