Trap-Seeking or Trap-Digging? Photoinjection of Hydrated Electrons into Aqueous NaCl Solutions

How to Probe Hydrated Dielectrons Experimentally: Ab Initio Simulations of the Absorption Spectra of Aqueous Dielectrons, Electron Pairs, and Hydride

In the radiation chemistry of water, two hydrated electrons (ehyd-) can react to form H2 and OH-. Experiments and simulations suggest that this reaction occurs through a mechanism involving colocalization of two ehyd-'s into the same solvent cavity, forming a hydrated dielectron ( ( e h y d ) 2 2 - ) intermediate, with aqueous hydride (H-) as a subintermediate. However, there has been no direct experimental observation of either ( e h y d ) 2 2 - or H-. Here, we present TD-DFT-based predictions for the absorption spectrum of open-shell-singlet and triplet ehyd- pairs, ( e h y d ) 2 2 - , and H-. We find that relative to ehyd-, triplet and open-shell singlet electron pairs show spectral shifts to the blue and red, respectively. Additionally, we find that ( e h y d ) 2 2 - absorbs even further to the red, and that H- has a charge-transfer-to-solvent-like transition at wavelengths several eV to the blue, providing a direct experimental handle with which to probe these species. We propose a three-pulse transient absorption experiment that should allow detection of ( e h y d ) 2 2 - and H-.

Simulating the Competitive Ion Pairing of Hydrated Electrons with Chaotropic Cations

Experiments show that the absorption spectrum of the hydrated electron (ehyd-) blue-shifts in electrolyte solutions compared with what is seen in pure water. This shift has been assigned to the ehyd-'s competitive ion-pairing interactions with the salt cation relative to the salt anion based on the ions' positions on the Hofmeister series. Remarkably, little work has been done investigating the ehyd-'s behavior when the salts have chaotropic cations, which should greatly change the ion-pairing interactions given that the ehyd- is a champion chaotrope. In this work, we remedy this by using mixed quantum/classical simulations to analyze the behavior of two different models of the ehyd- in aqueous RbF and RbI electrolyte solutions as a function of salt concentration. We find that the magnitude of the salt-induced spectral blue-shift is determined by a combination of the number of chaotropic Rb+ cations near the ehyd- and the number of salt anions near those cations so that the spectrum of the ehyd- directly reflects its local environment. We also find that the use of a soft-cavity ehyd- model predicts stronger competitive interactions with Rb+ relative to I- than a more traditional hard cavity model, leading to different predicted spectral shifts that should provide a way to distinguish between the two models experimentally. Our simulations predict that at the same concentration, salts with chaotropic cations should produce larger spectral blue-shifts than salts with kosmotropic cations. We also found that at high salt concentrations with chaotropic cations, the predicted blue-shift is greater when the salt anion is kosmotropic instead of chaotropic. Our goal is for this work to inspire experimentalists to make such measurements, which will help provide a spectroscopic means to distinguish between simulations models that predict different hydration structures for the ehyd-.

Roles of H-Bonding and Hydride Solvation in the Reaction of Hydrated (Di)electrons with Water to Create H2 and OH. J Chem Theory Comput 2024;20. [PMID: 39110603 PMCID: PMC11360129 DOI: 10.1021/acs.jctc.4c00780] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Even though single hydrated electrons (ehyd-'s) are stable in liquid water, two hydrated electrons can bimolecularly react with water to create H2 and hydroxide: ehyd- + ehyd- + 2H2O → H2 + 2OH-. The rate of this reaction has an unusual temperature and isotope dependence as well as no dependence on ionic strength, which suggests that cosolvation of two electrons as a single hydrated dielectron (e2,hyd2-) might be an important intermediate in the mechanism of this reaction. Here, we present an ab initio density functional theory study of this reaction to better understand the potential properties, reactivity, and experimental accessibility of hydrated dielectrons. Our simulations create hydrated dielectrons by first simulating single ehyd-'s and then injecting a second electron, providing a well-defined time zero for e2,hyd2- formation and offering insight into a potential experimental route to creating dielectrons and optically inducing the reaction. We find that e2,hyd2- immediately forms in every member of our ensemble of trajectories, allowing us to study the molecular mechanism of H2 and OH- formation. The subsequent reaction involves separate proton transfer steps with a generally well-defined hydride subintermediate. The time scales for both proton transfer steps are quite broad, with the first proton transfer step spanning times over a few ps, while the second proton transfer step varies over ∼150 fs. We find that the first proton transfer rate is dictated by whether or not the reacting water is part of an H-bond chain that allows the newly created OH- to rapidly move by Grotthuss-type proton hopping to minimize electrostatic repulsion with H-. The second proton transfer step depends significantly on the degree of solvation of H-, leading to a wide range of reactive geometries where the two waters involved can lie either across the dielectron cavity or more adjacent to each other. This also allows the two proton transfer events to take place either effectively concertedly or sequentially, explaining differing views that have been presented in the literature.

Dynamics of the charge transfer to solvent process in aqueous iodide

Charge-transfer-to-solvent states in aqueous halides are ideal systems for studying the electron-transfer dynamics to the solvent involving a complex interplay between electronic excitation and solvent polarization. Despite extensive experimental investigations, a full picture of the charge-transfer-to-solvent dynamics has remained elusive. Here, we visualise the intricate interplay between the dynamics of the electron and the solvent polarization occurring in this process. Through the combined use of ab initio molecular dynamics and machine learning methods, we investigate the structure, dynamics and free energy as the excited electron evolves through the charge-transfer-to-solvent process, which we characterize as a sequence of states denoted charge-transfer-to-solvent, contact-pair, solvent-separated, and hydrated electron states, depending on the distance between the iodine and the excited electron. Our assignment of the charge-transfer-to-solvent states is supported by the good agreement between calculated and measured vertical binding energies. Our results reveal the charge transfer process in terms of the underlying atomic processes and mechanisms.

Partial Molar Solvation Volume of the Hydrated Electron Simulated Via DFT

Different simulation models of the hydrated electron produce different solvation structures, but it has been challenging to determine which simulated solvation structure, if any, is the most comparable to experiment. In a recent work, Neupane et al. [J. Phys. Chem. B 2023, 127, 5941-5947] showed using Kirkwood-Buff theory that the partial molar volume of the hydrated electron, which is known experimentally, can be readily computed from an integral over the simulated electron-water radial distribution function. This provides a sensitive way to directly compare the hydration structure of different simulation models of the hydrated electron with experiment. Here, we compute the partial molar volume of an ab-initio-simulated hydrated electron model based on density-functional theory (DFT) with a hybrid functional at different simulated system sizes. We find that the partial molar volume of the DFT-simulated hydrated electron is not converged with respect to the system size for simulations with up to 128 waters. We show that even at the largest simulation sizes, the partial molar volume of DFT-simulated hydrated electrons is underestimated by a factor of 2 with respect to experiment, and at the standard 64-water size commonly used in the literature, DFT-based simulations underestimate the experimental solvation volume by a factor of ∼3.5. An extrapolation to larger box sizes does predict the experimental partial molar volume correctly; however, larger system sizes than those explored here are currently intractable without the use of machine-learned potentials. These results bring into question what aspects of the predicted hydrated electron radial distribution function, as calculated by DFT-based simulations with the PBEh-D3 functional, deviate from the true solvation structure.

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

We present time-resolved X-ray absorption spectra of ionized liquid water and demonstrate that OH radicals, H3O+ ions, and solvated electrons all leave distinct X-ray-spectroscopic signatures. Particularly, this allows us to characterize the electron solvation process through a tool that focuses on the electronic response of oxygen atoms in the immediate vicinity of a solvated electron. Our experimental results, supported by ab initio calculations, confirm the formation of a cavity in which the solvated electron is trapped. We show that the solvation dynamics are governed by the magnitude of the random structural fluctuations present in water. As a consequence, the solvation time is highly sensitive to temperature and to the specific way the electron is injected into water.

How Ions Break Local Symmetry: Simulations of Polarized Transient Hole Burning for Different Models of the Hydrated Electron in Contact Pairs with Na. J Phys Chem Lett 2023;14:3014-3022. [PMID: 36943261 DOI: 10.1021/acs.jpclett.3c00220]

The hydrated electron (e_aq^-) is known via polarized transient hole-burning (pTHB) experiments to have a homogeneously broadened absorption spectrum. Here, we explore via quantum simulation how the pTHB spectroscopy of different e_aq^- models changes in the presence of electrolytes. The idea is that cation-e_aq^- pairing can break the local symmetry and, thus, induce persistent inhomogeneity. We find that a "hard" cavity model shows a modest increase in the pTHB recovery time in the presence of salt, while a "soft" cavity model remains homogeneously broadened independent of the salt concentration. We also explore the orientational anisotropy of a fully ab initio density functional theory-based model of the e_aq^-, which is strongly inhomogeneously broadened without salt and which becomes significantly more inhomogeneously broadened in the presence of salt. The results provide a direct prediction for experiments that can distinguish between different models and, thus, help pin down the hydration structure and dynamics of the e_aq^-.

Ab Initio Studies of Hydrated Electron/Cation Contact Pairs: Hydrated Electrons Simulated with Density Functional Theory Are Too Kosmotropic

We have performed the first DFT-based ab initio MD simulations of a hydrated electron (e_aq^-) in the presence of Na⁺, a system chosen because ion-pairing behavior in water depends sensitively on the local hydration structure. Experiments show that e_aq^-'s interact weakly with Na⁺; the e_aq^-'s spectrum blue shifts by only a few tens of meV upon ion pairing without changing shape. We find that the spectrum of the DFT-simulated e_aq^- red shifts and changes shape upon interaction with Na⁺, in contrast with experiment. We show that this is because the hydration structure of the DFT-simulated e_aq^- is too ordered or kosmotropic. Conversely, simulations that produce e_aq^-'s with a less ordered or chaotropic hydration structure form weaker ion pairs with Na⁺, yielding predicted spectral blue shifts in better agreement with experiment. Thus, ab initio simulations based on hybrid GGA DFT functionals fail to produce the correct solvation structure for the hydrated electron.