NMR INVESTIGATION OF THE OXYETHYLENE UNIT ORDERING IN SOME RELATED DIALKOXY LATERALLY SUBSTITUTED NEMATOGENS

Laterally dialkylated nematogens containing a short terminal polyoxyet hylene chain present a wide nematic range starting from room temperatu re. The ordering of the oxyethylene (OE) unit has been studied by C-13 NMR in the liquid crystalline phase. The values of the order paramete rs were derived from the transient oscillations observed during variab le contact-time cross-polarization experiments. Along the oxyethylene chain, the order parameters decrease monotonically. These order parame ter values do not present the usual odd-even effect and are noticeably smaller than those found for terminal alkoxy chains. Entering the nem atic phase, the C-13 thermal evolution of the field-induced chemical s hift in the OE unit is far smaller than the one usually observed for t erminal alkyl or alkoxy chains, regardless of their non-negligible ord er parameters. In addition, no linear correlation between the order pa rameter and the field-induced chemical shift is observed for the carbo ns in the OE unit. This is an indication that the proportion between g auche- and trans-conformations in the OE unit is temperature dependent .