Non-Exponential Nuclear Spin-Lattice Relaxation in the Solid Phases of EMC

Measurements of the proton spin-lattice relaxation time in the solid phases of ethyl-[p-(p-methoxybenzilidene)-amino]-cinnamate (EMC) are reported. A model for the non-exponential relaxation observed is discussed as well as mathematical technique for treating the experimental data. This model, based on the assumption of the coexistence of two non-interacting groups of indentical spins, was tested using a model liquid-solid system. Results of the measurements on EMC indicate that the two possible solid phases coexist at low temperatures and that a slow annealing takes place near room temperature. The dominant spin-lattice relaxation mechanism at low temperatures is determined to be a rotational motion with an activation energy of ∼ 2.3 kcal/mole.