Methyl Group Rotation, H-1 Spin-Lattice Relaxation in an Organic Solid, and the Analysis of Nonexponential Relaxation
Journal of Chemical Physics
We report 1H spin-lattice relaxation measurements in polycrystalline 4,4′-dimethoxybiphenyl at temperatures between 80 and 300 K at NMR frequencies of ω0/2π = 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 1H 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 1H magnetization decay in a high-temperature nonexponential 1H 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.
Copyright (2012) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in J. Chem. Phys. 136, 054508 (2012) and may be found at http://jcp.aip.org/resource/1/jcpsa6/v136/i5/p054508_s1.
P. A. Beckmann and E. Schneider, J. Chem. Phys. 136, 054508 (2012).