NMR test of extended diffusion with internal rotation

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Abstract
The nuclear magnetic relaxation times for the ring and methyl deuteriums in mesitylene‐d12 and for the fluorine and deuterium in methyltrifluorosilane‐d3 have been measured as a function of temperature. These results together with the literature values for the chlorine and deuterium relaxation in tert‐butylchloride‐d9 [D. E. O’Reilly, E. M. Peterson, C. E. Scheie, and E. Seyfarth, J. Chem. Phys. 59, 3576 (1973)] have all been analyzed in terms of the theory of extended diffusion for symmetric top molecules with internal motion. These three cases are examples of low, intermediate, and high internal rotation barriers, respectively. For the intermediate and high barrier cases, the data are in good agreement with the predictions of the internal M diffusion theory for molecules with the corresponding barrier heights. However, for mesitylene‐d12 (V∼58 J/mole), the methyl deuterium correlation times are smaller than those predicted by the theory for this barrier, but are consistent with the predictions for the internal M diffusion theory with zero barrier height. This discrepancy is due to breakdown in treating the internal rotor with classical mechanics. When the more correct, quantum mechanical treatment is used, these data are also consistent with the internal M diffusion model.