INTERACTION POTENTIAL OF AL3+ IN WATER FROM FIRST PRINCIPLES CALCULATIONS

We present a parametrization of the interaction potential for Al3+ in water from first principles calculations. We have performed a critical study of the Al3+-water interaction using sequences of correlation co nsistent basis sets that approach the complete basis set limit and inc lude core-valence correlation effects. We suggest as minimum theoretic al requirements treatment of the electron correlation at the MP2 level of theory using a triple zeta quality basis set that accounts for the effect of core-valence correlation. The latter amounts for an increas e of similar to 5 kcal/mol (3%) to the stabilization energy, a shorten ing of 0.015 Angstrom in the Al-O distance, and an increase of 22 cm(- 1) in the harmonic frequency of the Al-O vibration. This is the first time that core-valence effects were investigated for this system. The stabilization energy of the Al3+(H2O) cluster is 201 kcal/mol and the corresponding Al-O bond length is 1.719 Angstrom at the MP2 level of t heory with the cc-pwCVQZ basis set. This minimum is metastable with re spect to the Al2+ + H2O+ asymptote since even the second ionization po tential (IF) of Al is larger than the first IP of water. The hexa-aqua cluster Al3+(H2O)(6) is, however, stable upon dissociation to Al3+(H2 O)(5) + H2O by 64.8 kcal/mol, demonstrating the capacity of ''effectiv e'' solvation in stabilizing the charge on the cation. The optimal str uctures of the n = 5 and 6 clusters (having C-2v and T-h symmetries, r espectively) and their harmonic vibrational frequencies are the first ones reported at the MP2 level with basis sets of this size. Core-vale nce correlation effects for the n = 6 cluster are found to be of simil ar magnitude with those observed for the n = 1 cluster. The stabilizat ion energy of the n = 6 cluster with respect to its fragments is 723.7 kcal/mol and the corresponding Al-O distance is 1.911 Angstrom. These results were used in order to parametrize a pairwise-additive interac tion potential for aluminum-water interaction that was grafted onto th e Toukan-Rahman interaction potential for water. The potential model r eproduces the nb initio results for Al3+(H2O)6 within 2.0 kcal/mol for the stabilization energy and 0.003 Angstrom for R(Al-O) distance. Usi ng this potential we estimated the enthalpy of solvation of Al3+ to be -1106i6 kcal/mol, therefore favoring the lower value of the experimen tally obtained data (-1115 and -1140 kcal/mol, respectively). In addit ion, we calculate the first peak of the Al-O radial distribution funct ion at 1.885 Angstrom, in excellent agreement with x-ray diffraction s tudies that suggest a peak at 1.882-0.004 Angstrom. We compute the fir st peak of the Al-H radial distribution function at 2.473 Angstrom and the average angle between the plane of a water molecule and the Al-O vector at -28.27 degrees. (C) 1997 American Institute of Physics.