$^2$H-NMR Detection of the Pressure Effect on Packing Modulations in a Columnar Liquid Crystal

The effect of high pressure on the columnar density modulation of the hexaoctylether of rufigallol, resulting from the incommensurability of the aromatic and aliphatic packing, is studied by 1D and 2D $^2$H-NMR spectroscopy. The mobility of the side chains can be used to probe the local packing density along the columns. We show that the numbers of molecules in regions of high and low packing density are strongly dependent both on temperature and on pressure. 2D-exchange NMR spectra show that the rate of exchange between these regions decreases considerably at increasing pressure, which is attributed to an increase of the length scale of the pinched regions. Our findings are consistent with the model proposed by de Gennes to resolve the incommensurability problem. A large decrease in the rate of molecular reorientation about the column axis is observed at the II-III phase transition.