MULTIPLE-QUANTUM NMR IN A MIXTURE OF LIQUID-CRYSTALS - DIFFERENTIAL COHERENCE DEVELOPMENT

A 2:1 molar mixture of 4’-(cyanophenyl)-4-n-heptylbenzoate and 4’,-(cyanophenyl)-4-n-butylbenzoate exhibits nematic liquid crystal behavior in the temperature range from 25 °C to 50 °C. The proton NMR spectra suggest that two regimes of dipolar interaction are present, i.e., those characteristic of nearly isolated proton spin pairs, presumably on the phenyl rings, and weakly coupled multispin clusters originating from the alkyl chain moieties comprising the remainder of the system. A solid-echo pulse sequence permits the observation and separation of these unique regions within the molecules on the basis of their differing relaxation properties. We have combined this solid echo selection technique with multiple-quantum NMR. Differential development of proton spin coherence can then be distinguished (in this case) for different segments of the same molecule. We find that over the course of the multiple-quantum preparation times, the phenyl proton spin pairs do not interact appreciably with the remaining protons of the molecule. © 1990, American Chemical Society. All rights reserved.