Dynamics Of Solvated Electrons In Liquid Medium Revealed By Theoretical Simulation Studies

The solvation dynamics and reactivity of excess electrons in bulk liquid medium have attracted great attention in many areas of physics, chemistry, and biology. Excess electrons in the liquid environment are localized and stabilized by the local rearrangement of the surrounding solvent molecules. Such solvated electrons are known to play an important role in systems such as biochemical reactions and atmospheric chemistry. Despite numerous studies over many years, little is known about the microscopic details of these electron-induced chemical processes, and interest in the fundamental processes involved in the reactivity of trapped electrons continues. In this work, we discuss our investigations of electrons solvated in various medium by means of ab initio molecular dynamics simulations. This approach allows us to characterize structural, dynamical, and reactive aspects of the solvated electron using all of the system’s valence electrons. The primary innovations are related as follows.1) Solvated electrons play very important role in many important processes occurring in complicated aqueous media, and thus realization and accurate characterization of their structures, states, and dynamics in solutions have still been of great interest. Using ab initio molecular dynamics simulations, we show that an excess electron can be more efficiently localized as a cavity-shaped state in aqueous glucose solution (AGS) than in water. In particular, compared with that (～1.5ps) in water, the localization time is considerably shortened by-0.7-1.2ps in three different AGSs (0.56,1.12, and2.87M). Although the radii of gyration of solvated electrons are almost equivalent to each other (2.6A) in four solutions, the formed solvated electron cavity in AGS becomes compact and can localize～80%of an excess electron which is considerably larger than that (～40-60%and occasionally～80%) in water. These observations are attributed to modification of the hydrogen-bonded network by introducing glucoses into water as promoters and stabilizers which allows the formation of voids around glucoses and thus favors the localization of excess electrons with high efficiency. This study provides important information about excess electrons in the physiological glucose medium and also proposes a new strategy to efficiently localize an excess electron in a stable cavity for further exploration of biological functions and processes.2) Details of excess electron (EE) localization in normal and glassy nucleobase solutions are unraveled using ab initio molecular dynamics simulations. The EE relaxations exhibit distinctly different dynamics, featuring different localization times towards the bases (direct transfer/Taq,160fs/Caq,600fs/Aaq, or900fs/Gaq) in normal solutions and are retarded in glassy ones. Localization leads to the formation of hydrated valence anions (T-aq, C-aq) or valence quasi-anions (A δ-aq, Gδ-aq) with different degrees and efficiencies of localization. Localization is driven by the electron-binding ability of the nucleobases and their dynamics are controlled by thermal fluctuations of the solutions. These results present new insights into the dynamics of electron localization towards nucleobases in solutions and the antiradiation mechanism of biological species at glassy temperatures.3) Using ab initio molecular dynamics simulations, we study the dehalogenation reaction of radiosensitizing drugs5-halo-deoxyuridines (XdUs) caused by excess electrons (EEs) in water solutions. The prehydrated EEs attach to neutral XdUs in a short time scale (-30fs), leading to the formation of unrelaxed XdU-anions. These anions exhibits distinctly different features during the relaxation: the FdU-and CldU-anions do not dissociate, while the BrdU-or IdU-anion dissociate into (Xδ-...dUδ-) rather than X-plus dU·. Free energy barriers for the cleavage of X-C5bonds suggest that IdU is the best-sensitizer among four XdUs. Our results present new insights of actions of XdUs in radiotherapy (XdU-â†'(Xδ-...dUδ-)â†'X-+dU·), which is helpful to understand the mechanism of their enhanced DNA damage and cell death during ionizing/ultraviolet radiation.4) We report an ab initio molecular dynamics simulation study of the solvation and dynamics of an excess electron in liquid acetonitrile (ACN). Four families of states are observed: a diffusely solvated state and three CAN core-localized states with monomer core, quasi-dimer (Ï€*-Rydberg mode) core, and dual-core/dimer core (a coupled dual-core). These core localized states cannot be simply described as the corresponding anions because only a part of the excess electron resides in the core molecule(s). The quasi-dimer core state actually is a mixture that features cooperative excess electron capture by the Ï€*and Rydberg orbitals of two ACNs. Well-defined dimer anion and solvated electron cavity were not observed in the5-10ps simulations, which may be attributed to slow dynamics of the formation of the dimer anion and difficulty of the formation of a cavity in such a fluxional medium. All of the above observed states have near-IR absorptions and thus can be regarded as the solvated electron states but with different structures, which can interpret the experimentally observed IR band. These states undergo continuous conversions via a combination of long-lasting breathing oscillation and core switching, characterized by highly cooperative oscillations of the electron cloud volume and vertical detachment energy. The quasi-dimer core and diffusely solvated states dominate the time evolution, with the monomer core and dual-core/dimer core states occurring occasionally during the breathing and core switching processes, respectively. All these oscillations and core switchings are governed by a combination of the electron-impacted bending vibration of the core CAN molecule(s) and thermal fluctuations.5) 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.