Interpretation of the Nuclear Spin-Lattice Relaxation-Time Behavior in the Nematic Mesophases

We give a coherent quantitative interpretation of both the frequency and temperature behavior of the proton spin-lattice relaxation time $T_1$ in the nematic phase of $p$-azoxyanisole. The frequency dependence of $T_1$ is dominated by the long-wavelength thermal fluctuations in the orientational order. The temperature dependence of $T_1$ has an important contribution from short-range phenomena involved in the motion of methyl protons. The activation energy for this motion is $W\approx 9$ kcal/mole.