title: Solvation structure of the halides from x-ray absorption spectroscopy.
abstract: Three-dimensional models for the aqueous solvation structures of chloride, bromide, and iodide are reported. K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near edge (MXAN) analyses found well-defined single shell solvation spheres for bromide and iodide. However, dissolved chloride proved structurally distinct, with two solvation shells needed to explain its strikingly different X-ray absorption near edge structure (XANES) spectrum. Final solvation models were as follows: iodide, 8 water molecules at 3.60 ± 0.13 Å and bromide, 8 water molecules at 3.40 ± 0.14 Å, while chloride solvation included 7 water molecules at 3.15 ± 0.10 Å, and a second shell of 7 water molecules at 4.14 ± 0.30 Å. Each of the three derived solvation shells is approximately uniformly disposed about the halides, with no global asymmetry. Time-dependent density functional theory calculations simulating the chloride XANES spectra following from alternative solvation spheres revealed surprising sensitivity of the electronic state to 6-, 7-, or 8-coordination, implying a strongly bounded phase space for the correct structure during an MXAN fit. MXAN analysis further showed that the asymmetric solvation predicted from molecular dynamics simulations using halide polarization can play no significant part in bulk solvation. Classical molecular dynamics used to explore chloride solvation found a 7-water solvation shell at 3.12 (-0.04/+0.3) Å, supporting the experimental result. These experiments provide the first fully three-dimensional structures presenting to atomic resolution the aqueous solvation spheres of the larger halide ions.
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, USA.
authors.forename: Matthew
authors.initials: M
authors.lastname: Antalek
authors.affiliation: Laboratori Nazionali di Frascati-INFN, P.O. Box 13, 00044 Frascati, Italy.
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Pace
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, USA.
authors.forename: Britt
authors.initials: B
authors.lastname: Hedman
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Keith O
authors.initials: KO
authors.lastname: Hodgson
authors.affiliation: CINECA, SCAI-SuperComputing Applications and Innovation Department, Via dei Tizii 6, 00185 Roma, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Chillemi
authors.affiliation: Laboratori Nazionali di Frascati-INFN, P.O. Box 13, 00044 Frascati, Italy.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Benfatto
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, USA.
authors.forename: Ritimukta
authors.initials: R
authors.lastname: Sarangi
authors.affiliation: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, USA.
authors.forename: Patrick
authors.initials: P
authors.lastname: Frank