Methyl group rotation, 1H spin-lattice relaxation in an organic solid, and the analysis of nonexponential relaxation.

We report (1)H spin-lattice relaxation measurements in polycrystalline 4,4'-dimethoxybiphenyl at temperatures between 80 and 300 K at NMR frequencies of ω(0)/2π = 8.50, 22.5, and 53.0 MHz. The data are interpreted in terms of the simplest possible Bloch-Wangsness-Redfield methyl group hopping model. Different solid states are observed at low temperatures. The (1)H spin-lattice relaxation is nonexponential at higher temperatures where a stretched-exponential function fits the data very well, but this approach is phenomenological and not amenable to theoretical interpretation. (We provide a brief literature review of the stretched-exponential function.) The Bloch-Wangsness-Redfield model applies only to the relaxation rate that characterizes the initial (1)H magnetization decay in a high-temperature nonexponential (1)H spin-lattice relaxation measurement. A detailed procedure for determining this initial relaxation rate is described since large systematic errors can result if this is not done carefully.