Spin–lattice relaxation in cross relaxed systems: Anisole

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Abstract
Spin−lattice relaxation has been monitored as a function of temperature in liquid anisole and its partially deuterated analogues C6D5OCH3 and C6H5OCD3, as pure liquids and in solution in C6D5OCD3. Conventional pulse and Fourier transform techniques have been used. The high resolution spectrum of C6H5OCH3 was found to contain three groups of lines, one associated with the methyl protons, one associated with the meta phenyl protons, and one associated with the ortho and para phenyl protons. The two groups of aromatic lines were found to have, within experimental error, identical spin−lattice relaxation times and activation energies. The contribution of intermolecular interactions to the relaxation mechanism was found to decrease with increasing temperature and was ? 36% for T ? 312°K. A method is discussed for determining the cross relaxation rate between the differently relaxing methyl and phenyl spins. This method utilizes average T1 values for C6H5OCH3, C6D5OCH3, and C6H5OCD3 and yields for anisole a positive cross relaxation rate as a function of temperature. The cross relaxation is found to proceed primarily by intramolecular interactions which cause methyl and phenyl spins to flip in the same sense. Correlation times for over−all molecular tumbling are calculated and compared to results from dielectric relaxation experiments. The comparison indicates that the over−all molecular tumbling may be near to the limit of large diffusive steps discussed by Ivanov [Sov. Phys.−JETP 18, 1041 (1964)]. A correlation time for the threefold methyl reorientation is also calculated. Activation energies were found for the species C6H5OCH3, C6D5OCH3, and C6H5OCD3 to be 2.77±0.08, 2.78±0.23, and 3.32±0.26 kcal/mole, respectively.