Solvation structure of the halides from x-ray absorption spectroscopy

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Understanding aqueous solvation structures is crucial for chemical processes.
• Previous models lacked atomic resolution for larger halide ions like chloride, bromide, and iodide.

Purpose of the Study:

• To determine the three-dimensional aqueous solvation structures of chloride, bromide, and iodide ions at atomic resolution.
• To elucidate the distinct solvation behaviors of these halide ions in water.

Main Methods:

• K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near edge (MXAN) analyses.
• Time-dependent density functional theory (TD-DFT) calculations.
• Classical molecular dynamics simulations.

Main Results:

• Bromide and iodide ions exhibit well-defined single solvation shells with 8 water molecules.
• Chloride ions display a unique two-shell solvation structure: 7 water molecules in the first shell and 7 in the second.
• Experimental findings align with molecular dynamics simulations for chloride solvation.

Conclusions:

• The study provides the first atomic-resolution, three-dimensional structures of aqueous solvation spheres for larger halide ions.
• Chloride's distinct two-shell solvation is confirmed, differing significantly from bromide and iodide.
• Electronic structure is sensitive to coordination number, highlighting the precision of the MXAN fitting.