摘要：

A detailed study of anisotropic ordering, line shapes, and relaxation is reported for the perdeuterated 2,2,6,6-tetramethyl-4-piperidone N-oxide (PD-Tempone) nitroxide radical in several liquid crystal solvents. The line width results are analyzed in terms of the Polnaszek, Bruno, and Freed (PBF) theory appropriately modified for anisotropic ordering both in the motional narrowing and slow tumbling region. The motional narrowing; results are usually consistent with isotropic rotational diffusion, but under a weak (asymmetric) ordering potential, 〈D002〉 ≈ -0.1, and activation energies characteristic of the twist viscous properties of the liquid crystal. Anomalous line shape behavior in the incipient slow tumbling region is observed, which is not explained by the extrapolation of the appropriate parameters from the motional narrowing region. This anomaly is discussed in terms of anisotropic viscosity and director fluctuations. The latter is predicted to be of negligible importance for the weakly ordered spin probe, as well as qualitatively of the wrong behavior. Anisotropic viscosity, while apparently able to "explain" the anomaly, leads to physically untenable conclusions. The anomaly is then discussed in terms of slowly fluctuating mtermolecular torques, leading to a frequency-dependent diffusion coefficient. While this latter may in part offer an explanation (when one distinguishes between torque components parallel and perpendicular to the director), the implied slowness of the fluctuating torques suggests, from general theory, a new model based upon a local solvent structure around the spin probe which may persist over longer periods than the reorientation time of the spin probe. A simple model calculation of the effects on the ESR relaxation is given. This limiting model is also appropriate for highly structured isotropic liquids. It is shown that such a model could have the same formal spectral effects as anisotropic rotational diffusion, and it would yield non-Debye-like spectral densities of the type that could potentially "explain" the observed incipient slow tumbling anomaly. More general theoretical approaches for the analysis of these effects are briefly discussed.

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