Lack of ionic strength effect in the recombination of hydrated electrons: (e−)aq + (e−)aq → 2(OH−) + H2

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-.

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.

Effect of Linear Energy Transfer on Cystamine's Radioprotective Activity: A Study Using the Fricke Dosimeter with 6-500 MeV per Nucleon Carbon Ions-Implication for Carbon Ion Hadrontherapy

(1) Background: Radioprotective agents have garnered considerable interest due to their prospective applications in radiotherapy, public health medicine, and situations of large-scale accidental radiation exposure or impending radiological emergencies. Cystamine, an organic diamino-disulfide compound, is recognized for its radiation-protective and antioxidant properties. This study aims to utilize the aqueous ferrous sulfate (Fricke) dosimeter to measure the free-radical scavenging capabilities of cystamine during irradiation by fast carbon ions. This analysis spans an energy range from 6 to 500 MeV per nucleon, which correlates with "linear energy transfer" (LET) values ranging from approximately 248 keV/μm down to 9.3 keV/μm. (2) Methods: Monte Carlo track chemistry calculations were used to simulate the radiation-induced chemistry of aerated Fricke-cystamine solutions across a broad spectrum of cystamine concentrations, ranging from 10-6 to 1 M. (3) Results: In irradiated Fricke solutions containing cystamine, cystamine is observed to hinder the oxidation of Fe2+ ions, an effect triggered by oxidizing agents from the radiolysis of acidic water, resulting in reduced Fe3+ ion production. Our simulations, conducted both with and without accounting for the multiple ionization of water, confirm cystamine's ability to capture free radicals, highlighting its strong antioxidant properties. Aligning with prior research, our simulations also indicate that the protective and antioxidant efficiency of cystamine diminishes with increasing LET of the radiation. This result can be attributed to the changes in the geometry of the track structures when transitioning from lower to higher LETs. (4) Conclusions: If we can apply these fundamental research findings to biological systems at a physiological pH, the use of cystamine alongside carbon-ion hadrontherapy could present a promising approach to further improve the therapeutic ratio in cancer treatments.

Solvated dielectrons from optical excitation: An effective source of low-energy electrons

Low-energy electrons dissolved in liquid ammonia or aqueous media are powerful reducing agents that promote challenging reduction reactions, but can also cause radiation damage to biological tissue. Knowledge of the underlying mechanistic processes remains incomplete, in particular with respect to the details and energetics of the electron transfer steps. Here, we show how ultraviolet (UV) photoexcitation of metal-ammonia clusters could be used to generate tunable low-energy electrons in situ. Specifically, we identified UV light-induced generation of spin-paired solvated dielectrons and their subsequent relaxation by an unconventional electron-transfer-mediated decay as an efficient low-energy electron source. The process is robust and straightforward to induce, with the prospect of improving our understanding of radiation damage and fostering mechanistic studies of solvated electron reduction reactions.

Assessment of Cystamine's Radioprotective/Antioxidant Ability under High-Dose-Rate Irradiation: A Monte Carlo Multi-Track Chemistry Simulation Study

(1) Background: cystamine and its reduced form cysteamine have radioprotective/antioxidant effects in vivo. In this study, we use an in vitro model system to examine the behavior of cystamine towards the reactive primary species produced during the radiolysis of the Fricke dosimeter under high dose-rate irradiation conditions. (2) Methods: our approach was to use the familiar radiolytic oxidation of ferrous to ferric ions as an indicator of the radioprotective/antioxidant capacity of cystamine. A Monte Carlo computer code was used to simulate the multi-track radiation-induced chemistry of aerated and deaerated Fricke-cystamine solutions as a function of dose rate while covering a large range of cystamine concentrations. (3) Results: our simulations revealed that cystamine provides better protection at pulsed dose rates compared to conventional, low-dose-rate irradiations. Furthermore, our simulations confirmed the radical-capturing ability of cystamine, clearly indicating the strong antioxidant profile of this compound. (4) Conclusion: assuming that these findings can be transposable to cells and tissues at physiological pH, it is suggested that combining cystamine with FLASH-RT could be a promising approach to further enhance the therapeutic ratio of cancer cure.

Ultra-short chain fluorocarboxylates exhibit wide ranging reactivity with hydrated electrons

Recent reports demonstrate that technologies generating hydrated electrons (e_aq^-; e.g., UV-sulfite) are a promising strategy for destruction of per- and polyfluoroalkyl substances, but fundamental rate constants are lacking. This work examines the kinetics and mechanisms of e_aq^- reactions with ultra-short chain (C2-C4) fluorocarboxylates using experimental and theoretical approaches. Laser flash photolysis (LFP) was used to measure bimolecular rate constants (k₂; M^-1 s^-1) for e_aq^- reactions with thirteen per-, and for the first time, polyfluorinated carboxylate structures. The measured k₂ values varied widely from 5.26 × 10⁶ to 1.30 × 10⁸ M^-1s^-1, a large range considering the minor structural changes among the target compounds. Molecular descriptors calculated using density functional theory did not reveal correlation between k₂ values and individual descriptors when considering the whole dataset, however, semiquantitative correlation manifests when grouping by similar possible initial reduction event such as electron attachment at the α-carbon versus β- or γ-carbons along the backbone. From this, it is postulated that fluorocarboxylate reduction by e_aq^- occurs via divergent mechanisms with the possibility of non-degradative pathways being prominent. These mechanistic insights provide rationale for contradictory trends between LFP-derived k₂ values and apparent degradation rates recently reported in UV-sulfite constant irradiation treatment experiments.

Scavenging of “dry” electrons prior to hydration by azide ions: effect on the formation of H2 in the radiolysis of water by 60Co γ-rays and tritium β-electrons

In this study, we use Monte Carlo track chemistry simulations to show that “dry” secondary electrons, precursors of the “hydrated” electron (e−aq), can be scavenged on the sub-picosecond time scale prior to hydration, by a high concentration (>0.1–1 M) of azide ions (N3−) in water irradiated with 60Co γ-rays and tritium β-electrons at 25 °C. This is a striking result, as N3− is known to react very slowly with e−aq. These processes tend to significantly reduce the yields of H2 as observed experimentally. For both energetic Compton electrons (“linear energy transfer”, LET ∼ 0.3 keV/µm), which are generated by the cobalt-60 γ-rays, and 3H β-electrons (LET ∼ 6 keV/µm), our H2 yield results confirm previous Monte Carlo simulations, which indicated the necessity of including the capture of the precursors to e−aq. Interestingly, our calculations show no significant changes in the scavenging of “dry” electrons at high azide concentrations in passing from γ-radiolysis to tritium β-radiolysis (i.e., with LET). This led us to the conclusion that the higher H2 yield observed experimentally for 3H β-electrons compared with 60Co γ-rays is mainly explained by the difference in the radiation track structures during the chemical stage (>1 ps). The higher LET of tritium β-electrons leads to more molecular products (H2 in this case) in tritium radiolysis than in γ-radiolysis. Finally, a value of ∼0.5 nm was derived for the reaction distance between N3− and the “dry” electron from the H2 yields observed in 60Co γ-radiolysis at high N3− concentrations.

A review of simulation codes and approaches for radiation chemistry

Historically, the field of radiation chemistry began shortly after the discovery of radioactivity, and its development has been closely related to discoveries in other related fields such as radiation and nuclear physics. Radiolysis of water and radiation chemistry have been very important in elucidating how radiation affects living matter and how it induces DNA damage. Nowadays, we recognize the importance of chemistry to understanding the effects of radiation on cells; however, it took several decades to obtain this insight, and much is still unknown. The radiolysis of water and aqueous solutions have been the subject of much experimental and theoretical research for many decades. One important concept closely related to radiation chemistry is radiation track structure. Track structure results from early physical and physicochemical events that lead to a highly non-homogenous distribution of radiolytic species. Because ionizing radiation creates unstable species that are distributed non-homogenously, the use of conventional reaction kinetics methods does not describe this chemistry well. In recent years, several methods have been developed for simulating radiation chemistry. In this review, we give a brief history of the field and the development of the simulation codes. We review the current methods used to simulate radiolysis of water and radiation chemistry, and we describe several radiation chemistry codes and their applications.

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.

Evaluation of the radioprotective ability of cystamine for 150 keV – 500 MeV proton irradiation: a Monte Carlo track chemistry simulation study

Cystamine, an organic diamino-disulfide, is among the best of the known radiation-protective compounds, although the underlying molecular mechanisms by which it operates remain poorly understood. This study aims to use the aqueous ferrous sulfate (Fricke) dosimeter to evaluate the protective properties of this compound when present during irradiation by fast incident protons in the energy range of 150 keV – 500 MeV, that is, for “linear energy transfer” (LET) values ranging from ∼72.3 to 0.23 keV/μm. The presence of cystamine in irradiated Fricke solutions prevents the oxidation of Fe2+ ions by the oxidizing species produced in the radiolysis of acidic water, resulting in reduced Fe3+ ion yields. A Monte Carlo computer code is used to simulate the radiation-induced chemistry of the studied Fricke–cystamine solutions under aerated conditions while covering a wide range of cystamine concentrations from 5 × 10−7 to 1 mol/L. Results indicate that the protective activity of cystamine is due to its radical-capturing ability, a clear signature of the strong antioxidant profile of this compound. In addition, our simulations show that at low and intermediate concentrations of cystamine, its protective efficiency decreases with increasing LET, which is consistent with previous work. This finding stems from differences in the geometry of the track structures that change from low-LET isolated spherical “spurs” to high-LET dense continuous cylindrical tracks as LET increases. This study concludes that Monte Carlo simulations represent a powerful method for understanding, at the molecular level, indirect radiation damage to complex molecules such as cystamine.

Self-radiolysis of tritiated water. 4. The scavenging effect of azide ions (N3−) on the molecular hydrogen yield in the radiolysis of water by 60Co γ-rays and tritium β-particles at room temperature

The effect of the azide ion N₃⁻ on the yield of molecular hydrogen in water irradiated with ⁶⁰Co γ-rays (∼1 MeV Compton electrons) and tritium β-electrons (mean electron energy of ∼7.8 keV) at 25 °C is investigated using Monte Carlo track chemistry simulations in conjunction with available experimental data. N₃⁻ is shown to interfere with the formation of H₂ through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H₂ yield in the diffusing radiation tracks. Chemical changes are observed in the H₂ scavengeability depending on the particular type of radiation considered. These changes can readily be explained on the basis of differences in the initial spatial distribution of primary radiolytic species (i.e., the structure of the electron tracks). In the “short-track” geometry of the higher “linear energy transfer” (LET) tritium β-electrons (mean LET ∼5.9 eV nm⁻¹), radicals are formed locally in much higher initial concentration than in the isolated “spurs” of the energetic Compton electrons (LET ∼0.3 eV nm⁻¹) generated by the cobalt-60 γ-rays. As a result, the short-track geometry favors radical–radical reactions involving hydrated electrons and hydrogen atoms, leading to a clear increase in the yield of H₂ for tritium β-electrons compared to ⁶⁰Co γ-rays. These changes in the scavengeability of H₂ in passing from tritium β-radiolysis to γ-radiolysis are in good agreement with experimental data, lending strong support to the picture of tritium β-radiolysis mainly driven by the chemical action of short tracks of high local LET. At high N₃⁻ concentrations (>1 M), our H₂ yield results for ⁶⁰Co γ-radiolysis are also consistent with previous Monte Carlo simulations that suggested the necessity of including the capture of the precursors to the hydrated electrons (i.e., the short-lived “dry” electrons prior to hydration) by N₃⁻. These processes tend to reduce significantly the yields of H₂, as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET ³H β-radiolysis, leading us to conclude that the increased amount of intra-track chemistry intervening at early time under these conditions favors the recombination of these electrons with their parent water cations at the expense of their scavenging by N₃⁻.

The effect of the azide ion on the yield of molecular hydrogen in water irradiated with ⁶⁰Co γ-rays and tritium β-electrons at 25 °C is investigated using Monte Carlo track chemistry simulations.

Radiolysis of Supercritical Water at 400°C: A Sensitivity Study of the Density Dependence of the Yield of Hydrated Electrons on the (eaq−+eaq−) Reaction Rate Constant

The temperature dependence of the rate constant (k) of the bimolecular reaction of two hydrated electrons (eaq−) measured in alkaline water exhibits an abrupt drop between 150°C and 200°C; above 250°C, it is too small to be measured reliably. Although this result is well established, the applicability of this sudden drop in k(eaq−+eaq−)) above ∼150°C to neutral or slightly acidic solution, as recommended by some authors, still remains uncertain. In fact, the recent work suggested that in near-neutral water the abrupt change in k above ∼150°C does not occur and that k should increase, rather than decrease, at temperatures greater than 150°C with roughly the same Arrhenius dependence of the data below 150°C. In view of this uncertainty of k, Monte Carlo simulations were used in this study to examine the sensitivity of the density dependence of the yield of eaq− in the low–linear energy transfer (LET) radiolysis of supercritical water (H2O) at 400°C on variations in the temperature dependence of k. Two different values of the eaq− self-reaction rate constant at 400°C were used: one was based on the temperature dependence of k above 150°C as measured in alkaline water (4.2×108  M−1 s−1), and the other was based on an Arrhenius extrapolation of the values below 150°C (2.5×1011  M−1 s−1). In both cases, the density dependences of our calculated eaq− yields at ∼60  ps and 1 ns were found to compare fairly well with the available picosecond pulse radiolysis experimental data (for D2O) for the entire water density range studied (∼0.15–0.6  g/cm3). Only a small effect of k on the variation of G(eaq−)) as a function of density at 60 ps and 1 ns could be observed. In conclusion, our present calculations did not allow us to unambiguously confirm (or deny) the applicability of the predicted sudden drop of k(eaq−+eaq−) at ∼150°C in near-neutral water.

Monte Carlo simulation of the radiolysis of the ceric sulfate dosimeter by low linear energy transfer radiation

The ceric sulfate dosimeter is based on the radio-induced reduction of Ce4+ in acidic medium. For low linear energy transfer (LET) radiation, the yield of Ce3+ is 2.4 molecules / 100 eV, regardless of the presence of oxygen. To investigate the reaction mechanisms of the ceric sulfate dosimeter, we simulated the chemical reaction kinetics curves and the evolution of G(Ce3+), G(O2), and G(H2) in the ceric sulfate solution with and without oxygen. Studies of G(Ce3+) as function of the initial concentration of Ce3+ and of the LET were also done. One important finding of this study is that •OH radicals are scavenged by the reaction •OH + HSO4– → SO4•– + H2O, rather than by the reaction •OH + Ce3+ → Ce4+ + OH–.

Chasing charge localization and chemical reactivity following photoionization in liquid water

The ultrafast dynamics of the cationic hole formed in bulk liquid water following ionization is investigated by ab initio molecular dynamics simulations and an experimentally accessible signature is suggested that might be tracked by femtosecond pump-probe spectroscopy. This is one of the fastest fundamental processes occurring in radiation-induced chemistry in aqueous systems and biological tissue. However, unlike the excess electron formed in the same process, the nature and time evolution of the cationic hole has been hitherto little studied. Simulations show that an initially partially delocalized cationic hole localizes within ~30 fs after which proton transfer to a neighboring water molecule proceeds practically immediately, leading to the formation of the OH radical and the hydronium cation in a reaction which can be formally written as H(2)O(+) + H(2)O → OH + H(3)O(+). The exact amount of initial spin delocalization is, however, somewhat method dependent, being realistically described by approximate density functional theory methods corrected for the self-interaction error. Localization, and then the evolving separation of spin and charge, changes the electronic structure of the radical center. This is manifested in the spectrum of electronic excitations which is calculated for the ensemble of ab initio molecular dynamics trajectories using a quantum mechanics/molecular mechanics (QM∕MM) formalism applying the equation of motion coupled-clusters method to the radical core. A clear spectroscopic signature is predicted by the theoretical model: as the hole transforms into a hydroxyl radical, a transient electronic absorption in the visible shifts to the blue, growing toward the near ultraviolet. Experimental evidence for this primary radiation-induced process is sought using femtosecond photoionization of liquid water excited with two photons at 11 eV. Transient absorption measurements carried out with ~40 fs time resolution and broadband spectral probing across the near-UV and visible are presented and direct comparisons with the theoretical simulations are made. Within the sensitivity and time resolution of the current measurement, a matching spectral signature is not detected. This result is used to place an upper limit on the absorption strength and/or lifetime of the localized H(2)O(+) ((aq)) species.

The identification of a solvated electron pair in the gaseous clusters of Na(-)(H2O)n and Li(-)(H2O)n

By first principles calculations, we explore the possibility that Na(-)(H(2)O)(n) and Li(-)(H(2)O)(n) clusters, which have been measured previously by photoelectron experiments, could serve as gas-phase molecular models for the solvation of two electrons. Such models would capture the electron-electron interaction in a solution environment, which is missed in the well-known anionic water clusters (H(2)O)(n) (-). Our results show that by n = 10, the two loosely bound s electrons in Li(-)(H(2)O)(n) are indeed detached from lithium, and they could exist in either the singlet (spin-paring) or the triplet (spin-coupling) state. In contrast, the two electrons would prefer to stay on the sodium atom in Na(-)(H(2)O)(n) and on the surface of the cluster. The formation of a solvated electron pair and the variation in solvation structures make these two cluster series interesting subjects for further experimental investigation.

Monte carlo simulations of site-specific radical attack to DNA bases

An atomistic biophysical model permitting the calculation of initial attacks to a 38-bp representation of B-DNA base moieties by water radicals is presented. This model is based on a previous radiation damage model developed by Aydogan et al. (Radiat. Res. 157, 38-44, 2002). Absolute efficiencies for radical attack to the 38-bp DNA molecule are calculated to be 41, 0.8 and 15% for hydroxyl radical ((.)OH), hydrogen radical (H(.)), and hydrated electron (e(aq))(,) respectively. Among the nucleobases, guanine is found to have the highest percentage (.)OH attack probability at 36%. Adenine, cytosine and thymine moieties have initial attack probabilities of 24, 18 and 22%, respectively. A systematic study is performed to investigate (.)OH attack probabilities at each specified attack site in four molecular models including free bases, single nucleotides, single base pairs, and the central eight base pairs of the 38-bp DNA molecule. Cytosine is the free base moiety for which the closest agreement is observed between the model prediction and the experimental data. The initial (.)OH attack probabilities for cytosine as the free base are calculated to be 72 and 28%, while experimental data are reported at 87 and 13% for the C5 and C6 positions on the base, respectively. In this study, we incorporated atomic charges to scale the site-specific (.)OH reaction rates at the individual atomic positions on the pyrimidine and purine bases. Future updates to the RIDNA model will include the use of electron densities to scale the reaction rates. With respect to reactions of the aqueous electron with DNA, a comparison of the initial distribution of electron attack sites calculated in this study and experimental results suggests an extremely rapid and extensive redistribution of the e(-)(aq) after their initial reactions with DNA.

Reactions of Hydrated Electron with Various Radicals: Spin Factor in Diffusion-Controlled Reactions

The reactions of hydrated electron (eaq-) with various radicals have been studied in pulse radiolysis experiments. These radicals are hydroxyl radical (*OH), sulfite radical anion (*SO3-), carbonate radical anion (CO3*-), carbon dioxide radical anion (*CO2-), azidyl radical (*N3), dibromine radical anion (Br2*-), diiodine radical anion (I2*-), 2-hydroxy-2-propyl radical (*C(CH3)2OH), 2-hydroxy-2-methyl-1-propyl radical ((*CH2)(CH3)2COH), hydroxycyclohexadienyl radical (*C6H6OH), phenoxyl radical (C6H5O*), p-methylphenoxyl radical (p-(H3C)C6H4O*), p-benzosemiquinone radical anion (p-OC6H4O*-), and phenylthiyl radical (C6H5S*). The kinetics of eaq- was followed in the presence of the counter radicals in transient optical absorption measurements. The rate constants of the eaq- reactions with radicals have been determined over a temperature range of 5-75 degrees C from the kinetic analysis of systems of multiple second-order reactions. The observed high rate constants for all the eaq- + radical reactions have been analyzed with the Smoluchowski equation. This analysis suggests that many of the eaq- + radical reactions are diffusion-controlled with a spin factor of 1/4, while other reactions with *OH, *N3, Br2*-, I2*-, and C6H5S* have spin factors significantly larger than 1/4. Spin dynamics for the eaq-/radical pairs is discussed to explain the different spin factors. The reactions with *OH, *N3, Br2*-, and I2*- have also been found to have apparent activation energies less than that for diffusion control, and it is suggested that the spin factors for these reactions decrease with increasing temperature. Such a decrease in spin factor may reflect a changing competition between spin relaxation/conversion and diffusive escape from the radical pairs.

Monte Carlo simulation study of the effects of acidity and LET on the primary free-radical and molecular yields of water radiolysis — Application to the Fricke dosimeter

Monte Carlo simulations are used to investigate the effects of acidity (pH) on the primary yields of various chemical species produced in the radiolysis of de-aerated aqueous sulfuric acid solutions over the range from neutral solution to 0.4 mol/L H2SO4. The effects of the quality of radiation, measured in terms of linear energy transfer (LET), have also been studied for LET varying from ~0.3 to 15 keV/µm at ambient temperature. Our results show that an increase in acidity (1 < pH < 4) leads to an increase in the yield [Formula: see text] of the "reducing" free radicals (hydrated electron and H• atom) and a slight increase in G·OH and [Formula: see text], while there is a slight decrease in [Formula: see text] At pH < 1, •OH radicals react with HSO4- anions to form SO4·– radicals, resulting in a steep decrease in G.OH. By contrast, in the range of pH from ~4 to 7, the calculated yield values are independent of sulfuric acid concentration. In both neutral water and 0.4 mol/L H2SO4 (pH 0.46) solutions, the primary molecular yields increase upon increasing LET to ~15 keV/µm with a concomitant decrease in those of free radicals. As an exception, GH. at first increases with LET, reaching a maximum near 6.5 keV/µm before decreasing steeply at higher LET. The results obtained are generally in good agreement with available experimental data over the whole acidity and LET ranges studied. Finally, as an application, we have simulated the radiation-induced oxidation of ferrous sulfate solutions in aerated aq. 0.4 mol/L H2SO4 (Fricke dosimeter) as a function of time up to ~50 s and addressed the effects of LET on the resulting ferric ion yield at 25 °C. The production of Fe3+ ions is highly sensitive to free-radical yields, especially H• atoms (via formation of HO2•), resulting in a marked decline of G(Fe3+) with increasing LET. The general trend of the observed variation of G(Fe3+) with radiation quality is well reproduced by our computed Fe3+ ion yield values.Key words: liquid water, acidic (H2SO4) aqueous solutions, radiolysis, free-radical and molecular yields, linear energy transfer (LET), Fricke dosimeter, Monte Carlo simulations.

Reaction between Diiodide Anion Radicals in Ionic Liquids

Photodetachment of electrons from iodide ions produced diiodide anion radicals in ionic liquids containing ammonium, pyrrolidinium, and piperidinium cations. The rates of reaction between diiodide anion radicals in molecular solvents such as H2O, methanol, and ethanol could be estimated by the Debye-Smoluchowski equation, which accounts for electrostatic interactions using dielectric constants for the molecular solvents. In contrast, the rates of reaction between diiodide anion radicals in the ionic liquids were close to the diffusion-limited rates for the neutral molecules, suggesting that electrostatic repulsion between the diiodide anion radicals is weakened by Coulombic shielding in the ionic liquids.

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.

Ultrafast dynamics for electron photodetachment from aqueous hydroxide

Charge-transfer-to-solvent reactions of hydroxide induced by 200 nm monophotonic or 337 and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, e(aq) (-), have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component ( approximately 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% versus 17.5% at 25 degrees C, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is <4 cm TW(-1) M(-1) (assuming unity quantum efficiency for the photodetachment). For [OH(-)] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10(-3) K(-1) and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.

La radiolyse de l'eau et des solutions aqueuses : historique et actualité

Experiments showing that water is decomposed by the action of high-energy radiations date back to the first days of the discovery of radioactivity, a century ago. On the occasion of this anniversary, we have attempted to give a comprehensive account of the radiation chemistry of water and its solutions since its origin, with special emphasis on the various physical and chemical stages that led to the present state of this science. To this aim, we describe the effect of different intervening factors on the molecular and radical yields, including dissolved solute concentration, pH, radiation intensity (or dose rate), type and energy of the radiation, presence of oxygen, temperature, phase, and pressure. We also discuss briefly the chemical behavior of the free radicals produced in radiolyzed aqueous solutions. A good, albeit incomplete, description of the phenomena is obtained that leads to various perspectives concerning, on the one hand, the development of this science and, on the other hand, its potential for applications.Key words : radical chemistry, dilution curve, water, hydrated electron, hydroxyl and superoxide radicals, free radicals, radiolysis, chain reactions, molecular and radical yields, cell survival, linear energy transfer.