Theory of hyperfine interactions in aqueous Mn3+-porphyrin system

The electronic structure and associated magnetic hyperfine properties of Mn3+-porphyrin with two ligand water molecules are studied using the unrestricted Hartree-Fock procedure with an aim to understand the observed anomalous water proton relaxivity associated with Mn3+ (TPPS4). Our results show that the electron spin distribution is strongly localized on the metal ion and the point dipole model seems to be a good approximation for the dipolar hyperfine interactions in this system. It appears that the anomalous relaxivity observed in the Mn3+ (TPPS4) is not due to the strong delocalization of the spin distribution associated with the metal ion and may result from unusual dynamical effects. (orig.)