  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.
