The solvent shell structure of aqueous iodide: X-ray absorption spectroscopy and classical, hybrid QM/MM and full quantum molecular dynamics simulations V.T. Pham a , I. Tavernelli b , C.J. Milne a , R.M. van der Veen a , P. D’Angelo c , Ch. Bressler a , M. Chergui a, * a Ecole Polytechnique Fédérale de Lausanne, Laboratoire de spectroscopie ultrarapide, ISIC, FSB-BSP, CH-1015 Lausanne, Switzerland b Ecole Polytechnique Fédérale de Lausanne, Laboratoire de chimie et biochimie computationnelles, ISIC, FSB-BSP, CH-1015 Lausanne, Switzerland c Dipartimento di Chimica, Università di Roma ‘‘La Sapienza”, Ple A. Moro 5, 00185 Roma, Italy article info Article history: Received 22 December 2009 In final form 19 March 2010 Available online 24 March 2010 Keywords: Aqueous halides Solvation shell X-ray absorption spectroscopy EXAFS Molecular dynamics DFT QM/MM abstract The L 3 X-ray absorption spectrum of aqueous iodide is reported, and its EXAFS is compared to theoretical spectra reconstructed from the radial distribution function of the iodide hydration obtained from classi- cal, hybrid Quantum Mechanics Molecular Mechanics, (QM/MM) and full quantum (density functional theory, DFT) molecular dynamics simulations. Since EXAFS is mainly sensitive to short distances around the iodide ion, it is a direct probe of the local solvation structure. The comparison shows that QM/MM simulations deliver a satisfactory description of the EXAFS signal, while nonpolarizable classical simula- tions are somewhat less satisfactory and DFT-based simulations perform poorly. We also identify a weak anisotropy of the water solvation shell around iodide, which may be of importance in electron photoejec- tion experiments. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The properties of solvated ions and the role of the surrounding water molecules are important in a large variety of chemical reac- tions and biochemical processes [1,2]. They are also key for trans- port of ionic solutes in water [3], and ionic hydration dynamics play a central role in several physiological processes such as ion transport through membranes, where the hydration shell reorga- nizes in the initial and final stages of the membrane-crossing mechanism [4]. Despite the importance of this subject, the struc- ture of the solvation shell and its dynamical behaviour are still in- tensely debated. Various experimental and theoretical studies have been carried out on halides in water aimed at obtaining a complete description of the solvent shell structure and dynamics. X-ray and neutron dif- fraction studies yield coordination numbers that are significantly scattered, varying by almost a factor of two for nearly all halides [5,6]. In the case of iodide, which is the system of interest here, the first peak of the I–O radial distribution function (RdF) was found to be in the range of 3.55–3.76 Å, and the coordination num- bers with oxygen atoms varied from 4 to 9. This spread of values underlines the difficulty of defining a solvation shell due to its diffuse character. To determine more precisely the solvent shell structure, hard X-ray absorption spectroscopy on the halides seems more appropriate, since by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) one probes the local structure around the atom of interest. Tanida et al. [7] recorded L 1 - and L 3 -edge XANES spectra of aqueous I  and compared them to simulated spectra assuming model geometries of clusters of water molecules around the ion. The spectra were satisfactorily described by taking into account the first hydration shell only. A somewhat similar approach was adopted by Merkling et al. [8] for both the XANES and the EXAFS of the bromide ion. They derived optimized geometries of [Br(H 2 O) n ]  (1 6 n 6 8) from quantum chemical calculations, and in order to reproduce the XANES spectra in a satisfactory way, they introduced statistical fluctuations, which in their case were ob- tained from snapshots of Monte Carlo (MC) simulations. The latter improvement underlines the complexity of the problem, which is already implicit from the above mentioned spread of coordination numbers and distances. In limiting their analysis to the first shell and to optimized geometries, the simulations of Tanida et al. [7] and Merkling et al. [8] are not sufficient to capture the dynamics of hydration in a bulk liquid and in particular the exchange with the molecules of the bulk. D’Angelo et al. [9] carried out a K-edge EXAFS study of bromide ions in aqueous solutions and combined them with classical molecular dynamics (CMD) simulations. They obtained a Br  –O RdF, which they used with the integral form of the EXAFS equation to simulate an EXAFS spectrum in good agreement with the experiment. The integral form of the EXAFS 0301-0104/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2010.03.023 * Corresponding author. Tel.: +41 21 693 0457x0447; fax: +41 21 693 0422. E-mail address: Majed.Chergui@epfl.ch (M. Chergui). Chemical Physics 371 (2010) 24–29 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys