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

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 $\mathrm{I}_3{}^{-}$ ion in water, methanol, and acetonitrile with subangstrom accuracy. The data reveal that $\mathrm{I}_3{}^{-}$ 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 $\mathrm{I}_3{}^{-}$ 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.

Figure 1

Candidate structures of $\mathrm{I}_3{}^{-}$ ion in solution.

Figure 2

Schematic of our experimental method (left) and the data analysis (right). Upon irradiation at 400 nm, $\mathrm{I}_3{}^{-}$ ion dissociates into $\mathrm{I}_2{}^{-}$ and I, and the temperature of the solution increases. By taking the difference between scattering patterns measured before and 100 ps after laser excitation, only the laser-induced changes are extracted with all other background contributions being eliminated. The structural information can be extracted by the maximum likelihood estimation using five parameters (see the Supplemental Material [30] for details). Three bond distances of $\mathrm{I}_3{}^{-}$ can be clearly identified, giving the exact structure of $\mathrm{I}_3{}^{-}$ ion in various solvents.

Figure 3

Difference scattering curves from the $\mathrm{I}_3{}^{-}$ photolysis in water and acetonitrile solutions. Experimental (black) and theoretical (red) curves using various candidate structures of $\mathrm{I}_3{}^{-}$ ion are compared. Residuals (blue) obtained by subtracting the theoretical curve from the experimental one are displayed at the bottom. (a) In water, $\mathrm{I}_3{}^{-}$ ion was found to have an asymmetric and bent structure. To emphasize the fine difference in fitting quality, the residuals shown were multiplied by a factor of 3. (b) In acetonitrile, $\mathrm{I}_3{}^{-}$ ion was found to have a symmetric and linear structure.

Figure 4

Structure reconstruction of $\mathrm{I}_3{}^{-}$ ion based on the extracted bond distances. The contribution of $\mathrm{I}_3{}^{-}$ alone (black solid line) was extracted (see the Supplemental Material [30] for details). Theoretical curves (red) were generated by a sum of three I–I distances (dashed lines). The residuals (blue solid line) are displayed at the bottom. (a) In water solution, the theoretical curve calculated from the asymmetric and bent structure gave the best fit to the experimental curve (top panel). When one average distance (3.16 Å) instead of two unequal distances was used, the broad feature in the experimental curve cannot be matched (middle panel). When a linear and asymmetric structure is used, the sum of two I–I distances (6.31 Å) do not match the $R_3$ (6.13 Å) determined from the experimental scattering curve, indicating the bent structure (bottom panel). (b) In acetonitrile solution, a symmetric and linear structure gave the best fit (top panel). If two unequal distances (3.15 Å and 2.84 Å) were used, the theoretical curve becomes broader than the experimental curve (middle panel). When a bent structure was used, the peak at 5.99 Å is shifted to a smaller value, giving a worse fit to the experimental curve (bottom). (c) The simulation using a higher DW factor is compared with the experimental curve. For clarity, $r^2\Delta \mathrm{S}(r)$ is used. If a higher DW factor with equal I–I bond lengths was used to fit the broad feature of the peak around 3.16 Å, the peak around 6.13 Å became too broad to fit the experimental curve and the entire quality of the fit became worse (blue).

Figure 5

Optimized structures and the relative energies of $\mathrm{I}_3{}^{-}$ ion with 34 explicit water molecules forming the first solvation shell by DFT method (see the Supplemental Material [30]). The optimized structure of $\mathrm{I}_3{}^{-}$ ion has a broken symmetry (asymmetric, bent) and is stabilized by $51.2\mathrm{kJ}/\mathrm{mol}$ compared with the linear symmetric one. The elongated iodine atom has a higher negative charge.