Reaction Of Hydrated Electron Research Articles

Exploring the Unusual Reactivity of the Hydrated Electron with CO2.

Many questions remain about the reactions of the hydrated electron despite decades of study. Of particular note is that they do not appear to follow the Marcus theory of electron transfer reactions, a feature that is yet to be explained. To investigate these issues, we used ab initio molecular dynamics (AIMD) simulations to investigate one of the better studied reactions, the hydrated electron reduction of CO2. The rate constant for the hydrated electron-CO2 reaction complex to react to form CO2- is for the first time estimated from AIMD simulations. Results at 298 and 373 K show the rate constant is insensitive to temperature, consistent with the low measured activation energy for the reaction, and the implications of this behavior are examined. The sampling provided by the simulations yields insight into the reaction mechanism. The reaction is found to involve both solvent reorganization and changes in the carbon dioxide structure. The latter leads to significant vibrational excitation of the bending and symmetric stretch vibrations in the CO2- product, indicating the reaction is vibrationally nonadiabatic. The former is estimated from the calculation of an approximate collective solvent coordinate and the free energy in this coordinate is determined. These results indicate that AIMD simulations can reasonably estimate hydrated electron reaction activation energies and provide new insight into the mechanism that can help illuminate the features of this unusual chemistry.

Integrating molecular descriptors for enhanced prediction: Shedding light on the potential of pH to model hydrated electron reaction rates for organic compounds

Hydrated electron reaction rate constant (ke-aq) is an important parameter to determine reductive degradation efficiency and to mitigate the ecological risk of organic compounds (OCs). However, OC species morphology and the concentration of hydrated electrons (e-aq) in water vary with pH, complicating OC fate assessment. This study introduced the environmental variable of pH, to develop models for ke-aq for 701 data points using 3 descriptor types: (i) molecular descriptors (MD), (ii) quantum chemical descriptors (QCD), and (iii) the combination of both (MD + QCD). Models were screened using 2 descriptor screening methods (MLR and RF) and 14 machine learning (ML) algorithms. The introduction of QCDs that characterized the electronic structure of OCs greatly improved the performance of models while ensuring the need for fewer descriptors. The optimal model MLR-XGBoost(MD + QCD), which included pH, achieved the most satisfactory prediction: R2tra = 0.988, Q2boot = 0.861, R2test = 0.875 and Q2test = 0.873. The mechanistic interpretation using the SHAP method further revealed that QCDs, polarizability, volume, and pH had a great influence on the reductive degradation of OCs by e-aq. Overall, the electrochemical parameters (QCDs, pH) related to the solvent and solute are of significance and should be considered in any future ML modeling that assesses the fate of OCs in aquatic environment.

A new perspective on predicting the reaction rate constants of hydrated electrons for organic contaminants: Exploring molecular structure characterization methods and ambient conditions

Hydrated electrons (eaq−) exhibit rapid degradation of diverse persistent organic contaminants (OCs) and hold great promise as a formidable reducing agent in water treatment. However, the diverse structures of compounds exert different influences on the second-order rate constant of hydrated electron reactions (keaq−), while the same OCs demonstrate notable discrepancies in keaq− values across different pH levels. This study aims to develop machine learning (ML) models that can effectively simulate the intricate reaction kinetics between eaq− and OCs. Furthermore, the introduction of the pH variable enables a comprehensive investigation into the impact of ambient conditions on this process, thereby improving the practicality of the model. A dataset encompassing 701 keaq− values derived from 351 peer-reviewed publications was compiled. To comprehensively investigate compound properties, this study introduced molecular descriptor (MD), molecular fingerprint (MF), and the integration of both (MD + MF) as model variables. Furthermore, 60 sets of predictive models were established utilizing two variable screening methodologies (MLR and RF) and ten prominent algorithms. Through statistical parameter analysis, it was determined that descriptors combined with MD and MF, the RF screening method, and the symbolism algorithm exhibited the best predictive efficacy. Importantly, the combination of descriptor models exhibited significantly superior performance compared to individual MF and MD models. Notably, the optimal model, denoted as RF - (MF + MD) - LGB, exhibited highly satisfactory predictive results (R2tra = 0.967, Q2tra = 0.840, R2ext = 0.761). The mechanistic explanation study based on Shapley Additive Explanations (SHAP) values further elucidated the crucial influences of polarity, pH, molecular weight, electronegativity, carbon–carbon double bonds, and molecular topology on the degradation of OCs by eaq−. The proposed modeling approach, particularly the integration of MF and MD, alongside the introduction of pH, may furnish innovative ideas for advanced reduction or oxidation processes (ARPs/AOPs) and machine learning applications in other domains.

Investigation of the Failure of Marcus Theory for Hydrated Electron Reactions.

Reactions of the hydrated electron with a wide variety of substrates have been found to exhibit unusually similar activation energies in a manner incompatible with Marcus electron transfer theory. Given the fundamental linear response assumption of Marcus theory, one possible explanation for this apparent failure is that the underlying free energy surfaces governing the reactions are not harmonic; i.e., hydrated electron structural fluctuations exhibit non-Gaussian behavior. In this work, we test this hypothesis by using simulations to calculate the hydrated electron vertical detachment energy distribution. We consider both cavity and noncavity models for the hydrated electron, between which the actual hydrated electron behavior is expected to lie. Our results identify a possible origin for non-Gaussian behavior of the hydrated electron but show that it is not of sufficient magnitude to explain the failure of Marcus theory to describe its reactions. Thus, other explanations must be sought.

Radiation induced degradation of ciprofloxacin and norfloxacin: Kinetics and product analysis

Ionizing radiation induced degradation of two fluoroquinolone antibiotics, ciprofloxacin and norfloxacin was investigated in 0.1 mmol dm–3 aqueous solutions. The rate constants, directly determined by pulse radiolysis, were 7.2 × 109 mol–1 dm3 s–1 and 5.4 × 109 mol–1 dm3 s–1 for •OH reaction and 8.2 × 109 mol–1 dm3 s–1 and 7.3 × 109 mol–1 dm3 s–1 for hydrated electron reaction with ciprofloxacin and norfloxacin, respectively. Absorption bands of several transient intermediates were observed on the pulse radiolysis spectra. For end product characterization samples were irradiated in a 60Co–γ irradiation chamber, they were analyzed by taking the UV–Vis spectra and by LC separation with MS detection. Based on their structures, most of the products presumably had lost their antibacterial activity. For both compounds products reflecting reaction on the quinolone part (hydroxylated molecules) or on the piperazinyl moiety (desethylene derivatives) were observed.

Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis.

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups.

Ionizing radiation induced degradation of monuron in dilute aqueous solution

The decomposition of monuron was investigated in dilute aqueous solutions using pulse radiolysis and γ-radiolysis in order to identify the intermediates and final products. The main reaction takes place between monuron and the hydroxyl radicals yielding hydroxycyclohexadienyl type radicals with a second order rate constant of (7.4±0.2)×109mol−1dm3s−1. In ∙OH reactions, the aminyl and phenoxyl radicals may also form. Dechlorination was observed in both hydroxyl radical and hydrated electron reactions. The ∙OH induced dechlorination reactions are suggested to occur through OH substitution or phenoxyl radical formation. The rate of oxidation is very high in the presence of dissolved oxygen. Some of the results are also supported by quantum chemical calculations.

Hidden chemistry of substituted aniline radical cations in water: a mechanistic study

Absolute rate constants for hydroxyl radical, azide radical, and hydrated electron reactions with a sulfa drug 4,4'‐diamino diphenyl sulfone (dapsone) in water have been evaluated using electron pulse radiolysis technique. Absolute rate constants for hydroxyl radical and azide radical were determined as (8.4 ± 0.3) × 109 and (5.6 ± 0.5) × 109 M−1 s−1, respectively. The reduction of dapsone with the hydrated electron occurred with rate constant of (9.2 ± 0.1) × 109 M−1 s−1. Hydroxyl radical reactions result in the synchronous formation of adduct as well as anilino radical. The interesting observation is that the yield of the anilino radical increases with increase in pH. Contrary to this, the yield of the adduct decreases with pH. We propose that hydroxyl radical adds predominantly to the aniline. In contrast, the reaction of azide radical with the dapsone suggests that the reaction occurs at the –NH2 moiety of the aniline ring. The free radical electron transfer from dapsone to parent radical cation of non‐polar solvent also results in the formation of anilino radical only suggesting that the radical cation of dapsone has a short lifetime. The reaction of hydrated electrons with the dapsone suggests that the reaction occurs at different reaction site. The experimental results supported by theoretical calculations of this study provide fundamental mechanistic parameters that probably decide the fate of the radical cation of aniline derivatives. Copyright © 2014 John Wiley & Sons, Ltd.

Free radical destruction of haloacetamides in aqueous solution

Haloacetamides are disinfection byproducts (DBPs) formed through chlorine/chloramine disinfection processes in drinking waters and have been recently highlighted in the US national Reconnaissance Survey. These species occur at low concentrations, but have been determined to have high cytotoxicity and mutagenicity and therefore may represent a human health hazard. Advanced oxidation/reduction processes (AO/RPs) which utilize free radical reactions are new alternatives to degrade these species. This study reports the absolute bimolecular reaction rate constants for 12 haloacetamides with •OH radical and hydrated electron. The •OH radical reaction rates varied from 5.18 × 107 to 1.14 × 1010 M−1 s−1, and hydrated electron reaction rates varied from 5.00 × 109 to 3.64 × 1010 M−1 s−1. Results obtained using an ion chromatograph indicated that ARPs are suitable for dehaloliazation of haloacetamides. These data are required for both evaluating AO/RPs for the destruction of these compounds and for studies of their fate and transport in surface waters where radical chemistry may be important in assessing their lifetime.

Radiation chemistry of salicylic and methyl substituted salicylic acids: Models for the radiation chemistry of pharmaceutical compounds

Salicylic acid and its derivatives are components of many medications and moieties found in numerous pharmaceutical compounds. They have been used as models for various pharmaceutical compounds in pharmacological studies, for the treatment of pharmaceuticals and personal care products (PPCPs), and, reactions with natural organic matter (NOM). In this study, the radiation chemistry of benzoic acid, salicylic acid and four methyl substituted salicylic acids (MSA) is reported. The absolute bimolecular reaction rate constants for hydroxyl radical reaction with benzoic and salicylic acids as well as 3-methyl-, 4-methyl-, 5-methyl-, and 6-methyl-salicylic acid were determined (5.86±0.54)×109, (1.07±0.07)×1010, (7.48±0.17)×109, (7.31±0.29)×109, (5.47±0.25)×109, (6.94±0.10)×109 (M−1s−1), respectively. The hydrated electron reaction rate constants were measured (3.02±0.10)×109, (8.98±0.27)×109, (5.39±0.21)×109, (4.33±0.17)×109, (4.72±0.15)×109, (1.42±0.02)×109 (M−1s−1), respectively. The transient absorption spectra for the six model compounds were examined and their role as model compounds for the radiation chemistry of pharmaceuticals investigated.

Isoniazid: Radical-induced oxidation and reduction chemistry

Isoniazid is a potent and selective therapeutic prodrug agent used to treat infections by Mycobacterium tuberculosis. Although it has been used clinically for over five decades its full mechanism of action is still being elucidated. Essential to its mechanism of action is the activation of isoniazid to a reactive intermediate, the isonicotinyl acyl radical, by the catalase-peroxidase KatG. The isonicotinyl acyl radical then reacts with NAD producing an inhibitor of the NADH-dependent enoyl ACP reductase responsible for mycolic acid synthesis as its primary target. However, the initial oxidation of isoniazid by KatG has also revealed alternative reaction pathways leading to an array of carbon-, oxygen-, and nitrogen-centered radical intermediates. It has also been reported that isoniazid produces nitric oxide in the presence of KatG and hydrogen peroxide. In this study, the temperature-dependent rate constants for the hydroxyl radical oxidation and solvated electron reduction of isoniazid and two model compounds have been studied. Based on these data the initial oxidation of isoniazid by the hydroxyl radical has been shown to predominantly occur at the primary nitrogen of the hydrazyl moiety, consistent with the postulated mechanism for the formation of the isonicotinyl radical. The hydrated electron reduction occurred mostly at the pyridine ring. Concomitant EPR spin-trap measurements under a variety of oxidizing and reducing conditions did not show any evidence of nitric oxide production as had been previously reported. Finally, examination of the transient absorption spectra obtained for hydrated electron reaction with isoniazid demonstrated for the first time an initial reduced transient identified as the isonicotinyl acyl radical produced from isoniazid.

Radical-based destruction of nitramines in water: kinetics and efficiencies of hydroxyl radical and hydrated electron reactions.

In support of the potential use of advanced oxidation and reduction process technologies for the removal of carcinogenic nitro-containing compounds in water reaction rate constants for the hydroxyl radical and hydrated electron with a series of low molecular weight nitramines (R(1)R(2)-NNO(2)) have been determined using a combination of electron pulse radiolysis and transient absorption spectroscopy. The hydroxyl radical reaction rate constant was fast, ranging from 0.54-4.35 × 10(9) M(-1) s(-1), and seen to increase with increasing complexity of the nitramine alkyl substituents suggesting that oxidation primarily occurs by hydrogen atom abstraction from the alkyl chains. In contrast, the rate constant for hydrated electron reaction was effectively independent of compound structure, (k(av) = (1.87 ± 0.25) × 10(10) M(-1) s(-1)) indicating that the reduction predominately occurred at the common nitramine moiety. Concomitant steady-state irradiation and product measurements under aerated conditions also showed a radical reaction efficiency dependence on compound structure, with the overall radical-based degradation becoming constant for nitramines containing more than four methylene groups. The quantitative evaluation of these efficiency data suggest that some (~40%) hydrated electron reduction also results in quantitative nitramine destruction, in contrast to previously reported electron paramagnetic measurements on these compounds that proposed that this reduction only produced a transient anion adduct that would transfer its excess electron to regenerate the parent molecule.

Effects of Microviscosity, Dry Electron Scavenging, and Protein Mobility on the Radiolysis of Albumen Hydrogel

Nanosecond pulse radiolysis experiments performed on the oxygen and nitrogen saturated thick fraction of egg white (which has an immeasurably high macroviscosity) produced a rate constant for hydrated electron reaction with oxygen equal to 1.7 × 10(10) M(-1) s(-1) at 18 °C, indicating that, due to the very low microviscosity, hydrated electrons are as mobile and reactive in the albumen hydrogel as in neat water. Also, the radiolytic yield for the hydrated electron (G-value) in the thick fraction of egg white (measured at the end of a 14 ns electron pulse) was found to be 86% of that determined in neat water, which can be attributed to the reaction of dry electrons with the protein constituents. Steady-state γ radiolysis studies on air-saturated thick and thin fraction (true solution) revealed that the immobility of egg white proteins disfavors reactions that result in molecular mass change.

Removing Estrogenic Steroids from Waters: The Role of Reducing Hydrated Electron Reactions

AbstractThe presence of both synthetic and natural steroids in waters is currently one of the most pressing concerns to water treatment utilities. The use of radical-based advanced oxidation/reduction processes (AO/RPs) to augment standard water treatment methods may be of interest. In support of the application of these AO/RP rate constants have been determined for the reaction of the reducing hydrated electron (e

Mechanism of azo dye degradation in Advanced Oxidation Processes: Degradation of Sulfanilic Acid Azochromotrop and its parent compounds in aqueous solution by ionizing radiation

Mechanistic studies were made on hydroxyl radical and hydrated electron reaction with Sulfanilic Acid Azochromotrop (SPADNS) as model azo dye in dilute aqueous solution. SPADNS contains 4,5-dihydroxynaphthalene-2,7-disulfonic acid part and 4-sulfophenylazo group. To establish the details of the reaction mechanism the reactions of two simpler molecules without 4-sulfophenylazo part were also studied: one of them contained one (in position 4, II), the other two (in positions 4 and 5, III) –OH groups. Hydroxyl radicals react with these molecules with radical addition to the naphthalene-2,7-disulfonic acid part. The adduct hydroxycyclohexadienyl type radical decays in radical–radical reactions, or undergoes a (pH dependent) water elimination to yield naphthoxy radical. The radical decay takes place on the ms timescale. Degradation efficiencies are 0.6−0.8. Hydrated electron in the case of the two simpler molecules reacts with the rings, while in the case of dye with the azo bond. Electron scavenging is followed by protonation, this reaction in the case of II and III yields cyclohexadienyl, while with the dye hydrazo radical. The efficiency of degradation with II and III is 0.2−0.6, while for SPADNS it is close to 1.

Radiolytic degradation of 2,4-dichlorophenoxyacetic acid in dilute aqueous solution: pH dependence

The reactions of hydroxyl radical and hydrated electron intermediates of water radiolysis were studied in the radiolytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) at pH values of 4, 6 and 8. The hydrated electron reactions are also suggested to contribute to the aromatic ring decomposition in addition to the highly effective hydroxyl radical reactions. The experimental results suggest also some contribution from the O 2 −• /HO 2 • pair to the degradation. The degradation efficiency was found to be the highest at pH 8 and the lowest at pH 6.

H atom yields in the radiolysis of water

Experimental measurements of molecular hydrogen including the use of isotopic techniques are combined with track model calculations to determine H atom yields in the radiolysis of water. The production of H atoms in the radiolysis of water is relatively small, but is important for fundamental considerations. H atoms are formed by the decay of excited states of water at the sub-picosecond timescale and by hydrated electron reactions during the diffusion-kinetic evolution of the ion track up to microseconds. The competition between H atom loss by combination reactions and its formation by reactions of the hydrated electron makes prediction of the H atom kinetics very difficult. H atom yields determined by subtraction of total molecular hydrogen production in neat water from that observed in the formate solutions is found to agree well with the more direct measurement of HD using deuterated formate as a scavenger. H atom yields are found to decrease with the evolution of the radiation track and decrease with increasing LET. Monte Carlo track model calculations are used to analyze the measured experimental yields and to elucidate the underlying H atom kinetics in the radiolysis of water.

Free Radical-Induced Redox Chemistry of Platinum-Containing Anti-cancer Drugs

Arrhenius parameters for the reactions of oxidizing hydroxyl radicals and reducing hydrated electrons with cisplatin, transplatin and carboplatin in aqueous solution have been determined using pulsed electron radiolysis and absorption spectroscopy techniques. Under physiological pH and chloride concentration conditions, hydroxyl radical reaction rate constants of (9.99 +/- 0.20) x 10(9), (8.38 +/- 0.55) x 10(9), and (6.03 +/- 0.08) x 10(9) M(-1) s(-1) at 24.0, 20.7 and 24.0 degrees C, respectively, with corresponding activation energies of 12.79 +/- 0.57, 13.88 +/- 1.14, and 14.35 +/- 0.56 kJ mol(-1) for these three reactions, were determined. These oxidations of cisplatin and transplatin to form a Pt(III) transient are significantly faster than reported previously at room temperature. The lower rate constant for carboplatin is consistent with hydroxyl radical reaction partitioning between reaction at the platinum center and the cyclobutanedicarboxylate ligand. The equivalent reductive hydrated electron reaction rate constants measured were (1.99 +/- 0.04) x 10(10) (24.0 degrees C), (1.77 +/- 0.08) x 10(10) (22.0 degrees C), and (8.92 +/- 0.06) x 10(9) M(-1) s(-1) (24.0 degrees C), with corresponding activation energies of 15.75 +/- 1.00, 19.74 +/- 1.82, and 19.99 +/- 0.34 kJ mol(-1). Again, the values determined for cisplatin and transplatin are faster than reported; however, all three values are consistent with direct reduction of the platinum center to form a Pt(I) moiety.

Free-Radical-Induced Oxidative and Reductive Degradation of Sulfa Drugs in Water: Absolute Kinetics and Efficiencies of Hydroxyl Radical and Hydrated Electron Reactions

Absolute rate constants and degradation efficiencies for hydroxyl radical and hydrated electron reactions with four different sulfa drugs in water have been evaluated using a combination of electron pulse radiolysis/absorption spectroscopy and steady-state radiolysis/high-performance liquid chromatography measurements. For sulfamethazine, sulfamethizole, sulfamethoxazole, and sulfamerazine, absolute rate constants for hydroxyl radical oxidation were determined as (8.3 +/- 0.8) x 10(9), (7.9 +/- 0.4) x 10(9), (8.5 +/- 0.3) x 10(9), and (7.8 +/- 0.3) x 10(9) M(-1) s(-1), respectively, with corresponding degradation efficiencies of 36% +/- 6%, 46% +/- 8%, 53% +/- 8%, and 35% +/- 5%. The reduction of these four compounds by their reaction with the hydrated electron occurred with rate constants of (2.4 +/- 0.1) x 10(10), (2.0 +/- 0.1) x 10(10), (1.0 +/- 0.03) x 10(10), and (2.0 +/- 0.1) x 10(10) M(-1) s(-1), respectively, with efficiencies of 0.5% +/- 4%, 61% +/- 9%, 71% +/- 10%, and 19% +/- 5%. We propose that hydroxyl radical adds predominantly to the sulfanilic acid ring of the different sulfa drugs based on similar hydroxyl radical rate constants and transient absorption spectra. In contrast, the variation in the rate constants for hydrated electrons with the sulfa drugs suggests the reaction occurs at different reaction sites, likely the different heterocyclic rings. The results of this study provide fundamental mechanistic parameters, hydroxyl radical and hydrated electron rate constants, and degradation efficiencies that are critical for the evaluation and implementation of advanced oxidation processes (AOPs).

Kinetics of hydrated electron reactions with phosphate anions: a laser photolysis study

The decay kinetics of hydrated electron (eaq−) formed upon photolysis of aqueous solutions of sodium pyrene-1,3,6,8-tetrasulfonate at λ = 337 nm in the presence of phosphate anions (up to 2 mol L−1) was studied by nanosecond laser-pulse photolysis in a wide range of pH (3.5–10) and ionic strength (I, up to 2 mol L−1) values. At high pH values, where the HPO42− ions dominate, the eaq− decay kinetics depends only slightly on phosphate concentration (rate constant for the reaction is at most 2·105 L mol−1 s−1). The H2PO4− ions react with eaq− at a rate constant of 2.8·106 L mol−1 s−1 (I = 0), which increases linearly with the parameter \(\exp ({{\sqrt I } \mathord{\left/ {\vphantom {{\sqrt I } {1 + \sqrt I }}} \right. \kern-\nulldelimiterspace} {1 + \sqrt I }})\) in accordance with the Debye-Huckel theory. The rate constant for quenching of eaq− by H3PO4 at pH ≤ 4 decreases linearly with the parameter \(\exp ({{\sqrt I } \mathord{\left/ {\vphantom {{\sqrt I } {1 + \sqrt I }}} \right. \kern-\nulldelimiterspace} {1 + \sqrt I }})\) due to the secondary salt effect and equals 1.6·109 L mol−1 s−1 at I = 0. The logarithm of the rate constant for quenching of eaq− by phosphates is linearly related to the number of the O-H bonds in the phosphate molecule.