Assignment and measurement of deuterium quadrupolar couplings in liquid crystals by deuterium-carbon NMR correlation spectroscopy

The performance of deuterium-carbon correlation (DECOR) spectroscopy for the assignment and measurement of deuterium quadrupolar couplings in fully deuterated liquid crystalline phases is analyzed. The experiment relies on a broadband polarization transfer step from deuterium to carbons. The efficiency of this step is demonstrated experimentally together with a brief study of cross-polarization dynamics. We show that resolution can be improved by removing the substantial contributions from heteronuclear dipolar couplings in a refocused version of the experiment. Quantification of the quadrupolar couplings is illustrated by the application to the thermotropic liquid crystal 5CB. A complete assignment is obtained by using both short-range and long-range correlations, allowing the individual measurement of all the aromatic couplings that are overlapping in the one-dimensional deuterium spectrum. The heteronuclear C-D couplings are shown to be measurable by direct inspection of the 2D spectra. By way of demonstration, we show that the experiment allows the measurement of quadrupolar couplings as a function of temperature.