Self-consistent reaction field calculations of aqueous Al3+, Fe3+, and Si4+: Calculated aqueous-phase deprotonation energies correlated with experimental In(K-a) and pK(a)

Reaction energies for the deprotonation of Al3+.6(H2O), Fe3+.6(H2O), and Si (OH)(4) were calculated using Hartree-Fock and density functional methods w ith 6-311+G(d,p) (for Al3+ and Si4+) and 6-311G(d) (for Fe3+) basis sets. T heoretical energies were calculated using a supermolecule approach (i.e., e xplicit hydration of the solute) combined with the Integral Equation Formal ism Polarized Continuum Model (IEFPCM; Cances et al., 1997, J. Chem. Phys. 107, 3032) and the Self-Consistent Isodensity Polarized Continuum Model (SC IPCM; Keith and Frisch, 1994, ACS Symp. Ser. 56, 22) in Gaussian 98. Tests on the effects of increasing the number of water molecules explicitly inclu ded in the supermolecule were also carried out. Additional water molecules in the energy minimizations of Al(OH)(3), Fe(OH)(3), and [Si(OH)(3)(OH2)]() resulted in 5-coordinate complexes for all three species. Correlations of deprotonation energies with observed ln(K-a) values are good for individua l cations. These correlations suggest that the combined supermolecule/conti nuum approach can give reliable pK(a) estimates, provided that the structur al optimization reflects the aqueous-phase solute and that the basis set in cludes polarization and diffuse functions. An estimate is made of the pK(a) of the reaction-[Si(OH)(3)(OH2)](+)((aq)) <----> Si(OH)(4(aq)) + H-(aq)(+) -which has not yet been measured. A value of pK(a) approximate to -2 is pre dicted in this study.