NUCLEAR AND ELECTRON-SPIN RELAXATION RATES IN SYMMETRICAL IRON, MANGANESE, AND GADOLINIUM IONS

Measurements of water H-1 spin-lattice relaxation rates as a function of magnetic field strength are reported for aqueous solutions of iron(III) and manganese(II) and X-band EPR measurements are reported for manganese(II), iron(III), and gadolinium(III) solutions that provide an improved understanding of what controls nuclear- and electron-spin relaxation in these electronically symmetric paramagnetic centers. The electron-spin relaxation rates of the iron(III) and gadolinium(III) aquo ions are higher than in other symmetrical complexes of these metal centers, which appears to be caused by intramolecular motions of the coordinated water molecules, The water proton relaxation rate at low magnetic field strengths in aqueous manganese(II) and iron(III) solutions increases with increasing perchloric acid concentration, which results in part from an increase in the metal-proton hyperfine coupling constant. In the iron(III) case, the additional relaxation efficiency is interpreted in terms of a change in the orientation of the coordinated water molecule that brings the protons closer to the metal center. The gadolinium(III) electron relaxation rate decreases with increasing perchloric acid and glycerol concentration, which is interpreted in terms of a change in the number of coordinated water molecules changing from 9 to 8. Electron-spin relaxation rates estimated from electron-spin resonance line widths of iron(III) and Gd(III) ions in symmetric complexes such as [FeF6](3-) or [FeCl4](-) long and demonstrate that construction of complex species with long electron-spin relaxation times that will be efficient water proton spin relaxation agents should be possible using iron(III) centers.