Interaction Potential of Al3+ In Water From First Principles Calculations

Journal Article · Sun Jun 15 04:00:00 EDT 1997 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.473866· OSTI ID:1543427

Wasserman, Evgeny ^[1]; Wasserman, Evgeny ^[1]; Rustad, James R. ^[2]; Xantheas, Sotiris S. ^[2]

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1. EMSL, Pacific Northwest National Lab
2. BATTELLE (PACIFIC NW LAB)

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 consistent basis sets that approach the complete basis set limit and include core-valence correlation effects. We suggest as minimum theoretical 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 increase of ~5 kcal/mol (3%) to the stabilization energy, a shortening of 0.015 Å 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 Å at the MP2 level of theory with the cc-pwCVQZ basis set. This minimum is metastable with respect to the Al2++H2O+ asymptote since even the second ionization potential (IP) of Al is larger than the first IP of water. The hexa-aqua cluster Al3+(H2O)6 is, however, stable upon dissociation to Al3+(H2O)5+H2O by 64.8 kcal/mol, demonstrating the capacity of “effective” solvation in stabilizing the charge on the cation. The optimal structures of the n=5 and 6 clusters (having C2v and Th symmetries, respectively) and their harmonic vibrational frequencies are the first ones reported at the MP2 level with basis sets of this size. Core-valence correlation effects for the n=6 cluster are found to be of similar magnitude with those observed for the n=1 cluster. The stabilization 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 Å. These results were used in order to parametrize a pairwise-additive interaction potential for aluminum–water interaction that was grafted onto the Toukan–Rahman interaction potential for water. The potential model reproduces the ab initio results for Al3+(H2O)6 within 2.0 kcal/mol for the stabilization energy and 0.003 Å for R(Al–O) distance. Using this potential we estimated the enthalpy of solvation of Al3+ to be -1106±6 kcal/mol, therefore favoring the lower value of the experimentally obtained data (-1115 and -1140 kcal/mol, respectively). In addition, we calculate the first peak of the Al–O radial distribution function at 1.885 Å, in excellent agreement with x-ray diffraction studies that suggest a peak at 1.882±0.004 Å. We compute the first peak of the Al–H radial distribution function at 2.473 Å and the average angle between the plane of a water molecule and the Al–O vector at -28.27°.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1543427
• Report Number(s): PNNL-SA-28613
• Journal Information: Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 23 Vol. 106
• Country of Publication: United States
• Language: English