Solvent Dynamics of Aqueous Halides before and after Photoionization

Abstract

Electron transfer reactions can be strongly influenced by solvent dynamics. We study the photoionization of halides in water as a model system for such reactions. There are no internal nuclear degrees of freedom in the solute, allowing the dynamics of the solvent to be uniquely identified. We simulate the equilibrium solvent dynamics for Cl–, Br–, I–, and their respective neutral atoms in water, comparing quantum mechanical/molecular mechanical (QM/MM) and classical molecular dynamics (MD) methods. On the basis of the obtained configurations, we calculate the extended X-ray absorption fine structure (EXAFS) spectra rigorously based on the MD snapshots and compare them in detail with other theoretical and experimental results available in the literature. We find our EXAFS spectra based on QM/MM MD simulations in good agreement with their experimental counterparts for the ions. Classical MD simulations for the ions lead to EXAFS spectra that agree equally well with the experiment when it comes to the oscillatory period of the signal, even though they differ from the QM/MM radial distribution functions extracted from the MD. The amplitude is, however, considerably overestimated. This suggests that to judge the reliability of theoretical simulation methods or to elucidate fine details of the atomistic dynamics of the solvent based on EXAFS spectra, the amplitude as well as the oscillatory period need to be considered. If simulations fail qualitatively, as does the classical MD for the aqueous neutral halogen atoms, the resulting EXAFS will also be strongly affected in both oscillatory period and amplitude. The good reliability of QM/MM-based EXAFS simulations, together with clear qualitative differences in the EXAFS spectra found between halides and their atomic counterparts, suggests that a combined theory and experimental EXAFS approach is suitable for elucidating the nonequilibrium solvent dynamics in the photoionization of halides and possibly also for electron transfer reactions in more complex systems.

Tracking Cavity Formation in Electron Solvation: Insights from X-ray Spectroscopy and Theory

“…While e aq – is rather transparent to X-rays, using computations of the K-edge X-ray absorption spectra (XAS), Li et al recently showed that X-rays could be used to probe the presence of the cavity around e aq – . They suggested using time-resolved X-ray absorption spectroscopy (trXAS) to track changes in the solvation structure of e aq – exploiting the high structural sensitivity of XAS. , X-rays have already demonstrated their exceptional suitability for uncovering local transient structures in liquid water. , But so far, experimental applications of X-ray spectroscopy to the solvated electron and its solvation shell remain absent. In this article, we report the most direct evidence of cavity formation with the help of trXAS.…”

Water Solvent Reorganization upon Ultrafast Resonant Stimulated X-ray Raman Excitation of a Metalloporphyrin Dimer

“…28−32 Changes in solvation shells and liquid structure following an optical actinic pulse 33−42 have been tracked and analyzed using molecular dynamics. 43,44 The ultrafast X-ray probe provides an adequate temporal resolution of the structural dynamics. 31,45−48 In this study, we combine local SXRS excitation on a solute molecule with an XRD probe of the structural solvent response.…”

“…This wavepacket can involve a large number of valence excited states covered by the large bandwidth of X-ray pulses. In another development, it is now possible to obtain diffraction images of noncrystalline molecular samples in the gas and liquid phases by time-resolved X-ray diffraction (XRD). − Changes in solvation shells and liquid structure following an optical actinic pulse − have been tracked and analyzed using molecular dynamics. , The ultrafast X-ray probe provides an adequate temporal resolution of the structural dynamics. ,− …”

Going beyond Radial Hydration Models: The Hidden Structures of Chloride and Iodide Aqua Ions Revealed by the Use of Lone Pairs

“…Therefore, many investigations have been dedicated to study aqueous solutions of halide ions and to provide a molecular level understanding of halide–water interactions . A plethora of different techniques have been adopted to characterize the hydration properties of halides, including classical molecular dynamics (MD) − and Monte Carlo simulations, QM/MM simulations, − ab initio simulations, ,,, Raman and IR spectroscopies, X-ray absorption spectroscopy, ,,, and neutron and X-ray diffraction . Nevertheless, the picture of halide solvation shell structure emerging from such works is inhomogeneous, and first shell coordination numbers and distances reported in the literature are very scattered .…”