ESR and Electronic Relaxation of Cr3+ and Fe3+ in Water and Water–Glycerol Mixtures

ESR intensities and linewidths at X band and linewidths at Q band of [Cr(H₂O)₆]³⁺ and [Fe(H₂O)₆]³⁺ in water‐glycerol mixtures are reported. At X band the linewidth increases with glycerol concentration and there is a concommitant decrease in the integrated intensity of the ESR line. The linewidth at Q band is independent of the glycerol content of the solvent. The results are interpreted in terms of the general theory of electronic relaxation. It is shown that the dominant electronic relaxation in solutions of Cr³⁺ and Fe³⁺ hydrates is due to modulation of the zero‐field splitting interaction. This interaction and its time dependence result from collisions of the hydrated complex with solvent molecules. Using equations derived for relaxation by rotational tumbling, a correlation time of about 5 × 10⁻¹² sec in water is found for both complexes. The decrease in the integrated intensity with increasing viscosity at X band is due to the smearing out of all but the $\frac{1}{2}\mathrm{\hspace{0.17em}\to \hspace{0.17em}-\hspace{0.17em}}\frac{1}{2}$ transitions. At Q band the Cr³⁺ resonance is essentially the $\frac{1}{2}\mathrm{\hspace{0.17em}\to \hspace{0.17em}-\hspace{0.17em}}\frac{1}{2}$ transition even in the aqueous solution. Linewidths for a number of other Cr³⁺ complexes are reported and discussed.