Cross correlation effects in the nuclear spin-lattice relaxation of polycrystalline o-, m-, and p -dimethoxybenzene

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Abstract
Nuclear spin‐lattice relaxation has been studied via pulsed NMR techniques in polycrystalline samples of o‐, m‐, and p‐dimethoxybenzene. Nonexponential relaxation has been observed in each compound at temperatures such that the spin‐spin relaxation time was constant at 12μsec and spin diffusion was rapid enough to maintain a single spin temperature. For each compound the nonexponentiality gradually decreased with decreasing temperature until a single spin‐lattice relaxation time characterized the relaxation at the lowest temperatures studied, consistent with the Hilt‐Hubbard theory for effects of cross correlations [Phys. Rev. 134, A392 (1964)]. Application of the theory yields correlation times and activation energies for hindered rotation of methyl protons. Values of 3.6 ± 0.1 and 1.9 ± 0.2 kcal/mole were determined for the activation energy for the para and meta isomer, respectively.