Hydrolysis of Ferric Ion in Water and Conformational Equilibrium

Reported here are results of theoretical calculations on the hexaaquoferric complex and deprotonated products to investigate the molecular mechanisms of hydrolysis of ferric ion in water. The combination of density functional electronic structure techniques and a dielectric continuum model for electrostatic solvation applied to the Fe(H$_2$O)$_6$$^{3+}$ complex yields -1020~kcal/mol (experimental values -1037~kcal/mol to -1019~kcal/mol) for the absolute free energy of the aqueous ferric ion. The predicted reaction energy change for the first hydrolysis is 2 kcal/mol (compared to 3 kcal/mol experimental). For the second hydrolysis, we found an unexpected low energy isomer of Fe(H$_2$O)$_4$(OH)$_2$$^{+}$ with five ligands in the inner sphere and one water outside. The cis and trans isomers are, respectively, slightly lower and higher in energy. Extrusion of the additional water to the outer sphere is nearly thermoneutral. The reaction free energy for the second hydrolysis is predicted in the range 16-18~kcal/mol, higher than the experimental value of 5~kcal/mol. Since the theoretical predictions for the second hydrolysis are higher than experimental values and novel structures were encountered, we argue that conformational entropy is important in these reactions.