Zero-field Splitting Interaction Research Articles

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 12 → − 12 transitions. At Q band the Cr3+ resonance is essentially the 12 → − 12 transition even in the aqueous solution. Linewidths for a number of other Cr3+ complexes are reported and discussed.

ESR and NMR of Mn(II) Complexes in Methanol

ESR spectra of Mn2+ in methanol solutions of Mn(ClO4)2 containing various amounts of LiCl, NaNCS, LiBr, LiNO3, and LiClO4 were studied. In all solutions, variations in the integrated intensity of the ESR signal upon addition of the electrolyte were observed. This effect was interpreted in terms of the formation of asymmetrically substituted octahedral Mn2+ complexes, in many of which the zero-field splitting interaction causes fast electronic relaxation and therefore an ESR signal too broad to observe. Some of the mixed solvent-anion complexes do however yield resolved ESR signals; in particular, a well-defined spectrum due to [Mn(MeOH)4(NCS)2] could be characterized. From the decrease of the ESR signal the equilibrium constants for the formation of monosubstituted octahedral complexes, [Mn(MeOH)5X]+, were estimated. The equilibrium constants for Cl−, NCS−, Br−, NO3−, and ClO4− are respectively >100, >100, 10, 5, and 1M−1. At high LiCl concentrations tetrahedral [MnCl4]2−, exhibiting resolved ESR lines, is formed. In the ESR spectrum of the system Mn(ClO4)2–LiCl–methanol there is, in addition to the intense sextuplet, also a very broad background signal which is attributed to the monochloro complex. From the over-all width of this signal an estimate of the zero-field splitting energy in this complex was made. Proton magnetic resonance measurements were made in methanol solutions of Mn(ClO4)2 containing various amounts of LiCl. There is a strong dependence of the proton linewidth on temperature and on LiCl concentration. The results were interpreted in terms of exchange of methanol molecules in and out of the solvation shells of [Mn(MeOH)6]2+ and the various mixed chloro complexes, but only for [Mn(MeOH)6]2+could a quantitative analysis be made. The kinetic parameters for the exchange reaction of this complex are 1 / τM(25°C) = 9.5 × 105sec−1 and ΔE‡ = 7.4 kcal mole−1. In the monochloro complex the exchange rate is about twice as high.