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.     

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.     

Selective Photoelectrochemical Reduction of Aqueous CO2 to CO by Solvated Electrons

Reduction of CO₂ by direct one‐electron activation is extraordinarily difficult because of the ?1.9 V reduction potential of CO₂. Demonstrated herein is reduction of aqueous CO₂ to CO with greater than 90 % product selectivity by direct one‐electron reduction to CO₂^.? by solvated electrons. Illumination of inexpensive diamond substrates with UV light leads to the emission of electrons directly into water, where they form solvated electrons and induce reduction of CO₂ to CO₂^.?. Studies using diamond were supported by studies using aqueous iodide ion (I^?), a chemical source of solvated electrons. Both sources produced CO with high selectivity and minimal formation of H₂. The ability to initiate reduction reactions by emitting electrons directly into solution without surface adsorption enables new pathways which are not accessible using conventional electrochemical or photochemical processes.     

Emission phenomena in electrochemical systems: Investigation of solvated and delocalized electrons in polar media

The principles of using the results of studies of emission phenomena in electrochemical systems for determination of the energy levels of excess (solvated and delocalized) electrons in electrolyte solutions are formulated. The energy characteristics of excess electrons in hexamethylphosphoric triamide have been estimated on the basis of data obtained by the author and literature data (results of photo- and thermoelectron emission measurements and equilibria studies in solutions of solvated electrons). The energy levels of solvated electron and bi-electron, their photoionization energies and solvent reorganization energies have been found. The nature of the absorption spectra is discussed.     

Dynamics of Electrons in Ammonia Cages: The Discovery System of Solvation

Two centuries ago solvated electrons were discovered in liquid ammonia and a century later the concept of the solvent cage was introduced. Here, we report a real time study of the dynamics of size‐selected clusters, n=20 to 60, of electrons in ammonia, and, for comparison, that of electrons in water cages. Unlike the water case, the observed dynamics for ammonia indicates the formation, through a 100 fs temperature jump, of a solvent collective motion in a 500 fs relaxation process. The agreement of the experimental results—obtained for a well‐defined n, gated electron kinetic energy, and time delay—with molecular dynamics theory suggests the critical and different role of the kinetic energy and the librational motions involved in solvation.     

On the diffusive nature of solvated electron transport in polar liquids

We quantitatively examine the validity of the relationship μ = Cη^-1 between the mobility (μ) of excess electrons in pure polar solvents and the viscosity (η) of the solvent. In the majority of cases, the solvent electron mobilities do follow the η^-1 behavior, indicating a diffusive mechanism of electron transport in these liquids. Application of Eyring's theory of absolute reaction rates to diffusion and viscosity enables us to estimate the radii of solvated electrons in polar liquids.     

Drift Mobility of Electrons in Pyrazoline-Containing Copolymers

Drift mobility of electrons in copolymers containing pyrazolyl fragment in the main or side chain of the copolymer is studied experimentally using the time-of-flight method. The mobility is an exponential function of the applied electric field, and its values for the copolymers range from 5 × 10^–7 to 2 × 10^–5 cm² V^–1 s^–1 at the field strength ranging from 5 × 10⁴ to 1 × 10⁶ V cm^–1. Based on the measured mobilities and the molecule parameters, which were obtained by a PM3 quantum-chemical calculation, the role of copolymer fragments in the transport of charge carriers is discussed.     

Hot Electrons at Solid–Liquid Interfaces: A Large Chemoelectric Effect during the Catalytic Decomposition of Hydrogen Peroxide

The study of energy and charge transfer during chemical reactions on metals is of great importance for understanding the phenomena involved in heterogeneous catalysis. Despite extensive studies, very little is known about the nature of hot electrons generated at solid–liquid interfaces. Herein, we report remarkable results showing the detection of hot electrons as a chemicurrent generated at the solid–liquid interface during decomposition of hydrogen peroxide (H₂O₂) catalyzed on Schottky nanodiodes. The chemicurrent reflects the activity of the catalytic reaction and the state of the catalyst in real time. We show that the chemicurrent yield can reach values up to 10^?1 electrons/O₂ molecule, which is notably higher than that for solid–gas reactions on similar nanodiodes.     

Manipulating the production and recombination of electrons during electron transfer: Femtosecond control of the charge-transfer-to-solvent (CTTS) dynamics of the sodium anion

The scavenging of a solvated electron represents the simplest possible electron-transfer (ET) reaction. In this work, we show how a sequence of femtosecond laser pulses can be used to manipulate an ET reaction that has only electronic degrees of freedom: the scavenging of a solvated electron by a single atom in solution. Solvated electrons in tetrahydrofuran are created via photodetachment using the charge-transfer-to-solvent (CTTS) transition of sodide (Na(-)). The CTTS process ejects electrons to well-defined distances, leading to three possible initial geometries for the back ET reaction between the solvated electrons and their geminate sodium atom partners (Na(0)). Electrons that are ejected within the same solvent cavity as the sodium atom (immediate contact pairs) undergo back ET in approximately 1 ps. Electrons ejected one solvent shell away from the Na(0) (solvent-separated contact pairs) take hundreds of picoseconds to undergo back ET. Electrons ejected more than one solvent shell from the sodium atom (free solvated electrons) do not recombine on subnanosecond time scales. We manipulate the back ET reaction for each of these geometries by applying a "re-excitation" pulse to promote the localized solvated electron ground state into a highly delocalized excited-state wave function in the fluid's conduction band. We find that re-excitation of electrons in immediate contact pairs suppresses the back ET reaction. The kinetics at different probe wavelengths and in different solvents suggest that the recombination is suppressed because the excited electrons can relocalize into different solvent cavities upon relaxation to the ground state. Roughly one-third of the re-excited electrons do not collapse back into their original solvent cavities, and of these, the majority relocalize into a cavity one solvent shell away. In contrast to the behavior of the immediate pair electrons, re-excitation of electrons in solvent-separated contact pairs leads to an early time enhancement of the back ET reaction, followed by a longer-time recombination suppression. The recombination enhancement results from the improved overlap between the electron and the Na(0) one solvent shell away due to the delocalization of the wave function upon re-excitation. Once the excited state decays, however, the enhanced back ET is shut off, and some of the re-excited electrons relocalize even farther from their geminate partners, leading to a long-time suppression of the recombination; the rates for recombination enhancement and relocalization are comparable. Enhanced recombination is still observed even when the re-excitation pulse is applied hundreds of picoseconds after the initial CTTS photodetachment, verifying that solvent-separated contact pairs are long-lived, metastable entities. Taken together, all these results, combined with the simplicity and convenient spectroscopy of the sodide CTTS system, allow for an unprecedented degree of control that is a significant step toward building a full molecular-level picture of condensed-phase ET reactions.     

Moving solvated electrons with light: nonadiabatic mixed quantum/classical molecular dynamics simulations of the relocalization of photoexcited solvated electrons in tetrahydrofuran (THF)

Motivated by recent ultrafast spectroscopic experiments [Martini et al., Science 293, 462 (2001)], which suggest that photoexcited solvated electrons in tetrahydrofuran (THF) can relocalize (that is, return to equilibrium in solvent cavities far from where they started), we performed a series of nonequilibrium, nonadiabatic, mixed quantum/classical molecular dynamics simulations that mimic one-photon excitation of the THF-solvated electron. We find that as photoexcited THF-solvated electrons relax to their ground states either by continuous mixing from the excited state or via nonadiabatic transitions, approximately 30% of them relocalize into cavities that can be over 1 nm away from where they originated, in close agreement with the experiments. A detailed investigation shows that the ability of excited THF-solvated electrons to undergo photoinduced relocalization stems from the existence of preexisting cavity traps that are an intrinsic part of the structure of liquid THF. This explains why solvated electrons can undergo photoinduced relocalization in solvents like THF but not in solvents like water, which lack the preexisting traps necessary to stabilize the excited electron in other places in the fluid. We also find that even when they do not ultimately relocalize, photoexcited solvated electrons in THF temporarily visit other sites in the fluid, explaining why the photoexcitation of THF-solvated electrons is so efficient at promoting recombination with nearby scavengers. Overall, our study shows that the defining characteristic of a liquid that permits the photoassisted relocalization of solvated electrons is the existence of nascent cavities that are attractive to an excess electron; we propose that other such liquids can be found from classical computer simulations or neutron diffraction experiments.     

Inverse solvent effects in carbocation carbanion combination reactions: the unique behavior of trifluoromethylsulfonyl stabilized carbanions

Second-order rate constants for the reactions of the trifluoromethylsulfonyl substituted benzyl anions 1a-e (CF3SO2CH(-)-C6H4-X) with the benzhydrylium ions 2f-j and structurally related quinone methides 2a-e have been determined by UV-vis spectroscopy. The reactions proceed approximately 10-40 times faster in methanol than in DMSO leading to the unique situation that these carbocation carbanion combinations are faster in protic than in dipolar aprotic media. The pK(a) values of some benzyl trifluoromethylsulfones were determined in methanol (1c-H, 17.1; 1d-H, 16.0; 1e-H, 15.0) and found to be 5 units larger than the corresponding values in DMSO. Rate and equilibrium measurements thus agree that the trifluoromethylsulfonyl substituted benzyl anions 1a-e are more effectively solvated by ion-dipole interactions in DMSO than by hydrogen bonding in methanol. Br?nsted correlations show that in DMSO the trifluoromethylsulfonyl substituted carbanions 1 are less nucleophilic than most other types of carbanions of similar basicity, indicating that in DMSO the intrinsic barriers for the reactions of the localized carbanions 1 are higher than those of delocalized carbanions, including nitroalkyl anions. The situation is reversed in methanol, where the reactions of the localized carbanions 1 possess lower intrinsic barriers than those of delocalized carbanions as commonly found for proton-transfer processes. As a consequence, the relative magnitudes of intrinsic barriers are strongly dependent on the solvent.     

Electrochemical Generation of Hot Electrons in Fully Aqueous Solutions at Tetrahedral Amorphous Carbon Thin Film Electrodes and Electrochemiluminescence Immunoassay of Serum Amyloid A

Electrochemistry of hot electrons in fully aqueous solutions at tetrahedral amorphous carbon thin film electrodes is discussed. The generation of these highly reducing chemical species was confirmed by normal pulse voltammetry and several electrochemiluminescent systems. Electron transfer into pre-existing solvent cavities was observed at approximately −2.65 V vs. Ag/AgCl (sat.). Electrogenerated hot electrons were utilized as chemiluminescent mediators in heterogeneous sandwich immunoassay of Serum Amyloid A. The calibration curve was linear over four orders of magnitude and the detection limit was 85 ng L⁻¹ that demonstrates the efficiency of hot electron generation at this electrode material.     

Photoconductivity of presolvated electrons in ethanol glass at 4 K

Photoconductivity of localized electrons, which are not solvated, in γ-irradiated ethanol glass at 4 K was studied. Photocurrents of 10^?13 A or larger were stably detected in the wavelength range of 900–2200 nm. The wavelength dependence of the photocurrent reveals that pre-solvated electrons probably have no bound excited states.     

电子自旋、微观可区分性及化学键

本文从分析电子自旋磁矩 (磁极 )的空间性质入手,讨论了电子的可区分性。通过讨论2个电子自旋组态的 8种形式,其中,包括 4种磁极吸引的耦合态、4种磁矩排斥的非耦合态,同理,电子轨旋运动也存在 4种耦合态。自旋耦合、轨旋全耦合需要 8个电子,所以元素周期性为 8音律。磁矩耦合是形成化学键的第一要求,第二才是异核吸引作用 ;化学键的广义表达语言应该是:化学键只能由磁矩耦合的电子组成。对电子的波粒二象性和测不准原理进行了新的理论解释,并讨论了波粒二象性和测不准现象的物理模型。该模型与电子的微观可区分性相一致。     

The solvation of two electrons in the gaseous clusters of Na- (NH3)n and Li- (NH3)n

Alkali metal ammonia clusters, in their cationic, neutral, and anionic form, are molecular models for the alkali-ammonia solutions, which have rich variation of phases with the solvated electrons playing an important role. With two s electrons, the Na(-)(NH(3))(n) and Li(-)(NH(3))(n) clusters are unique in that they capture the important aspect of the coupling between two solvated electrons. By first principles calculations, we demonstrate that the two electrons are detached from the metal by n = 10, which produces a cluster with a solvated electron pair in the vicinity of a solvated alkali cation. The coupling of the two electrons leads to either the singlet or triplet state, both of which are stable. They are also quite distinct from the hydrated anionic clusters Na(-)(H(2)O)(n) and Li(-)(H(2)O)(n), in that the solvated electrons are delocalized and widely distributed among the solvent ammonia molecules. The Na(-)(NH(3))(n) and Li(-)(NH(3))(n) series, therefore, provide another interesting type of molecular model for the investigation of solvated electron pairs.     

Unearthing the Mechanism of Prebiotic Nitrile Bond Reduction in Hydrogen Cyanide through a Curious Association of Two Molecular Radical Anions

HCN is clearly associated with the prebiotic chemical evolution of life. It has been known for decades that the radiolysis of HCN solutions produces sugars, amino acids and nucleobases. Remarkably, recent experimental studies have shown that the photolytic reduction of aqueous HCN by a photoredox reagent [Cu(CN)₃]^2? specifically yields sugars, which are the essential building blocks of RNA. Although a mechanistic understanding of such reductions with solvated electrons is poor, the general consensus is that they involve neutral free radicals. We show herein through the use of electronic structure studies and molecular simulations that the reduction of the nitrile bond of HCN is initiated through the formation of a molecular dipole‐bound anion from the photoredox reagent. Our theoretical studies show how HCN binds to the photoexcited reagent and then extracts an electron from the reagent and is ultimately detached as a dipole‐bound anion. The dipole‐bound anionic form of [HCN]^? can easily convert into a solvated valence‐bound form of [HCN]^?. After the formation of solvated [HCN]^?, an extraordinary chemical event ensues through a counter‐intuitive coupling of two valence‐bound anions to form a solvated molecular dianionic intermediate, [HCN]₂^2?. Finally, a proton‐coupled electron transfer occurs within the dianionic entity to complete the reduction. This mechanistic scenario is applicable to the reduction of other prebiotic nitrile species and avoids neutral radical‐based pathways, thereby preventing the proliferation of reactive species and preserving chemical selectivity. Furthermore, we show how such similar nitrile reduction pathways operate to yield the sugar precursors.     

Inter- or intramolecular electron transfer between triruthenium clusters: we’ll cross that bridge when we come to it

The purpose of this review is to examine the fundamental differences between intermolecular self-exchange vs. intramolecular ET in mixed-valence complexes based on similar triruthenium structural units. The role of orbital overlap between ancillary ligands of the electron donor and acceptor are considered in self-exchange reactions which are found to be strongly adiabatic and again in bridged mixed-valence systems. The method of infrared (IR) reflectance spectroelectrochemistry for the determination of extremely fast (10¹¹–10¹³ s^?1) ET rate constants is reviewed as a tool to provide quantitative information about the time scales of localization and delocalization. The role of internal vibrations of the bridging ligand in strongly delocalized mixed-valence ions is investigated by resonance Raman and IR spectroscopies. The role of solvent dipolar relaxation times in determining the rates of ultrafast intramolecular ET reactions is reviewed in the context of inorganic mixed-valence chemistry. Finally, the concept of Robin–Day Class II/III “borderline” complexes is considered, and a concise definition of the localized to delocalized transition is provided in terms of the relative contributions of external solvent and internal complex ion vibrational modes to ET.     

Modification of Particle Filled Polymers with High Energy Electrons Under In-Stationary Conditions of Melt Mixing

Summary: Polymer modification with high energy electrons is well-established in polymer industry and used for degradation, cross-linking, grafting, curing, and polymerization. These applications use local and temporal precise input of energy in order to generate excited atoms or molecules and ions for subsequent molecule changes via radical induced chemical reactions. In the present study, high energy electrons have been used to modify polyolefine (polyethylene and polypropylene) systems in presence of a grafting agent under stationary and in-stationary conditions. Polymer modification with high energy electrons under stationary conditions characterizes a process where required absorbed dose is applied to polymers in solid state and at room temperature. Polymer modification with high energy electrons under in-stationary conditions is a novel process where required absorbed dose is applied in molten state during melt mixing process. In this novel process, the penetration depth of electrons is limited to a part of mixing volume. The total mixing volume is modified due to the change of polymer mass within the penetration depth of electrons during mixing process. A 1.5 MeV electron accelerator has been directly coupled to a banbury mixing chamber in order to study this novel process. In comparison to the stationary process, the main differences are working at higher temperature, absence of any crystallinity, intensive macromolecular mobility as well as intensive mixing during dose application. The influence of both processes on mechanical properties and flame resistance of polymer composites is discussed.     

The impedance of solutions of AOT/water micelles in hexane

Broadband dielectric spectroscopy was used to study the electric properties of solutions of reverse AOT/water micelles in hexane. An analysis of the frequency dependences of the complex electric modulus allowed us to find the region of frequencies in which dc-conductivity was observed and exclude the region of electrode effects. At frequencies f of ～ 10⁴Hz, the field dependences of dc-conductivity changed from linear (the Ohm law) to quadratic (the Mott law) as the volume fraction of micelles increased. This was evidence of a strengthening of the effect of current limitation by a volume charge. The upper and lower limits of the drift mobility of carriers μ responsible for dc-conductivity were estimated as 0.1 cm^?2 V^?1 s^?1 < μ < 0.3 cm^?2 V^?1 s^?1, which was close to the mobility of electrons in hexane. This allowed us to relate the nature of current carriers to that of free electrons; the activation energy of electron creation was found to be E _a ≈ 0.41 eV. The electron lifetime up to its trapping by acceptors was estimated. The results obtained and the literature data on the rate constants of such reactions led us to conclude that micelles were capable of absorbing acceptor impurities from solvents (additional solvent purification).