Nuclear magnetic resonance proton dipolar order relaxation in the selectively deuterated nematic para-azoxyanizole

Abstract

Larmor frequency dependent measurements of Zeeman (T1Z) and dipolar order (T1D) relaxation times provide experimental information on the molecular dynamics in liquid crystals. However, at present, a comprehensive theoretical expression relating T1D with the spectral densities of the molecular motions is not available for liquid crystals. In fact, recent relaxation studies in nematic thermotropic liquid crystals have shown that the traditional model of isolated phenyl proton pairs predicts a relaxation of the dipolar order noticeably slower than the observed one. In this work we show that the failure cannot be assigned exclusively to the assumption of isolated spin-pairs. With this aim, we study the dipolar order relaxation in the nematic PAAd6 (methyl deuterated para-azoxyanizole). After calculating a generalized expression for T1D valid for an arbitrary number of spins, we found that the contributions from multispin interactions and correlations are negligible. This means that, from the point of view of the traditional weak collision theory of relaxation, PAAd6 provides an example of an ensemble of isolated spin-pairs. Nevertheless, after measuring T1D over a broad frequency range (103–3×107 Hz), we found a faster relaxation than predicted by the two-spin model, even in this compound. We conclude, therefore, that the failure of the model should be ascribed to basic assumptions of the traditional semiclassical model of spin-lattice relaxation in liquid crystals