Motional Correlation Time in the Electron Spin Relaxation of S6 Spin State Ions in Solution

The electron spin resonance (ESR) linewidth of Mn2+ and Fe3+ ions was measured as a function of temperature in various solvents which can reach high viscosity coefficients before solidification. With increasing correlation time, fast-motion narrowing is no longer verified and the linewidth passes through a maximum value corresponding to ω0τc = 1. Evaluation of τc allows a quantitative check of the zero-field splitting relaxation process. It has been found that modulation of the zero-field splitting in most cases is not the only effective process in relaxing the spin system, even at room temperature. Evaluation of the hydrodynamical radii for Debye tumbling of the complexes gives results which are always smaller than expected. However, the dependence of the correlation time on the size of the complex strongly suggests that rotation of the zero-field splitting tensor is responsible for the line broadening.