Hydrated Electron Production by Reaction of Hydrogen Atoms with Hydroxide Ions: A First-Principles Molecular Dynamics Study

Electrolyzed-Reduced Water: Review I. Molecular Hydrogen Is the Exclusive Agent Responsible for the Therapeutic Effects

Numerous benefits have been attributed to alkaline-electrolyzed-reduced water (ERW). Sometimes these claims are associated with easily debunked concepts. The observed benefits have been conjectured to be due to the intrinsic properties of ERW (e.g., negative oxidation-reduction potential (ORP), alkaline pH, H₂ gas), as well enigmatic characteristics (e.g., altered water structure, microclusters, free electrons, active hydrogen, mineral hydrides). The associated pseudoscientific marketing has contributed to the reluctance of mainstream science to accept ERW as having biological effects. Finally, through many in vitro and in vivo studies, each one of these propositions was examined and refuted one-by-one until it was conclusively demonstrated that H₂ was the exclusive agent responsible for both the negative ORP and the observed therapeutic effects of ERW. This article briefly apprised the history of ERW and comprehensively reviewed the sequential research demonstrating the importance of H₂. We illustrated that the effects of ERW could be readily explained by the known biological effects of H₂ and by utilizing conventional chemistry without requiring any metaphysical conjecture (e.g., microclustering, free electrons, etc.) or reliance on implausible notions (e.g., alkaline water neutralizes acidic waste). The H₂ concentration of ERW should be measured to ensure it is comparable to those used in clinical studies.

Hydrogen atom in water from ambient to high temperatures

The aqueous hydrogen atom is studied with molecular dynamics simulations from ambient temperature to near the critical point. The radial distribution functions find a hydrogen atom coordination number of about 13 water molecules at 300 K to about 4 water molecules at 646 K. The radial and angular distribution functions indicate that first-shell water molecules tend to orient to maximize hydrogen bonding interactions with other water molecules. These orientational tendencies diminish with temperature. The calculated diffusion coefficient agrees very well with experimental results known near ambient temperatures. It fits a simple activation model to about 575 K, above which the diffusion becomes much faster than predicted by the fit. To temperatures of at least 500 K there is evidence for caging on a time scale of about 1 ps, but the evidence disappears at very high temperatures. Values of the aqueous hydrogen hyperfine coupling constant are obtained by averaging the results of density functional calculations on clusters extracted from the simulations. The hyperfine coupling calculations do not agree well with experiment for reasons that are not understood now, pointing to the need for further research on this problem.

Structure, dynamics, and reactivity of hydrated electrons by ab initio molecular dynamics

Understanding the properties of hydrated electrons, which were first observed using pulse radiolysis of water in 1962, is crucial because they are key species in many radiation chemistry processes. Although time-resolved spectroscopic studies and molecular simulations have shown that an electron in water (prepared, for example, by water photoionization) relaxes quickly to a localized, cavity-like structure ∼2.5 Å in radius, this picture has recently been questioned. In another experimental approach, negatively charged water clusters of increasing size were studied with photoelectron and IR spectroscopies. Although small water clusters can bind an excess electron, their character is very different from bulk hydrated species. As data on electron binding in liquid water have become directly accessible experimentally, the cluster-to-bulk extrapolations have become a topic of lively debate. Quantum electronic structure calculations addressing experimental measurables have, until recently, been largely limited to small clusters; extended systems were approached mainly with pseudopotential calculations combining a classical description of water with a quantum mechanical treatment of the excess electron. In this Account, we discuss our investigations of electrons solvated in water by means of ab initio molecular dynamics simulations. This approach, applied to a model system of a negatively charged cluster of 32 water molecules, allows us to characterize structural, dynamical, and reactive aspects of the hydrated electron using all of the system's valence electrons. We show that under ambient conditions, the electron localizes into a cavity close to the surface of the liquid cluster. This cavity is, however, more flexible and accessible to water molecules than an analogous area around negatively charged ions. The dynamical process of electron attachment to a neutral water cluster is strongly temperature dependent. Under ambient conditions, the electron relaxes in the liquid cluster and becomes indistinguishable from an equilibrated, solvated electron on a picosecond time scale. In contrast, for solid, cryogenic systems, the electron only partially localizes outside of the cluster, being trapped in a metastable, weakly bound "cushion-like" state. Strongly bound states under cryogenic conditions could only be prepared by cooling equilibrated, liquid, negatively charged clusters. These calculations allow us to rationalize how different isomers of electrons in cryogenic clusters can be observed experimentally. Our results also bring into question the direct extrapolation of properties of cryogenic, negatively charged water clusters to those of electrons in the bulk liquid. Ab initio molecular dynamics represents a unique computational tool for investigating the reactivity of the solvated electron in water. As a prototype, the electron-proton reaction was followed in the 32-water cluster. In accord with experiment, the molecular mechanism is a proton transfer process that is not diffusion limited, but rather controlled by a proton-induced deformation of the excess electron's solvent shell. We demonstrate the necessary ingredients of a successful density functional methodology for the hydrated electron that avoids potential pitfalls, such as self-interaction error, insufficient basis set, or lack of dispersion interactions. We also benchmark the density functional theory methods and outline the path to faithful ab initio simulations of dynamics and reactivity of electrons solvated in extended aqueous systems.

A statistical mechanical theory of proton transport kinetics in hydrogen-bonded networks based on population correlation functions with applications to acids and bases

Extraction of relaxation times, lifetimes, and rates associated with the transport of topological charge defects in hydrogen-bonded networks from molecular dynamics simulations is a challenge because proton transfer reactions continually change the identity of the defect core. In this paper, we present a statistical mechanical theory that allows these quantities to be computed in an unbiased manner. The theory employs a set of suitably defined indicator or population functions for locating a defect structure and their associated correlation functions. These functions are then used to develop a chemical master equation framework from which the rates and lifetimes can be determined. Furthermore, we develop an integral equation formalism for connecting various types of population correlation functions and derive an iterative solution to the equation, which is given a graphical interpretation. The chemical master equation framework is applied to the problems of both hydronium and hydroxide transport in bulk water. For each case it is shown that the theory establishes direct links between the defect's dominant solvation structures, the kinetics of charge transfer, and the mechanism of structural diffusion. A detailed analysis is presented for aqueous hydroxide, examining both reorientational time scales and relaxation of the rotational anisotropy, which is correlated with recent experimental results for these quantities. Finally, for OH(-)(aq) it is demonstrated that the "dynamical hypercoordination mechanism" is consistent with available experimental data while other mechanistic proposals are shown to fail. As a means of going beyond the linear rate theory valid from short up to intermediate time scales, a fractional kinetic model is introduced in the Appendix in order to describe the nonexponential long-time behavior of time-correlation functions. Within the mathematical framework of fractional calculus the power law decay ∼t(-σ), where σ is a parameter of the model and depends on the dimensionality of the system, is obtained from Mittag-Leffler functions due to their long-time asymptotics, whereas (stretched) exponential behavior is found for short times.