The Hydration Structure of Sn(II): An ab initio Quantum Mechanical Charge Field Molecular Dynamics Study Len Herald V. Lim, Thomas S. Hofer, Andreas B. Pribil, and Bernd M. Rode* Theoretical Chemistry DiVision, Institute of General, Inorganic and Theoretical Chemistry, UniVersity of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria ReceiVed: NoVember 11, 2008; ReVised Manuscript ReceiVed: January 23, 2009 The structural properties of the hydrated Sn 2+ ion have been investigated using ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulations at double- HF quantum mechanical level. The results from the work significantly extend previous study using QM/MM MD simulation and are in good agreement with X-Ray and EXAFS diffraction experiments. The data indicate a set of characteristics for the first hydration shell uncommon among metal ions. Although frequent ligand exchange prevents the formation of a well defined structure, more detailed analyses reveal an asymmetric distribution of ligands around Sn(II). An average of eight water molecules coordinate with the Sn 2+ ion and are distributed at proximal and distal positions that are distinguishable from the second hydration shell and manifest dissimilar degrees of lability. 1. Introduction Tin is one of the commonly used components of various materials ranging from industrial coating to fungicides. 1 Al- though a wealth of applications of tin compounds exists, little is known about the intrinsic behavior of its ion in aqueous solution. An earlier study on hydrated Sn(II) using a quantum mechanic/molecular mechanic molecular dynamics (QM/MM MD) approach revealed a high flexibility of the first solvation shell and unusual ligand dynamics in this region. 2 An average of 8 coordinated water molecules were found at a distance of ∼2.53 Å. However, significant tailing in the radial distribution function up to a distance of ∼3.8 Å had suggested rather irregular local ligand configuration. Earlier experimental inves- tigations using X-ray diffraction and EXAFS showed two distinct bond lengths at ∼2.35 and ∼2.9 Å, respectively. 3 This type of behavior was not observed for the first hydration shell of Pb 2+ , 4 which also belongs to the fourth main group of the periodic table. As such, the Sn 2+ ion poses an intriguing case in understanding metal ion solvation in aqueous solution. Recent investigations have proven the usefulness of ab initio MD simulations in obtaining realistic data on solvation structures of ions, 5 especially when experimental data are limited. Al- though the degree at which solvation properties are reproduced by ab initio QM/MM MD has had considerable impact on the study of metal ions in solution, 6 careful attention in the preparation of solute-solvent potentials may not sufficiently reflect charge transfer and polarization effects. The quantum mechanical charge field molecular dynamics (QMCF MD) 7,8 improve the standard QM/MM approach by removing potential construction requirements and introducing a sophisticated handling of Coloumbic interactions. Recent applications of the QMCF MD ansatz produced results in excellent agreement with EXAFS and LAXS data. 9,10 Because the lability of the first hydration shell of Sn 2+ indicates the strong influence of electron density distribution, a detailed analysis focused on this region is of particular interest for the interpretation and rationalization of underlying attributes. The solvation behavior of the Sn 2+ ion, in terms of structure and dynamics, thus requires combined methods of analysis at quantum mechanical level applied to a sufficiently large subsystem. 2. Methods Similar to the QM/MM framework, 11-13 the method of ab initio QMCF MD appropriately partitions the system into two regions. Partitioning is done such that computational treatment at the levels of quantum mechanics (core zone) and classical mechanics (MM zone) can be realized to achieve an efficient compromise between speed and accuracy. As the difference in the QM and MM formalisms lead to inconsistencies in the description of microscopic state properties, schemes to bridge the gap are employed (e.g., the introduction of a transition layer between core and MM zones) ensuring a continuous evolution of forces for the whole system. A simple illustration of the partioning scheme is shown in Figure 1. Formally, QMCF handles the interaction of QM and MM particles via Coloumbics plus non-Coloumbics, where appropri- ate. Force evaluation for the different regions are defined as: * Corresponding author: E-mail: Bernd.M.Rode@uibk.ac.at; phone: +43- 512-507-5160; fax: +43-512-507-2714. Figure 1. The partitioning scheme in the QMCF framework. J. Phys. Chem. B 2009, 113, 4372–4378 4372 10.1021/jp809937h CCC: $40.75  2009 American Chemical Society Published on Web 03/02/2009