Nonexponential nuclear spin-lattice relaxation in polycrystalline dimethyl sulfone

Abstract

Spin-lattice relaxation has been monitored via pulse techniques in polycrystalline dimethyl sulfone (DMS) and a 1:1 polycrystalline solution of DMS in the perdeutero analogue. The temperature range covered for DMS was between −90.1 and +75.8°C, while for the 1:1 solution it was between −111.4 and +72.8°C. The relaxation was found to be consistently nonexponential, with exponential relaxation being approached at the lowest temperature. The extent of the nonexponentiality is intermediate between that expected from the effects of cross correlations and that expected from a simple averaging of the exponential behavior of randomly oriented single crystals. A parameter δ=τnull/Tlav is introduced, which may be used to judge the extent to which cross correlations are increasing the observed nonexponentiality beyond that expected from a simple averaging of the exponential behavior of randomly oriented single crystals. Further, it is suggested, in situations where cross correlations do not completely determine the nonexponentiality, that the initial slopes of the decay plots be used to determine correlation times for methyl reorientation since these initial slopes are independent of cross correlation effects. Correlation times for methyl reorientation in DMS have been determined and yield an activation energy of 4.22±0.07 kcal/mole.