Simplification of deuterium spectra of liquid crystals by the refocussing of proton–deuterium dipolar couplings

Abstract

It is shown that spin-echo spectra of deuterium nuclei in liquid crystalline samples are in practice independent of effects arising from proton–deuterium dipolar couplings. The proton decoupling effect of echo experiments is demonstrated on a sample of [4-²H]-pyridine dissolved in a nematic solvent and the form of the spectrum is also calculated in order to determine the magnitude of the residual effects from deuterium—proton coupling. Deuterium spin-echo experiments are also described on three pure liquid crystals. For 4,4′-dimethoxy-[²H₆]-azoxybenzene it is shown that the echo spectrum is simplified sufficiently for both quadrupolar and dipolar splittings to be resolved. Comparing normal with echo spectra and also measuring T₁ values of deuteriums in the alkyl-chain deuterated mesogens 4′([²H₁₁]-n-pentyl)- and 4′-([²H₁₇]n-octyl)-4-cyanobiphenyl suggests that these experiments are a possible means of assigning deuterium resonances to an alkyl-chain position in partially deuterated mesogens.