Electronic and nuclear relaxation in solutions of transition metal ions with spinS=3/2 and 5/2

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Abstract
Electronic relaxation in solutions of solvated Fe3+, Mn2+ (S=5/2) and Cr3+ (S=3/2) is controlled by modulation of the quadratic zero field splitting interaction. The modulation is caused by collisions of the hydrated complex with bulk solvent molecules. Theoretical expressions are derived for the electronic transversal and longitudinal relaxation times T 2e and T 1e and are used to interpret the E.S.R. data of these ions. The analysis yields results for the zero field splitting constants and for the mean lifetime between collisions. The latter are found to be in the range 4–9 × 10-12 s. The nuclear relaxation rate of the solvent nuclei are affected by the dipole-dipole and scalar interaction with the unpaired electrons of the paramagnetic ions. The usual equations for nuclear relaxation due to these interactions are modified to take into account the existence of several T 1e s and T 2e s. These equations are used to analyse proton relaxation data in aqueous solutions of Cr3+, Fe3+ and Mn2+. The E.S.R. and N.M.R. methods give good agreement for Cr3+ and Fe3+ but only moderate agreement for Mn2+.