Solvent-dependent molecular structure of ionic species directly measured by ultrafast x-ray solution scattering
Kyung Hwan Kim, Jae Hyuk Lee, Joonghan Kim, Shunsuke Nozawa, Tokushi Sato, Ayana Tomita, Kouhei Ichiyanagi, Hosung Ki, Jeongho Kim, Shin-ichi Adachi, and Hyotcherl Ihee
Accepted
Ionic species often plays important roles in chemical reactions occurring in water and other solvents, but it has been elusive to determine its solvent-dependent molecular structure with atomic resolution. 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 sub-angstrom 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 DFT calculation using 34 explicit water molecules, confirming that the origin of the symmetry breaking is the hydrogen-bonding interaction between the solute and solvent molecules.