| Abstract
| - Reported here are results of theoretical calculations onFe(H2O)63+,Fe(H2O)5(OH)2+, threeisomers of Fe(H2O)4(OH)2+, andFe(H2O)3(OH)2+,which investigate the molecular mechanisms of hydrolysis of ferricionin water. The combination of density functional electronicstructure techniques and a dielectric continuummodel for electrostatic solvation applied to theFe(H2O)63+ complex yields anestimate of −1020 kcal/mol(experimental values −1037 to −1019 kcal/mol) for the absolute freeenergy of the aqueous ferric ion. Thepredicted free energy change for the first hydrolysis reaction issurprisingly close to the experimental value(2 kcal/mol predicted compared to 3 kcal/mol experimental). Forthe second hydrolysis reaction, we foundan unexpected low-energy isomer ofFe(H2O)4(OH)2+with five ligands in the inner sphere and one wateroutside. The hexacoordinate cis and transisomers are, respectively, slightly lower and higher inenergy.Calculations on the pentacoordinate speciesFe(H2O)3(OH)2+suggest that extrusion of the outer-sphere wateris nearly thermoneutral. The reaction free energy for the secondhydrolysis is predicted in the range 16−18kcal/mol, higher than the experimental value of 5 kcal/mol.Because the theoretical predictions are higherthan experimental values, and novel structures were encountered amongproducts of the second hydrolysis,we argue that conformational entropy is an important omission in thistheoretical treatment of net reactionfree energies. A fuller cataloging of low-energy hydrolysisproducts and direct calculations of partition functionsof the isolated complexes should help in modeling equilibriumspeciation in groundwaters.
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