NONEXPONENTIAL RELAXATION OF DIPOLAR NUCLEI IN POLYCRYSTALLINE SAMPLES

Nuclear magnetic relaxation for spin I = 1 2 nuclei interacting with quadrupolar nuclei (spin S = 3 2) through dipolar coupling is discussed. The dipolar nuclear spin-lattice relaxation rate [[T1(I)]-1 is derived as a function of the angle β between the external magnetic field and the principal direction of the electric field gradient at the site of the quadrupolar nuclei. The T1 results are strongly dependent on the angle β in a frequency range near the quadrupolar spin transition frequencies. This anisotropy of the relaxation rates for single crystals can translate into nonexponentiality of the magnetization recovery in spin-lattice relaxation time measurements in polycrystalline samples. As examples of this case, the nonexponential relaxation of fluorine nuclei in trichlorofluoromethane and protons in trichloromethane (chloroform) is presented and compared with the theory. © 1992.