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

On the role of collective and local molecular fluctuations in the relaxation of proton intrapair dipolar order in nematic 5CB

“…Starting from the Redfield high-temperature relaxation theory, 13,14 using the spin-temperature hypothesis and considering only the spectral densities related to autocorrelation of the internuclear spin vectors, it is possible to obtain that any initial nonequilibrium condition for the observable H D o relaxes to thermal equilibrium with the lattice with an intrapair dipolar order relaxation time T 1D−R given by 25 FIG. Starting from the Redfield high-temperature relaxation theory, 13,14 using the spin-temperature hypothesis and considering only the spectral densities related to autocorrelation of the internuclear spin vectors, it is possible to obtain that any initial nonequilibrium condition for the observable H D o relaxes to thermal equilibrium with the lattice with an intrapair dipolar order relaxation time T 1D−R given by 25 FIG.…”

“…27 This model only takes into account the interaction between the ortho-protons in the benzene rings of the molecules. 25 This result provides a tool for evaluating the contributions from intramolecular as well as intermolecular relaxation mechanisms within the hightemperature limit. 24,25 A generalized expression for T 1D , valid for an arbitrary number of spins, in the framework of the Redfield hightemperature relaxation theory, and using the spintemperature hypothesis, was calculated in an attempt of explaining the FC experiments.…”

“…[19][20][21][22] This difficulty is even more limiting in confined nematic and smectic phases. 24,25 This feature makes T 1D measurements a powerful tool for studying hydrodynamic fluctuations in mesophases because of the wide range of Larmor frequencies where this relaxation mechanism can be clearly observed. This fact generally makes the collection of reliable values for the spectral densities associated with the cooperative motions difficult.…”

“…Most of the experimental results were obtained in the 1970s and 1980s, [26][27][28][29][30][31][32][33] when the only available theoretical approach was the standard two-spin relaxation model. 24,25 A generalized expression for T 1D , valid for an arbitrary number of spins, in the framework of the Redfield hightemperature relaxation theory, and using the spintemperature hypothesis, was calculated in an attempt of explaining the FC experiments. The failure of this model was first suggested after angular-dependent measurements, 32,33 and was clearly confirmed more recently by T 1D ͑͒ experiments.…”

“…Nevertheless, this approach cannot account for the predominance of the OFD, since it provides only negligible corrections to the standard two-spin model. 25 for evaluating the contributions resulting from the Redfield hightemperature relaxation theory. Furthermore, it was shown that the Redfield high-temperature theory does not account for the strong effect of the OFD in the relaxation of the intrapair order.…”