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The temperature dependence of the rate constant for the reaction of dichloride anion radical ( Cl 2 - • ) with atomic hydrogen (H • ) in water up to 75 °C has been determined by pulse radiolysis of deaerated 0.1 M HCl solution. The room temperature value is (6.1±0.6)×10 9 M −1 s −1 . The activation energy of (13.2±0.6) kJ mol −1 is less than 16.7 kJ mol −1 , expected for the diffusion-controlled reaction. Based on the temperature dependence of the rate constant for the reactions H • + Cl 2 - • and H • +Cl 2 , derived in this work, and on that reported earlier ( Szala-Bilnik et al., 2014 ) for Cl 2 - • + Cl 2 - • , we show that a value of (10±2) M −1 s −1 determined by Hartig and Getoff (1982) for k (H • +H 2 O) in water at 25 °C is overestimated by at least two orders of magnitude. [ABSTRACT FROM AUTHOR]

The rate constant for the disproportionation of Cl− 2 in water has been determined for the temperature range 22–87°C using pulse radiolysis of 0.1M NaCl+1mM HClO4 aqueous solution. The rate constant for the decay of Cl− 2 has been found to be 2×(7.35±0.53)×108 M−1 s−1 at 22°C (at zero ionic strength). The determined activation energy, E a =10.89±0.37kJmol−1, is less than expected for diffusion-controlled reactions. A good fit to the Noyes equation (1/k obs=1/k diff+1/k react) has been obtained assuming the reaction step is activationless with k react=A′T. [ABSTRACT FROM AUTHOR]

Understanding the properties of the • OH radical in aqueous environments is essential for biochemistry, atmospheric chemistry, and the development of green chemistry technologies. In particular, the technological applications involve knowledge of microsolvation of the • OH radical in high temperature water. In this study, the classical molecular dynamics (MD) simulation and the technique based on the construction of Voronoi polyhedra were used to provide 3D characteristics of the molecular vicinity of the aqueous hydroxyl radical ( • OH aq ). The statistical distribution functions of metric and topological features of solvation shells represented by the constructed Voronoi polyhedra are reported for several thermodynamic states of water, including the pressurized high-temperature liquid and supercritical fluid. Calculations showed a decisive influence of the water density on the geometrical properties of the • OH solvation shell in the sub- and supercritical region: with the decreasing density, the span and asymmetry of the solvation shell increase. We also showed that the 1D analysis based on the oxygen-oxygen radial distribution functions (RDFs) overestimates the solvation number of • OH and insufficiently reflects the influence of transformations in the hydrogen-bonded network of water on the structure of the solvation shell.

The rate of formation of dichloride anions (Cl 2 •- ) in dilute aqueous solutions of HCl (2-100 mmol·kg -1 ) was measured by the technique of pulse radiolysis over the temperature range of 288-373 K. The obtained Arrhenius dependence shows a concentration averaged activation energy of 7.3 ± 1.8 kJ·mol -1 , being half of that expected from the mechanism assuming the • OHCl - intermediate and supporting the ionic equilibrium-based mechanism, i.e., the formation of Cl 2 •- in the reaction of • OH with a hydronium-chloride (Cl - ·H 3 O + ) contact ion pair. Assuming diffusion-controlled encounter of the hydronium and chloride ions and including the effect of the ionic atmosphere, we showed that the reciprocal of τ, the lifetime of (Cl - ·H 3 O + ), follows an Arrhenius dependence with an activation energy of 23 ± 4 kJ·mol -1 , independent of the acid concentration. This result indicates that the contact pair is stabilized by hydrogen bonding interaction of the solvent molecules. We also found that at a fixed temperature, τ is noticeably increased in less-concentrated solutions ( m HCl < 0.01 m). Since this concentration effect is particularly pronounced at near ambient temperatures, the increasing pair lifetime may result from the solvent cage effect enhanced by the presence of large supramolecular structures (patches) formed by continuously connected four-bonded water molecules.

The solvent-composition dependence of quenching triplet states of benzophenone ( 3 BP) by anisole in acetonitrile-water (ACN-H 2 O) mixtures was investigated by laser flash photolysis over the water mole fraction ( x w ) increasing from 0 to 0.92. Single exponential decay of 3 BP was observed over the whole composition range. The quenching rate constant consistently increased with the water content but increased far more rapidly with x w > 0.7. The water-triggered electron-transfer (ET) mechanism was confirmed by a steeply growing quantum yield of the benzophenone ketyl radical anion, escaping back-ET when the partial water volume exceeded the acetonitrile one. The water-content influence on the 3 BP quenching rate was described by a kinetic model accounting for the microheterogeneous structure of the ACN-H 2 O mixtures and the very different solubility of the reactants in the solvent components. According to the model, the ET mechanism occurs at a rate constant of 1.46 × 10 9 M -1 s -1 and is presumably assisted by the ACN-H 2 O hydrogen-bonding interaction.

A new mechanism for the dichloride radical anion (Cl 2 •- ) formation in diluted acidic chloride solutions is proposed on the grounds of pulse radiolysis measurements of the optical absorption growth at 340 nm and the density functional theory and Hartree-Fock computations. We show that the rate of • OH conversion into Cl 2 •- is determined by the equilibrium concentration of the ionic pair H 3 O + ·Cl - . According to the proposed mechanism, the diffusional encounter of • OH and H 3 O + ·Cl - is followed by fast concerted charge/proton transfer ( k(25 °C) = 6.2 × 10 12 s -1 ) to yield Cl • , which then reacts with Cl - to produce Cl 2 •- . The mechanism has been confirmed by the observed first-order growth of the Cl 2 •- absorption and a direct proportionality of the rate constant to the activities of H 3 O + and Cl - ions. The salt effect on the rate of Cl 2 •- formation is due to the ionic strength effect on the equilibrium H 3 O + + Cl - ⇄ H 3 O + ·Cl - .

Flexible models of the radical and water molecules including short-range interaction of hydrogen atoms have been employed in molecular dynamic simulation to understand mechanism of (●)OH hydration in aqueous systems of technological importance. A key role of H-bond connectivity patterns of water molecules has been identified. The behavior of (●)OH(aq) strongly depends on water density and correlates with topological changes in the hydrogen-bonded structure of water driven by thermodynamic conditions. Liquid and supercritical water above the critical density exhibit the radical localization in cavities existing in the solvent structure. A change of mechanism has been found at supercritical conditions below the critical density. Instead of cavity localization, we have identified accumulation of water molecules around (●)OH associated with the formation of a strong H-donor bond and diminution of non-homogeneity in the solvent structure. For all the systems investigated, the computed hydration number and the internal energy of hydration Δ(h)U showed approximately linear decrease with decreasing density of the solvent but a degree of radical-water hydrogen bonding exhibited non-monotonic dependence on density. The increase in the number of radical-water H-acceptor bonds is associated with diminution of extended nets of four-bonded water molecules in compressed solution at ~473 K. Up to 473 K, the isobaric heat of hydration in compressed liquid water remains constant and equal to -40 ± 1 kJ mol(-1)., (© 2012 American Institute of Physics)

Classical molecular dynamics (MD) simulation of ˙OH in liquid water at 37 °C has been performed using flexible models of the solute and solvent molecules. We derived the Morse function describing the bond stretching of the radical and the potential for ˙OH-H(2)O interactions, including short-range interactions of hydrogen atoms. Scans of the potential energy surface of the ˙OH-H(2)O complex have been performed using the DFT method with the B3LYP functional and the 6-311G(d,p) basis set. The DFT-derived partial charges, ±0.375e, and the equilibrium bond-length, 0.975 Å, of ˙OH resulted in the dipole moment of 1.76 D. The radical-water radial distribution functions revealed that ˙OH is not built into the solvent structure but it rather occupies distortions or cavities in the hydrogen-bonded network. The solvent structure at 37 °C has been found to be the same as that of pure water. The hydration cage of the radical comprises 13-14 water molecules. The estimated hydration enthalpy -42 ± 5 kJ mol(-1) is comparable with the experimental value -39 ± 6 kJ mol(-1) for 25 °C. Inspection of hydrogen bonds showed the importance of short-range interaction of hydrogen atoms and indicated that neglect of the angular condition greatly overestimates the number of the H-acceptor radical-water bonds. The mean number ̅n = 0.85 of radical-water H-bonds has been calculated using geometric definition of H-bond and ̅n = 0.62 has been obtained when the energetic condition, E(da)≤-8 kJ mol(-1), was additionally considered. The continuous lifetimes of 0.033 ps and 0.024 ps have been estimated for the radical H-donor and the H-acceptor bonds, respectively. Within statistical uncertainty the radical self-diffusion coefficient, (2.9 ± 0.6) × 10(-9) m(2) s(-1), is the same as (3.1 ± 0.5) × 10(-9) m(2) s(-1) calculated for water in solution and in pure solvent. To the best of our knowledge, this is the first study of the ˙OH(aq) properties at a biologically relevant body temperature., (This journal is © the Owner Societies 2011)

Assembling of water molecules via hydrogen bonding has been studied by molecular dynamics simulations using flexible potential model. The relationship between the number of H-bonds per molecule, n(HB), the size of H-bonded nets, k, and the size of patches of four-bonded molecules, k(4), has been examined for several thermodynamic states of water ranging from ambient to supercritical conditions. Two kinds of structural inhomogeneity have been found: the patchlike associated with the mean n(HB)> 2.0 and the clusterlike for n(HB)< 1.9. In compressed water up to ~473 K patches coexist with less ordered nets, both constituting the gel-like H-bonded network. The size of patches steeply decreases with the increasing temperature and the decreasing density of water. The inhomogeneity resulting from the presence of patches disappears above 473 K. This feature is associated with the rapid increase in the fraction of unbound molecules and with the breakage of the gel-like network into a variety of H-bonded clusters leading to the clusterlike structural inhomogeneity. In contrast to the patchlike inhomogeneity an increase in temperature and a decrease in density make this kind of inhomogeneity more pronounced. A degree of connectivity of H-bonds has been characterized by a parameter P(g) defined as the total fraction of molecules belonging to the H-bonded clusters of size k ≥ 5. The simulation-derived values of P(g) agree well with the predictions of the random bond theory giving the explicit expression for P(g) as a function of the mean n(HB). Going from ambient to supercritical conditions, we have found that the patchlike inhomogeneity is connected with the very slight reduction in P(g), whereas the clusterlike inhomogeneity generates a steep linear decrease of P(g) with the decreasing mean n(HB). The self-diffusion coefficient calculated for the thermodynamic states of water showing the clusterlike inhomogeneity has occurred to be inversely proportional to the density. We have also found that the clusterlike inhomogeneity is associated with the linear correlation between P(g) and the macroscopic properties of water: the static dielectric constant, the viscosity, and the density. The provided relationships allow one to estimate the degree of connectivity of hydrogen bonds from the measured macroscopic quantities.