Solvated and Stabilized Electrons in Radiation-Chemical Processes

The electrons solvated in metal-ammonia solutions are relatively stable; by contrast, hydrated electrons are very unstable and have been discovered only recently during radiolysis of water. They can be regarded as the simplest radicals. Radiation chemical production of solvated electrons has proved to be a particularly elegant method for the qualitative and quantitative investigation of the reactions between these electrons and numerous compounds, whose rates are partly controlled by diffusion. It has been possible in some cases to identify optically and ESR-spectroscopically the resulting short-lived products (radical-anions). The similarity between the physical and chemical properties of electrons solvated in solutions and those of electrons stabilized in the solid phase suggests that the two species are identical.     

Formation and Properties of Solvated Electrons

In the formulation of many chemical reactions, electrons are regarded as readily transferable particles, though their participation in these reactions cannot be directly observed. However, the discovery that electrons can be produced in various ways in suitable solutions and that they are stabilized by solvation and can thus be studied directly has recently led to a rapid growth of interest in these, the simplest and most reactive particles of chemistry. The solvated electron has physical properties that permit its detection by various methods even at very low concentrations, so that it is also possible to follow its many reactions, most of which are extremely fast.     

Polarographic behavior of methyl benzenesulfinates

The polarographic reduction of methyl benzenesulfinate is irreversible and involves a fission of the sulfur-oxygen bond. In aqueous solutions the ester is reduced by 4 electrons per molecule to thiphenol and methanol. The main electrochemical process in DMF is a splitting of the S?O linkage by 2 electrons. Coulometric experiments in the aprotic solvent yield products (thiophenolate ion, benzenesulfinate ion and methanol), which originate from the electrode reaction as well as from secondary reactions. Disporportionation of benzenesulfenate ion and basic hydrolysis of the original ester were considered as possible secondary reactions.     

The chemistry of positronium. Inhibition and reactivity of solutes with electrons

Examination of the inhibition of positronium formation in aqueous solutions, by a wider range of solutes shows that inhibiting power is not determined only by the rate constant for the reaction of the solute with aquated electrons, nor by the ability of the solute to inhibit formation of aquated electrons in fast pulse radiolysis. The inhibition by selected solutes in different solvents has been investigated. Inhibition is not necessarily greater in the less polar solvents. The data suggest that the spur model for positronium formation in solutions is probably only one of the mechanisms involved. Both inhibition and quenching reactions of solutes are sensitive to the structure of the solute; the maleate and fumarate anions showing quantitatively very different behaviour.     

甲醇溶液辉光放电等离子体电解

甲醇溶液辉光放电等离子体电解过程出现明显的非法拉第定律现象, 主要产物是氢气和甲醛, 还有少量一氧化碳、甲烷、乙烷、丙烷、1,3,5-三噁烷和水等, 产物和产量受放电极性和辅助电解质及放电电压等因素的影响. 在甲醇溶液电导率为11.40 mS·cm-1, 放电电压700 V 条件下, 阳极气体产量为55.90 mol/(mol electrons), 阴极气体产量为707.90 mol/(mol electrons), 阴极气体产量是阳极气体产量的12.66 倍, 气相产物中氢气含量在86%(molar fraction)以上. 在等离子体层中甲醇分解过程和其它类型的等离子体分解过程类似, 蒸汽鞘层中的加速电子是引发辉光放电过程非法拉第定律现象的决定因素. 阴极辉光放电过程中等离子体-溶液界面上的主要活性物种是中性粒子和电子,阳极辉光放电过程中等离子体鄄溶液界面上的主要活性物种是中性粒子和正离子. 辅助电解质对产物的影响主要是通过影响界面上发生的后续反应过程来表现.     

Anti-inhibition and Promotion of Radiolytic Hydrogen Formation in Water by Hydroxonium Ions

The ability of hydroxonium ions to enhance the yield of radiolytic hydrogen in aqueous potassium nitrate and potassium chloride solutions is shown. The proposed explanation of the effect is based on the concept of a presolvated electron as a hydrogen precursor. The interception of electrons by H⁺ _aq ions yielding the weakly bound transient species (H⁺ _aq...e^–) retards the hydration of electrons, thus providing a possibility of their longer involvement in hydrogen formation reactions and, hence, enhancement of the yield of hydrogen. The revealed effect is similar to the phenomenon known in positronium chemistry as hydrogen anti-inhibition occurring in a nonpolar liquid containing an electron scavenger, when a second solute that, unlike the first solute, weakly binds electrons is added.     

Physical and Chemical Properties of Dissolved Electrons (Excess Electrons)

Electrons produced in a gaseous, liquid, or solid solvent are called dissolved electrons or excess electrons. These excess electrons can exist as quasi-free particles of high mobility in a delocalized state, comparable with electrons in a metal; or as bound particles of low mobility they can be localized within narrow limits—in a solvent cavity formed by repulsive forces. Localized electrons can also be solvated like normal ions. Characteristically, such solvated electrons exhibit broad and extensive absorption spectra in the visible to near infrared spectral range. The localized and delocalized states of the excess electrons can be in equilibrium with each other, such that a continuous transition of the properties between the limiting extremes can be observed. The reactions of the excess electrons with suitable acceptors (substrates) are initiated by an attachment-detachment equilibrium A + e^? ? A^? which is followed by further chemical rearrangements. The rate constants of these reactions vary by more than 15 powers of ten depending on the substrates and the solvents. Most of the properties of excess electrons in solution can be interpreted by means of a model which is easily understandable but quantitatively evaluated only with considerable effort.     

Kinetic and mechanistic investigations of multielectron transfer reactions induced by stored electrons in TiO2 nanoparticles: a stopped flow study

The kinetics and the mechanism of various multielectron transfer reactions initiated by stored electrons in TiO(2) nanoparticles have been investigated employing the stopped flow technique. Moreover, the optical properties of the stored electrons in the TiO(2) nanoparticles have been studied in detail following the UV (A) photolysis of deaerated aqueous suspensions of TiO(2) nanoparticles in the presence of methanol. The reduction of common electron acceptors that are often present in photocatalytic systems such as O(2), H(2)O(2), and NO(3)(-) has been investigated. The experimental results clearly show that the stored electrons reduce O(2) and H(2)O(2) to water by multielectron transfer processes. Moreover, NO(3)(-) is reduced via the transfer of eight electrons evincing the formation of ammonia. On the other hand, the reduction of toxic metal ions, such as Cu(II), has been studied mixing their respective anoxic aqueous solutions with those containing the electrons stored in the TiO(2) particles. A two-electron transfer is found to occur, indicating the reduction of the copper metal ion into its non toxic metallic form. Other metal ions, such as Zn(II) and Mn(II), could not be reduced by TiO(2) electrons, which is readily explained on the bases of their respective redox potentials. The underlying reaction mechanisms are discussed in detail.     

Local radiolytic effectiveness of Auger electrons of iodine-125 in benzene-iodine solutions

The gas-chromatographic analysis of benzene-iodine solutions containing molecular iodine-125 after various storage times showed that no radiolytic products appeared while some products such as benzene, iodobenzene, biphenyl, and iodobiphenyl, were detected under the same analytical conditions in our recent studies concerning the reactions of iodine atoms activated by L shell photoelectric ionization in benzene-iodine solutions, and the self-radiolysis of iodobenzene labelled with iodine-125¹⁵. On the other hand, our theoretical calculations showed that if iodine molecules were uniformly distributed in benzene, iodobenzene should be detected as a radiolytic product due to the local absorption of Auger electrons of iodine-125. The absence of any radiolytic product clearly demonstrated that iodine aggregates have an important role in the absorption of Auger electrons of iodine-125 in benzene-iodine solutions. In addition, theoretical calculations showed that if the iodine aggregation is taken into account the experimental result agrees well with theoretical calculations.     

Determination of Some Catecholamines by Coulometric Titration and Cyclic Voltammetry

Catecholamines quantitatively react with electrogenerated halogens. The reactions of adrenaline with bromine and iodine involve three electrons, and the reactions of dopamine with chlorine and bromine involve two electrons. Possible reaction schemes are proposed. Adrenaline and dopamine are reversibly oxidized at graphite and glassy-carbon electrodes in acid media at 0.7 and 0.58 V, respectively, to form corresponding quinones. Determination limits and linearity ranges of oxidation current were found as functions of catecholamine concentrations. The relative standard deviation was 1–4% for model solutions. A procedure was proposed for the electrochemical determination of adrenaline and dopamine in pharmaceuticals.__________Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 7, 2005, pp. 753–758.Original Russian Text Copyright © 2005 by Ziyatdinova, Budnikov.     

A theory of elementary photophysical processes with the participation of excess electrons in polar liquids

The main concepts of the new theory of processes with the participation of excess electrons in polar liquids are considered. The theory takes into account that (1) polar liquids are electrostatically inhomogeneous (local potentials on molecules are different) and (2) a molecule can accept an electron for a short time to produce an anion in an unstable state with a certain energy and lifetime. A discrete model of a substance consisting of molecules with constant dipole moments is used. Excess electrons in a liquid are described by energy distribution density, and the behavior of electrons, by quantum mechanics equations. The experimental data on the photoionization of water and aqueous solutions of salts and the low threshold energy of photons (～6.5 eV) at which solvated electrons appear in water are explained. The absorption spectra of water with excess electrons at the first and subsequent time moments after their photogeneration are reproduced theoretically. The dependence of the photoemission of solvated electrons from potassium-ammonia solutions on the energy of photons is interpreted. The continuous spectrum of spontaneous radiation of solvated electrons in liquid ammonia and water is calculated. The optical absorption spectra of solvated electrons in such polar liquids as water and ammonia are reproduced.     

Protonated (protosolvated) onium ions (onlum dications)

In this short review we have shown the importance of protosolvation of onium ions (containing non-bonded pairs of electrons) in superacid catalyzed reactions. Such activation can result in unusual reactions such as aromatic alkylation with Meerwein’s salts, aliphatic nitration with nitronium ion, alkylation of saturated hydrocarbons, greatly enhanced activity of acyl cations, etc. Possibly such phenomena may be operative in hydroxylation reactions using protonated hydrogen peroxide in strong acid solutions. Even the reactivity of halonium ions could be enhanced by protosolvation. Consequently, electrophilic protosolvation may play a significant role in strogg acid catalyzed reactions.     

Polarographische Untersuchungen an Solochromviolett RS in Pufferlösungen in Abwesenheit und in Gegenwart oberflächenaktiver Stoffe (SAS)

The polarographic behaviour of solochrome violet RS is investigated in buffer solutions of varying pH in the absence and presence of surfactants (triton 100-X and dodecylbenzene-sulphonate). The reduction proceeds irreversibly along a single wave in acid solutions and two waves in alkaline ones. The electrode reaction corresponds to four electrons in media of pH<2.5 and pH>8.0; in media of pH 4 5.5 two electrons are consumed. In solution of pH 2.5 4 or 5.5–8.0, both reactions contribute in varying magnitudes. The addition ofSAS causes the inhibition of reduction beyond the 2-electrons stage in acid solutions; the polarograms comprise one wave in case ofDBS and two waves of equal height in presence of triton. In alkaline media the polarogram comprises three waves due to the splitting of the main reduction wave. The kinetic parameters of the electrode reaction are also determined.     

Charge permutation reactions in tandem mass spectrometry

Central to the tandem mass spectrometry experiment is the process that gives rise to product ions, i.e. the reaction intermediate to stages of mass analysis. Changes in mass or charge of the parent ion (or both) are generally readily detected by all forms of tandem mass spectrometry. Charge changing, or charge permutation, reactions have a long history in mass spectrometry. However, with the advent of new ionization methods, such as electrospray ionization, and the expansion of tandem mass spectrometry instrumentation to include ion trapping instruments, the past decade has seen a major increase in the types of charge permutation reactions that can be studied. Most charge permutation reactions involve electrons or protons as the charge mediating agents. This report, therefore, provides an overview of charge permutation reactions involving protons or electrons. Particular emphasis is placed on processes that involve interactions of precursor ions with gaseous neutral species, electrons or oppositely charged ions. Charge permutation reactions involving electron gain/loss are described first according to a rough order of the energy required for the reaction beginning with the most endoergic reactions and ending with the most exoergic reactions. An analogous approach is then taken with charge permutation reactions involving proton gain/loss. Important charge permutation reactions discussed herein, among others, include those referred to as charge inversion, charge stripping, electron capture dissociation, collision-induced ionization and charge separation. These reaction types, and others described herein, are the subjects of active research and are also finding use in many current areas of application. Copyright © 2004 John Wiley & Sons, Ltd.     

Electron transfer between colloidal ZnO nanocrystals

Colloidal ZnO nanocrystals capped with dodecylamine and dissolved in toluene can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction-band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals (e(-)(CB):ZnO-S) with a solution of uncharged large nanocrystals (ZnO-L) caused changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e(-)(CB):ZnO-L + ZnO-S, showed that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetics studies monitoring the change in the UV band-edge absorption showed that reactions of 50 μM nanocrystals were complete within the 5 ms mixing time of the instrument. Similar results were obtained for the reaction of charged nanocrystals with methyl viologen (MV(2+)). These and related results indicate that the electron-transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for studies of electron transfer involving semiconductor nanostructures.     

Characterization of potassium and sodium-potassium alloy solutions containing metal anions and complexed cations by means of NMR and ESR techniques

The influence of a complexing agent, kind of solvent and temperature on the stability and ionic composition of potassium and sodium-potassium alloy solutions containing metal anions and complexed cations as well as solvated electrons are discussed basing on the analysis of alkali metal NMR and ESR spectra. Surprisingly it seems that the stability of metal solutions in tetrahydrofuran at ambient temperature is inversely proportional to the durability of K+ complex in the case of five studied ligands. The most stable metal solutions were obtained using 15-crown-5. It was shown that the characteristic blue colour of metal solutions is not connected with the presence of solvated electrons but with metal anions.     

VUV photolysis of aqueous solutions of nitrate and nitrite

Water homolyses upon vacuum-uv excitation into HO^* radicals, hydrogen atoms and with lower efficiency, hydrated electrons. These primary species induce a series of reactions partially depleting nitrate and nitrite from aqueous solutions. Depletion rates depend on the presence of dissolved oxygen and temperature. Nitrate, nitrite, peroxynitrite and N₂O were identified as reaction products after irradiation of, either, nitrite and nitrate in aqueous solutions. A reaction mechanism is proposed in accord with the experimental facts and with the evidence given in the literature, where NO₂ ^* and NO^* are key intermediates. NO₃ ^?, NO₂ ^?, NO₂ ^*, NO^* O₂NO₂ ^?, ONO₂ ^? and N₂O, seem to be interrelated by many redox reactions and reaction equilibria where pH and the availability of electrons determine their occurrence. The proposed mechanism is supported by a computer program with which the observed experimental behavior could be simulated.     

The effect of encounters involving ions,excited molecules,and neutral radicals in a track on the delayed fluorescence of irradiated alkane solutions

A computer model has been developed to simulate the processes of the excited states formation in irradiated alkane solutions. The model includes the charge and energy transfer reactions as well as intratrack encounters involving excess electrons, radical ions, the excited states of molecules, and neutral radicals including spin effects. The model is applied to visualize the contact interactions and to establish their effect on the delayed fluorescence decay in nanosecond time domain after pulsed irradiation. The model predicts no significant influence of neutral alkyl radicals on the delayed fluorescence.     

Explicitly correlated second-order perturbation theory using density fitting and local approximations

Three major obstacles in electronic structure theory are the steep scalings of computer time with respect to system size and basis size and the slow convergence of correlation energies in orbital basis sets. Three solutions to these are, respectively, local methods, density fitting, and explicit correlation; in this work, we combine all three to produce a low-order scaling method that can achieve accurate MP2 energies for large systems. The errors introduced by the local approximations into the R12 treatment are analyzed for 16 chemical reactions involving 21 molecules. Weak pair approximations, as well as local resolution of the identity approximations, are tested for molecules with up to 49 atoms, over 100 correlated electrons, and over 1000 basis functions.     

A novel non-radioactive electron source for ion mobility spectrometry

Typical ion mobility spectrometers work by employing a radioactive source to provide electrons with high energy to ionize the analytes in a series of chemical reactions. General security as well as regulatory concerns related to the use of radioactivity resulted in a need for a different ionization source which on the other hand produces ions in a similar manner as a radioactive source because the mechanisms are well known. Here we introduce a novel non-radioactive electron source which is capable of providing high energy electrons in a way that is similar to beta radiating substances yielding correspondingly similar peak spectra.