Interaction potential of Al3+ in water from first principles calculations

Abstract

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°.

Structure and dynamics of the hydration shells of the Zn2+ ion from ab initio molecular dynamics and combined ab initio and classical molecular dynamics simulations

“…Therefore, many-body interactions, including polarization, are included in a parameter free way as the simulation proceeds. For highly charged ions, it has been demonstrated that AIMD works reliably [12][13][14]21 and we will show that there is very good agreement between simulated extended x-ray absorption fine structure ͑EXAFS͒ spectra and the results of measurements.…”

“…9͒ based on empirical potentials or by ab initio molecular dynamics ͑AIMD͒. 10,11 Modeling charged ions in solution is challenging because the water molecules surrounding the ion are strongly polarized; [12][13][14] we have recently reported water dipole moments increasing by Ϸ1 D relative to bulk water values for the first hydration shells of the Al 3+ and Fe 3+ ions. 14 In this article, we report simulation of the properties of the Zn 2+ ion.…”

“…[18][19][20] However, developing an empirical potential to describe such an extreme polarization is still very difficult. 13,18 In the simulations reported here, the forces between atoms are calculated directly from the electronic Schrödinger equation ͑first principles or AIMD simulations͒. Therefore, many-body interactions, including polarization, are included in a parameter free way as the simulation proceeds.…”

Coupling a polarizable water model to the hydrated ion–water interaction potential: A test on the Cr3+ hydration

“…40,41 Also, the methodology has recently been extended by proposing a low-cost technique for the generalization of the HIW potential to other small and highly charged cations like Be 2ϩ , Mg 2ϩ , and Al 3ϩ . 42 The validity of this approach has also recently been tested by other groups [43][44][45][46] and was pointed out by Cordeiro et al 47 in an earlier Cu 2ϩ hydration study. In particular, Bleuzen et al 43 have studied the water-exchange mechanism for the second hydration shell of the Cr 3ϩ , assuming in the ionwater interaction potential the existence of the hexahydrate and selecting the nonelectrostatic parameters of the ionsolvent potential in such a way that MD simulations produced the closer results to experimental properties.…”

Shape and size of simple cations in aqueous solutions: A theoretical reexamination of the hydrated ion via computer simulations

“…[20][21][22][23][24][25] Their results also point out the advantages of this approach to describe a wide set of physicochemical properties of ionic solutions.…”