NMR Spin—Lattice Relaxation in Solid Solutions of C6H6 and C6D6: Evidence for Concerted Motions by Adjacent Molecules

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Abstract
The NMR spin—lattice relaxation time T1 has been investigated as a function of isotopic substitution in cyrstalline benzene. 1,3,5‐C6H3D3 and six solid solutions of C6H6 and C6D6 were studied between 140°K and the melting point. The concentration dependence of T1 was used to separate the effects of intra‐ and intermolecular relaxation. The experimental intramolecular T1 contribution agreed quite closely with that calculated from known molecular dimensions. Comparison of the intra‐ and intermolecular correlation times are shown to give information regarding the relative motion of neighboring molecules in the solid. Independent molecular ``jumps'' are found to be relatively unimportant in crystalline benzene, and a free‐volume model, leading to the simultaneous motion of adjacent molecules, is discussed.