Molecular Dynamics Simulation of Fe 2+ (aq) and Fe 3+ (aq) Sami Amira, † Daniel Spångberg, † Michael Probst, ‡ and Kersti Hermansson* ,† Materials Chemistry, The Ångstro ¨m Laboratory, Uppsala UniVersity, Box 538, S-751 21 Uppsala, Sweden, and Institut fu ¨r Ionenphysik, Leopold Franzens UniVersita ¨t, Technikerstrasse 25, A-6020 Innsbruck, Austria ReceiVed: April 2, 2003; In Final Form: July 7, 2003 Molecular dynamics simulations of single-ion Fe 2+ (aq) and Fe 3+ (aq) solutions have been performed with two rigid-water models (SPC and SPC/E) and a newly constructed SPC-based flexible-water model (SPC+CCL). The SPC+CCL water model in combination with effective Fe 2+ and Fe 3+ ion-water potentials manages to reproduce many experimental structural and dynamical properties of the solutions. Special attention is given to the large ion-induced frequency shifts of the OH stretching bands, which are also well reproduced by the SPC+CCL model. 1. Introduction Solvent structure and solvent exchange are important keys to understanding the dynamics of chemical reactions in solution. The motion of solvent molecules and the way they arrange themselves around reacting species critically affect the energetics and kinetics of the chemical reactions. The experimental determination of solvation structure and dynamics is generally not a trivial task; here computer simulations (Monte Carlo, molecular dynamics (MD)) can help provide precise molecular- level information. An extensive compilation by Ohtaki and Radnai 1 summarizes experimental and theoretical results for aqueous ionic solutions until 1993. In this paper, we are concerned with the solvation structure and the ionic and molecular dynamics of dilute aqueous solutions of Fe 2+ and Fe 3+ . Previous theoretical studies of ferrous and ferric ions in aqueous solutions discuss solvation- shell structure, 2-7 ion and water diffusion, 7 hydrolysis, 8 charge- transfer reactions, 9-11 ion-pair interactions, 12-14 ligand acidity, 15 many-body effects, 2,3,16 and solvent isotope effects. 4,5 Recently, the Fe 2+ ion in aqueous solution has been studied by the Car- Parrinello molecular dynamics technique 17,18 with the focus on chemical reactions in solution. Also very recently, the diffusion properties and librational and vibrational motions of water molecules in the first and second hydration shells of Fe 2+ and Fe 3+ were studied by ab initio QM/MM molecular dynamics simulations, where the ions and water molecules inside the first shell were treated quantum-mechanically and those outside it by a force-field method. 19 Most of the classical simulations have used the Fe 2+ and Fe 3+ ion-water potentials developed by Curtiss et al. 2 or Kuharski et al., 11 combined with rigid-water models (SPC, SPC/E), except for the paper by Floris et al., 3 in which an effective pair potential based on the polarizable continuum model was used. Flexible- water Fe 2+ (aq) and Fe 3+ (aq) simulations have been published by Curtiss et al., 2 and both rigid- and flexible-water simulations have been published by Gua `rdia et al., 7 who combined the ion- water potential by Curtiss et al. with the SPC 20 and the flexible SPC water models by Toukan and Rahman (SPC-TR) 21 to study the effect of flexibility on the structural and dynamical properties of the solution. Kneifel et al. used both the flexible Bopp- Jancso ´ -Heinzinger (BJH) water model 22 (in ref 4) and a flexible SPC model (SPC combined with a modified version of the intramolecular part of the BJH model) (in ref 5) in studies that focused on solvent isotope effects. In the present work, the structure and dynamics of water molecules around the Fe 2+ and Fe 3+ ions, as well as the ionic diffusion, are studied using a new flexible water model (SPC+CCL) combined with the ion-water potential by Curtiss et al. We have chosen to describe the iron-water interaction by the existing off-the-shelf effective iron-water potentials by Curtiss et al. because they are known to give good results together with the SPC-based water models. We compare the results using the SPC+CCL model with those from the rigid- body SPC and SPC/E water models. The emphasis in this paper lies on the performance of the new flexible water model. Our study includes an elaborate discussion of the O-H stretching frequency, a property which is most-sensitive to the details of the intra- and intermolecular potentials. The OH stretching frequency of water molecules in the first hydration sphere of trivalent ions is known from experiment to down-shift as much as approximately -850 cm -1 compared to the gas-phase value. 23 The layout of the paper is as follows. The interatomic potentials and the MD simulations are described in the Method section. In the Method section, we also introduce the new flexible-water model used in our simulations. The results are presented and discussed in the Results and Discussion section, where the structural and dynamical properties, as well as the intramolecular vibrations, are reported. 2. Method 2.1. The Interatomic Potentials. Both flexible-water and rigid-water MD simulations were performed. In all cases, the intermolecular ion-water and water-water interactions were described by means of pair-additive potentials. We have recently constructed a simple flexible-water model, which was found to satisfactorily reproduce many of the properties of pure liquid water, including the OH vibrational band (both its absolute position and its shift with respect to the gas phase 24 ). In this model (SPC+CCL), the H 2 O-H 2 O interactions are expressed as a simple sum of inter- and * To whom correspondence should be addressed. E-mail: kerst@ mkem.uu.se. † Uppsala University. ‡ Leopold Franzens Universita ¨t. 496 J. Phys. Chem. B 2004, 108, 496-502 10.1021/jp034855k CCC: $27.50 © 2004 American Chemical Society Published on Web 12/10/2003