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Rksumk.-Les temps de relaxation nucleaire, T I Z et T I D , et les coefficients de diffusion Dl, et DL ont kt6 mesures en fonction de la temperature dans ethyl p-(p-Cthoxybenzylid6ne)-amino-cinnamate (EBAC) et dans TBBA. Les rksultats montrent que la diffusion contribue aux vitesses de relaxation dans les phases smectiques A, C et H de TBBA et les phases A et B de EBAC. La diffusion perpendiculairement au directeur est clairement du type difSusion liquide dans les phases A et C. Dans la phase B de EBAC, d'autre part, la diffusion se fait par sauts comme dans un pseudorbseau.

Statistical models that have been used so far for the description of relaxation and dynamic polarization in spin systems are extended to more general stochastic processes. The CAMPBELL theorem provides a means of calculating a great number of spectral intensity functions. Almost any measured frequency-and temperature-dependence of relaxation rates and dynamic polarization coupling parameters can thus be interpreted from simple molecular physical concepts.

Two methods are utilized to study the relaxation behaviour and molecular motion in solutions of free radicals in toluene: the investigation of longitudinal and transversal solvent proton relaxation at a properly chosen frequency (48 MHz), and the determination of enhancement factors of the dynamic proton polarization over a large frequency range. The temperature dependence of the quantities concerned is measured between +70°C and —120°C. The results can be interpreted in terms of a pure dipole-dipole interaction between proton and electron spins whose time dependence is provided by translational random motions of the molecules and rotational tumbling of associated complexes. The activation energies for both processes are 3.1 kcal/mole and 7.8 kcal/mole, respectively. The solvation is only very weak. The relative contribution of rotational tumbling is R=0.26, from which the average co-ordination number is estimated to be less than one.

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