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

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.