Proton magnetic relaxation of manganese (II) tetrakis(4-sulfophenyl)porphine ion in water.

Water proton nuclear magnetic spin-lattice relaxation rates are reported as a function of magnetic field strength for aqueous solutions of manganese tetrakis(4-sulfophenyl)porphine complexes. The manganese(III) complex displays relaxation that is remarkably independent of temperature at low magnetic field and a magnetic field dependence that is characteristic of the electron spin relaxation rates, making a contribution to the correlation time that dominates the electron-nuclear coupling. The manganese(II) complex is much more effective in relaxing water protons, but the usual models of first coordination sphere and outer-sphere relaxation fail to account for the magnitude and the magnetic field dependence of the relaxation rates. The data suggest that the delocalization of the electron density into the ligand system provides an increase in the effectiveness of what may be called the outer-sphere paths for water proton relaxation.