Solvation structure of the halides from x-ray absorption spectroscopy

Matthew Antalek,Ritimukta Sarangi,Britt Hedman,Giovanni Chillemi,Elisabetta Pace,Maurizio Benfatto,Patrick Frank,Keith O Hodgson

doi:10.1063/1.4959589

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.

Highlights

The solvation structure of the halides, especially chloride, bromide, and iodide, remains under intense investigation, as recent reviews indicate.1–4 Ion solvation is an important subject of itself,5 but solvation structure impacts, e.g., the dynamics of aqua-ion migration and electrode kinetics.6 Halide ion solvation governs the Hofmeister series defining protein salting-in and salting-out,7–10 while the immediate structure of dissolved halide influences the extended structure of water.11–13In all cases, solvating water molecules were found oriented so as to each produce a single HO–H· · ·X− hydrogen bond
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
Timedependent density functional theory calculations simulating the chloride X-ray absorption near edge structure (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

Summary

Introduction

The solvation structure of the halides, especially chloride, bromide, and iodide, remains under intense investigation, as recent reviews indicate. Ion solvation is an important subject of itself, but solvation structure impacts, e.g., the dynamics of aqua-ion migration and electrode kinetics. Halide ion solvation governs the Hofmeister series defining protein salting-in and salting-out, while the immediate structure of dissolved halide influences the extended structure of water.11–13In all cases, solvating water molecules were found oriented so as to each produce a single HO–H· · ·X− hydrogen bond. The solvation structure of the halides, especially chloride, bromide, and iodide, remains under intense investigation, as recent reviews indicate.. Including anomalous X-ray diffraction (AXD) and extended X-ray absorption fine structure (EXAFS) analyses have generally confirmed the earlier results.. Including anomalous X-ray diffraction (AXD) and extended X-ray absorption fine structure (EXAFS) analyses have generally confirmed the earlier results.2,4,16–28 These room temperature studies have a large error in the first shell determination. For example at room temperature, the EXAFS structural approach, extremely powerful, has an estimation accuracy of ±25% in coordination number (CN) and ±0.03 Å in bond length, indicating that the consensus halides solvation structure derived from EXAFS is CN = 6 ± 2

Methods

Results

Conclusion