On the molecular spin density and the electrostatic potential as determinants of the relaxivity of metalloporphyrins

The origin of the unexpectedly large relaxivity of a manganese metalloporphyrin is explored through computer simulations that include a description of the molecular spin density and the molecular electrostatic potential. These molecular properties describe not only the distribution of unpaired electrons in the coordination complex, but also a major driving force responsible for the dynamic behavior of the water molecules. By comparing the computed properties for the manganese complex with those of its congeneric iron complex, we gain insight into the origin of the unusually large relaxivity of the manganese metalloporphyrin. In the process, we learn how to use the computed molecular properties to formulate rules on how to modify the chemical structure of the metalloporphyrins to improve their relaxivity. Specifically, we show how to spatially direct the molecular spin density by the splitting of d orbitals and by the delocalization of electronic spin across unsaturated rings. We also learn how to attract water protons to the areas of high spin density by designing electrostatic focusing fields.