Mixed Quantum/Classical Molecular Dynamics Simulations of the Hydrated Dielectron: The Role of Exchange in Condensed-Phase Structure, Dynamics, and Spectroscopy

Molecular Dynamics Characterization of Dielectron Hydration in Liquid Water with Unique Double Proton Transfers

Radiation chemistry of water and aqueous solutions has always been an interesting scientific issue owing to involving electronic excitations, ionization of solvated species, and formation of radiolytic species and many elementary reactions, but the underlying mechanisms are still poorly understood. Here, we for the first time molecular dynamics characterize the hydration dynamics of two correlated electrons and their triggered unique phenomena in liquid water associated with radiolysis of water using the combined hybrid functional and nonlocal dispersion functional. Hydration of two electrons may experience two distinctly different mechanisms, one forming a spin-paired closed-shell unicaged dielectron hydrate (e₂^2-_aq) and the other forming a spin-paired metastable open-shell bicaged hydrated electron pair (e^-_aq···e^-_aq) which exhibits intriguing antiferromagnetic spin coupling dynamics (in a range of -40 cm^-1 to -500 cm^-1). e^-_aq···e^-_aq can recombine to e₂^2-_aq through a unique solvent fluctuation-controlled gradual-flowing mechanism, and enlarging fluctuation can promote the conversion. Interestingly, we directly observe that e₂^2-_aq as the precursor can trigger hydrogen evolution via unique continuous spontaneous double proton transfer to the dielectron with a short-lived H^-_aq intermediate, but e^-_aq···e^-_aq does not directly. This is the first direct observation for the connection between e₂^2-_aq and spontaneous hydrogen evolution including participation of H^-_aq in aqueous solution, bridging relevant experimental phenomena. This work also evidences an unnoticed process, the double proton transfer mediated charge separation, and presents the first detailed analysis regarding the evolution dynamics of e₂^2-_aq for the understanding of the radiolysis reactions in aqueous solutions.

Excess dielectron in an ionic liquid as a dynamic bipolaron

We report an ab initio molecular dynamics simulation study on the accommodation of a dielectron in a pyridinium ionic liquid in both the singlet and triplet state. In contrast to water and liquid ammonia, a dielectron does not prefer to reside in cavity-shaped structures in the ionic liquid. Instead, it prefers to be distributed over more cations, with long-lived diffuse and short-lived localized distributions, and with a triplet ground state and a low-lying, open-shell singlet excited state. The two electrons evolve nonsynchronously in both states via a diffuse-versus-localized interconversion mechanism that features a dynamic bipolaron with a modest mobility, slightly lower than a hydrated electron. This work presents the first detailed study on the structures and dynamics of a dielectron in ionic liquids.

Probing the interactions of the solvated electron with DNA by molecular dynamics simulations: II. bromodeoxyuridine-thymidine mismatched DNA

The interaction of solvated electrons (e(-)(aq)) with DNA results in various types of DNA lesions. The in vitro and in vivo sensitisation of DNA to (e(-)(aq))-induced damage is achieved by incorporation of the electron-affinity radiosensitiser bromodeoxyuridine (BUdR) in place of thymidine. However, in DNA duplexes containing single-stranded regions (bulged BUdR-DNA), the type of lesion is different and the efficiency of damage is enhanced. In particular, DNA interstrand crosslinks (ICL) form at high efficiency in bulged DNA but are not detectable in completely duplex DNA. Knowledge about the processes and interactions leading to these differences is obscure. Previously, we addressed the problem by applying molecular modelling and molecular dynamics (MD) simulations to a system of normal (BUdR.A)-DNA and a hydrated electron, where the excess electron was modelled as a localised e(-)(H2O6) anionic cluster. The goal of the present study was to apply the same MD simulation to a wobble DNA-e(-)(aq) system, containing a pyrimidine-pyrimidine mismatched base pair, BUdR.T. The results show an overall dynamic pattern similar to that of the e(-)(aq) motion around normal DNA. However, the number of configuration states when e(-)(aq)) was particularly close to DNA is different. Moreover, in the (BUdR.T)-wobble DNA system, the electron frequently approaches the brominated strand, including BUdR, which was not observed with the normal (BUdR.A)-DNA. The structure and exchange of water at the sites of e(-)(aq) immobilisation near DNA were also characterised. The structural dynamics of the wobble DNA is prone to more extensive perturbations, including frequent formation of cross-strand (cs) interatomic contacts. The structural deviations correlated with e(-)(aq) approaching DNA from the major groove side, with sodium ions trapped deep in the minor groove. Altogether, the obtained results confirm and/or throw light on dynamic-structure determinants possibly responsible for the enhanced radiation damage of wobble DNA.

Nonadiabatic Molecular Dynamics Simulations of Correlated Electrons in Solution. 1. Full Configuration Interaction (CI) Excited-State Relaxation Dynamics of Hydrated Dielectrons

The hydrated dielectron is composed of two excess electrons dissolved in liquid water that occupy a single cavity; in both its singlet and triplet spin states there is a significant exchange interaction so the two electrons cannot be considered to be independent. In this paper and the following paper,we present the results of mixed quantum/classical molecular dynamics simulations of the nonadiabatic relaxation dynamics of photoexcited hydrated dielectrons, where we use full configuration interaction (CI) to solve for the two-electron wave function at every simulation time step. To the best of our knowledge, this represents the first systematic treatment of excited-state solvation dynamics where the multiple-electron problem is solved exactly. The simulations show that the effects of exchange and correlation contribute significantly to the relaxation dynamics. For example, spin-singlet dielectrons relax to the ground state on a time scale similar to that of single electrons excited at the same energy, but spin-triplet dielectrons relax much faster. The difference in relaxation dynamics is caused by exchange and correlation: The Pauli exclusion principle imposes very different electronic structure when the electrons' spins are singlet paired than when they are triplet paired, altering the available nonadiabatic relaxation pathways. In addition, we monitor how electronic correlation changes dynamically during nonadiabatic relaxation and show that solvent dynamics cause electron correlation to evolve quite differently for singlet and triplet dielectrons. Despite such differences, our calculations show that both spin states are stable to excited-state dissociation, but that the excited-state stability has different origins for the two spin states. For singlet dielectrons, the stability depends on whether the solvent structure can rearrange to create a second cavity before the ground state is reached. For triplet dielectrons, in contrast, electronic correlation ensures that the two electrons do not dissociate, even if the dielectron is artificially kept from reaching the ground state. In addition, both singlet and triplet dielectrons change shape dramatically during relaxation, so that linear response fails to describe the solvation dynamics for either spin state. In the following paper (Larsen, R. E.; Schwartz, B. J. J. Phys. Chem. B 2006, 110, 9692), we use these simulations to calculate the pump-probe spectroscopic signal expected for photoexcited hydrated dielectrons and to predict an experiment to observe hydrated dielectrons directly.

Nonadiabatic Molecular Dynamics Simulations of Correlated Electrons in Solution. 2. A Prediction for the Observation of Hydrated Dielectrons with Pump−Probe Spectroscopy

The hydrated dielectron is a highly correlated, two-electron, solvent-supported state consisting of two spin-paired electrons confined to a single cavity in liquid water. Although dielectrons have been predicted to exist theoretically and have been used to explain the lack of ionic strength effect in the bimolecular reaction kinetics of hydrated electrons, they have not yet been observed directly. In this paper, we use the extensive nonadiabatic mixed quantum/classical excited-state molecular dynamics simulations from the previous paper to calculate the transient spectroscopy of hydrated dielectrons. Because our simulations use full configuration interaction (CI) to determine the ground and excited state two-electron wave functions at every instant, our nonequilibrium simulations allow us to compute the absorption, stimulated emission (SE), and bleach spectroscopic signals of both singlet and triplet dielectrons following excitation by ultraviolet light. Excited singlet dielectrons are predicted to display strong SE in the mid infrared and a transient absorption in the near-infrared. The near-infrared transient absorption of the singlet dielectron, which occurs near the peak of the (single) hydrated electron's equilibrium absorption, arises because the two electrons tend to separate in the excited state. In contrast, excitation of the hydrated electron gives a bleach signal in this wavelength region. Thus, our calculations suggest a clear pump-probe spectroscopic signature that may be used in the laboratory to distinguish hydrated singlet dielectrons from hydrated electrons: By choosing an excitation energy that is to the blue of the peak of the hydrated electron's absorption spectrum and probing near the maximum of the single electron's absorption, the single electron's transient bleach signal should shrink or even turn into a net absorption as sample conditions are varied to produce more dielectrons.

Full Configuration Interaction Computer Simulation Study of the Thermodynamic and Kinetic Stability of Hydrated Dielectrons

The hydrated electron is a unique solvent-supported state comprised of an excess electron that is confined to a cavity by the surrounding water. Theoretical studies have suggested that two-electron solvent-supported states also can be formed; in particular, simulations indicate that two excess electrons could pair up and occupy a single cavity, forming a so-called hydrated dielectron. Although hydrated dielectrons have not been observed directly by experiment, their existence has been posited to explain the lack of an ionic strength effect in hydrated electron bimolecular annihilation [Schmidt, K. H.; Bartels, D. M. Chem. Phys. 1995, 190, 145]. To determine whether dielectrons may be created in the laboratory, we use thermodynamic integration (TI), combined with mixed quantum/classical molecular dynamics simulation, to examine the thermodynamic stability of hydrated electrons and dielectrons. For the dielectron calculations, we solve the two-electron quantum problem using full configuration interaction. Our results suggest that hydrated dielectrons are thermodynamically unstable relative to separated (single) hydrated electrons, although we also show that increasing the pressure could drive the equilibrium toward the formation of dielectrons. Because the simulations suggest that hydrated dielectrons are kinetically stable, we also examine a scenario for creating metstable, nonequilibrium populations of dielectrons, which involves the capture of a newly injected electron by a preexisting, equilibrated hydrated electron. These calculations, which allow for the full nonadiabatic relaxation of the injected electron, show that hydrated electrons may indeed act as trapping sites for unequilibrated electrons, so that capture may be a viable mechanism for creating dielectrons. We suggest possible experimental procedures to create such nonequilibrium hydrated dielectrons using either pulse radiolysis or ultrafast spectroscopic techniques.

The role of solvent structure in the absorption spectrum of solvated electrons: Mixed quantum/classical simulations in tetrahydrofuran

In polar fluids such as water and methanol, the peak of the solvated electron's absorption spectrum in the red has been assigned as a sum of transitions between an s-like ground state and three nearly degenerate p-like excited states bound in a quasispherical cavity. In contrast, in weakly polar solvents such as tetrahydrofuran (THF), the solvated electron has an absorption spectrum that peaks in the mid-infrared, but no definitive assignment has been offered about the origins of the spectrum or the underlying structure. In this paper, we present the results of adiabatic mixed quantum/classical molecular dynamic simulations of the solvated electron in THF, and provide a detailed explanation of the THF-solvated electron's absorption spectrum and electronic structure. Using a classical solvent model and a fully quantum mechanical excess electron, our simulations show that although the ground and first excited states are bound in a quasispherical cavity, a multitude of other, nearby solvent cavities support numerous, nearly degenerate, bound excited states that have little Franck-Condon overlap with the ground state. We show that these solvent cavities, which are partially polarized so that they act as electron trapping sites, are an inherent property of the way THF molecules pack in the liquid. The absorption spectrum is thus assigned to a sum of bound-to-bound transitions between a localized ground state and multiple disjoint excited states scattered throughout the fluid. Furthermore, we find that the usual spherical harmonic labels (e.g., s-like, p-like) are not good descriptors of the excited-state wave functions of the solvated electron in THF. Our observation of multiple disjoint excited states is consistent with femtosecond pump-probe experiments in the literature that suggest that photoexcitation of solvated electrons in THF causes them to relocalize into solvent cavities far from where they originated.