Hyperfine interactions and relaxivities in divalent and trivalent aquoion systems

The electronic structures of aquoions involving the divalent paramagnetic ion Mn⁺² and the two trivalent ions Fe⁺³ and Cr ⁺³ have been studied using the unrestricted Hartree-Fock procedure, with six H₂O molecules surrounding the metal ion. From the results of our investigations, good agreement is found between the metal-oxygen distances obtained from the present calculation and those from hydrated ionic crystals and aqueous solutions. From the analysis of the charges on the various atoms of the cluster, the covlaent bonding between the metal ion and water molecules is found to be negligible in the Mn⁺² system and significant for Fe ⁺³ and Cr⁺³. The spin populations however indicate strong localization on the metal ions in all three cases. Very good agreement is found between the calculated contact proton hyperfine constants for all three systems and measured values and for the ¹⁷O hyperfine constant in the Mn ⁺² aquoion system, the only one of those systems for which it is available. The contributions of the dipolar hyperfine fields at the proton sites to the proton relaxivities are studied and it is shown that in all three cases, the use of the point dipole model in earlier literature to explain the data on these relaxivities is well justified. Physical reasons for the various features of our results are discussed.