Solvent-Dependent Molecular Structure of Ionic Species Directly Measured by Ultrafast X-Ray Solution Scattering

Abstract

Ionic species often play important roles in chemical reactions occurring in water and other solvents, but it has been elusive to determine the solvent-dependent molecular structure with atomic resolution. The triiodide ion has a molecular structure that sensitively changes depending on the type of solvent and its symmetry can be broken by strong solute-solvent interaction. Here, by applying pump-probe x-ray solution scattering, we characterize the exact molecular structure of I3 ion in water, methanol, and acetonitrile with subangstrom accuracy. The data reveal that I3 ion has an asymmetric and bent structure in water. In contrast, the ion keeps its symmetry in acetonitrile, while the symmetry breaking occurs to a lesser extent in methanol than in water. The symmetry breaking of I3 ion is reproduced by density functional theory calculations using 34 explicit water molecules, confirming that the origin of the symmetry breaking is the hydrogen-bonding interaction between the solute and solvent molecules.