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 I(3)(-) ion in water, methanol, and acetonitrile with subangstrom accuracy. The data reveal that 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 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.

Solvent‐Dependent Structure of the I3− Ion Derived from Photoelectron Spectroscopy and Ab Initio Molecular Dynamics Simulations

“…The charge distribution analysis in Figure 2 was carried out by sampling 1000 snapshots from the trajectory for which we also performed I 4 d XPS analysis, as discussed in the next paragraph. We have also calculated the averaged bond lengths extracted from the entire trajectory and these are presented in Table 1, in which we have also mentioned the corresponding values obtained by Kim et al [14] It is evident that our values match well with the experimental values. Another indicator of the distortion of the I 3 À ion is the I-I-I angle.…”

“…Within the framework of DFT, we relate the core-level shifts with the structural and electronic properties of the I 3 À ion in different solvents, namely, water, acetonitrile, methanol, and ethanol. Additionally, our results are in good agreement with the direct experimental structural information reported by Kim et al [14] Thus, our approach is an alternative and useful methodology for establishing the relationship between solvent-induced structural and electronic changes in the I 3 À ion, which has been at the center of attention for decades, as outlined above.…”

“…[8] This important topic has also attracted theoretical approaches based on Monte Carlo or molecular dynamics (MD) simulations to complement experimental observations. [11][12][13] It is worthwhile mentioning that, in spite of all these studies, the direct experimental observation of solvent-induced symmetry breaking of the triiodide ion came recently from a report by Kim et al [14] Although useful information on the structures of ionic species in solution can, in principle, be obtained from X-ray scattering, ångstrçm-level precision is highly impaired by background scattering from the solvent molecules. Interestingly, in the approach of Kim et al based on time-resolved ultrafast Xray solution scattering, they were able to demonstrate, with sub-ångstrçm accuracy, the degree of symmetry breaking of the above ion in various solvents and gave conclusive evidence that the solvation effects, particularly in water, led to a highly asymmetric and bent geometry for the ion.…”

“…Interestingly, in the approach of Kim et al based on time-resolved ultrafast Xray solution scattering, they were able to demonstrate, with sub-ångstrçm accuracy, the degree of symmetry breaking of the above ion in various solvents and gave conclusive evidence that the solvation effects, particularly in water, led to a highly asymmetric and bent geometry for the ion. [14] Our unique standpoint, however, in approaching this pertinent problem has been based on the combination of core-level Xray photoelectron spectroscopy (XPS) and ab initio MD and spectrum simulations. This allows us to relate structural and electronic effects of solvation.…”

Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation

“…The I 3 − chains exhibit high structural malleability in solution 15,16 and in the solid state 17 as a result of electrostatic interactions with their surroundings. The solidstate absorption spectrum of TBAT is remarkably more complex than that in ethanol solution (see Fig.…”