PULSED NMR-STUDY OF MOLECULAR-DYNAMICS IN LIQUID-CRYSTAL 40.7

Molecular dynamics governing the nuclear magnetic relaxation times in a liquid crystal, butyloxybenzylidene heptylaniline (40.7), is studied using proton magnetic relaxation spectroscopy. These results are compared with those of similar studies on other compounds belonging to the same homologues series (viz. 40.8, 40.6, and 40.5). Earlier studies indicate that the frequency dispersion of the spin-lattice relaxation time in typical NMR frequency ranges is governed by order director fluctuations (ODF) in 40.8, self-diffusion (SD) in 40.5, whereas by both ODF and SD in 40.6. Molecular reorientations contribute to a frequency-independent component in all these compounds. The present studies show that SD and molecular reorientations effectively mediate the relaxation mechanism in 40.7 in the nematic and the smectic A phases and their relative contributions are quantified. The relaxation data and the line width measurements at magic angle orientation are used to obtain dynamical information on the smectic B phase. These results are supported by dipolar relaxation time measurements. From the temperature-dependence studies the molecular parameters associated with these dynamical processes are obtained. © 1990.