ESR Linewidths in Solution. II. Analysis of Spin—Rotational Relaxation Data

In Part I of this series the residual linewidths for vanadyl acetylacetonate in toluene were found to be proportional to (T/η), where η is the viscosity, and independent of applied field. Since the solutions are dilute, spin exchange is not important. The residual linewidth is attributed to a spin—rotational relaxation mechanism, and calculations based on a perturbation theory of spin—rotational interactions and Hubbard's theory of spin—rotational relaxation yield results in reasonably good agreement with experiment. This relaxation mechanism is probably significant for many dilute S = ½ systems in liquids. For systems with weak anisotropic intermolecular interactions, e.g. ClO2 in CCl4, the correlation time τω for angular momentum and the spin—rotational contribution to the linewidth appear to be larger than those predicted by Hubbard's theory.