ESR Study of Complex Formation and Electronic Relaxation of Fe3+ in Aqueous Solutions

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Abstract
ESR measurements of aqueous solutions of Fe3+ containing various electrolytes were made at X‐ and Q‐band frequencies. It was found that the line shape and integrated intensity depend on the pH and on the type and concentrations of counteranions in the solution. Addition of LiCl and NH4NCS result in reduction in the integrated intensity and subsequent disappearance of the ESR signal. This is interpreted in terms of the formation of monosubstituted complexes which give signals too broad to observe. From the results, formation constants have been calculated. Successive addition of NH4F results in the formation of complexes of the type [Fe(H2O)6−nFn]3−n having different ESR linewidth ranging from 10 to 1000 G. There is a strong reduction in the linewidth on going from X‐ band to Q‐band frequencies. The results are explained in terms of electronic relaxation via the modulation of the zero field splitting (zfs) parameter. From the analysis of the data it was possible to derive values for the zfs interactions and the associated correlation times. The results indicate that the modulation of the zfs interaction is due to collisions between complex and solvent molecules rather than diffusional tumbling.

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