Measurement of Carbon−Proton Dipolar Couplings in Liquid Crystals by Local Dipolar Field NMR Spectroscopy

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.