Translational, rotational, and spin-rotational contributions to spin lattice relaxation in C₆H₆ and 1,3,5- C₆H₃D₃

Abstract

Measurements of proton T₁ in mixtures of C₆H₆ - C₆D₆ and 1,3,5 - C₆H₃D₃ - C₆D₆ have been made over the temperature range of 5° to 150°C and are analyzed to obtain separately the inter-molecular and intra-molecular contributions to relaxation. The temperature dependence of the inter-molecular part is the same as that of the measured C₆H₆ self-diffusion coefficient in both liquids. Agreement between theory and experiment can be obtained for the inter-molecular part by calculating the average distance of closest approach between spins in different molecules on the basis of the geometry of the benzene molecule and the possible collision geometries, if no rotational correlation is assumed between molecules. The intra-molecular contribution in C₆H₆ exhibits an anomalous behavior in both the magnitude and the temperature dependence predicted by dipolar relaxation theory. The inclusion of a spin-rotational interaction with a spin-rotational constant of 0.41 kc and the inclusion of a microviscosity coefficient of 0.081 lead to agreement between theory and experiment for both magnitude and temperature dependence up to 300°C for C₆H₆. In 1,3,5 - C₆H₃D₃, an additional anomaly in the dipolar intra-molecular contribution cannot be explained in terms of the relaxation mechanisms investigated.