Hydrated Electron Research Articles

Radiolytic yields of solvated electrons in ionic liquid and its solvation dynamics at low temperature

We present an investigation of the solvated electron in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (P14NTf2) using pulse radiolytic techniques. Temperature-dependent studies reveal that the yield of the solvated electron decreases with decreasing temperature. The lower initial yield measured indicates that we have a loss of some electrons before they become fully solvated. There may be a high probability that the excess dry electrons (pre-solvated electron) react before the electron solvation is completed because the solvation dynamics is slowing down with decreasing temperature.

Water-structure based mechanistic view on the bimolecular decay of the hydrated electron

The reaction between hydrated electrons (e−aq) is postulated to occur via proton transfer when electrons occupy neighbouring cavities in HB network. Subsequent recombination of a spatially correlated pair (H•, e−aq) leading to the formation of H2 is specific to backward scattering in rigid HB network stiffen by the presence of supramolecular structures of continuously connected four-bonded molecules. Disappearance of backward scattering between 150 and 200°C favours dissociation of (H•, e−aq). The postulated mechanism explains slower decay of e−aq in alkaline hydrogenated solutions above 150°C.

Role of Quantum Vibrations on the Structural, Electronic, and Optical Properties of 9-Methylguanine.

In this work, we report theoretical predictions of the UV-absorption spectra of 9-methylguanine using time dependent density functional theory (TDDFT). Molecular dynamics simulations of the hydrated DNA base are peformed using an empirical force field, Born-Oppenheimer ab initio molecular dynamics (AIMD), and finally path-integral AIMD to understand the role of the underlying electronic potential, solvation, and nuclear quantum vibrations on the absorption spectra. It is shown that the conformational distributions, including hydrogen bonding interactions, are perturbed by the inclusion of nuclear quantum effects, leading to significant changes in the total charge and dipole fluctuations of the DNA base. The calculated absorption spectra using the different sampling protocols shows that the inclusion of nuclear quantum effects causes a significant broadening and red shift of the spectra bringing it into closer agreement with experiments.

The effect of air on solvated electron chemistry at a plasma/liquid interface

Plasmas in contact with liquids initiate complex chemistry that leads to the generation of a wide range of reactive species. For example, in an electrolytic configuration with a cathodic plasma electrode, electrons from the plasma are injected into the solution, leading to solvation and ensuing reactions. If the gas contains oxygen, electronegative oxygen molecules may react with the plasma electrons via attachment to reduce the electron flux to the solution reducing the production of solvated electrons or produce reactive oxygen species that quickly scavenge solvated electrons in solution. Here, we applied a total internal reflection absorption spectroscopy technique to compare the concentration of solvated electrons produced in solution by an argon plasma containing various amounts of oxygen, nitrogen, and air. Our measurements indicate that in oxygen or air ambients, electron attachment in the plasma phase greatly attenuates the electron flux incident on the liquid surface. The remaining electrons then solvate but are quickly scavenged by reactive oxygen species in the liquid phase.

Photoredox Properties of Homoleptic d6 Metal Complexes with the Electron‐Rich 4,4′,5,5′‐Tetramethoxy‐2,2′‐bipyridine Ligand

AbstractA synthetic procedure leading to 4,4′,5,5′‐tetramethoxy‐2,2′‐bipyridine [(MeO)4bpy] was developed, and the first three metal complexes with this ligand were synthesized. A few ligand precursor compounds, the final ligand, and its homoleptic iron(II) complex were characterized structurally by X‐ray diffraction. The combination of cyclic voltammetry, optical absorption, luminescence, and transient absorption spectroscopy provided detailed insight into the electronic structure of the entire series of homoleptic FeII, RuII, and OsII complexes. The ruthenium(II) complex is a more potent photoreductant than the [Ru(bpy)3]2+ parent compound by approximately 0.4 V as confirmed by 3MLCT excited‐state quenching experiments with a relatively mild oxidant, 1‐chloro‐4‐nitrobenzene. In the presence of methanesulfonic acid in CH3CN, the photoexcited [Ru{(MeO)4bpy}3]2+ complex is able to undergo proton‐coupled electron transfer (PCET) with acetophenone to yield a ketyl radical. Chemically robust and potent photoreductants are of interest for the phototriggering of electron‐transfer reactions, for example, in photoredox catalysis, in dye‐sensitized solar cells, in fundamental studies of (proton‐coupled) electron transfer, or for the generation of solvated electrons.

Photodetachment, electron cooling, and recombination, in a series of neat aliphatic room temperature ionic liquids.

Transient absorption following photodetachment of a series of neat methyl-alkyl-pyrrolidinium bis(trifluoromethylsulfonyl)amides at 6.20 eV was measured with sub-picosecond time resolution in the visible and near-IR portions of the spectrum. This series spans the onset of structuring in the liquids in the form of polarity alternation. Excitation promotes the electron into a delocalized state with a very large reactive radius. Strong transient absorption is observed in the visible spectrum with a ∼700 fs lifetime, and much weaker, long-lived absorption is observed in the near-IR spectrum. Absorption in the visible is shown to be consistent with the hole, and absorption in the near-IR is assigned to the free solvated electron. Yield of free electrons is estimated at ∼4%, is insensitive to the size of the cation, and is determined in less than 1 ps. Solvation of free electrons depends strongly on the size of the cation and correlates well with the viscosity of the liquid. In addition to radiolytic stability of the aliphatic cations, ultrafast, efficient recombination of separated charge in NTf2 (-) based ionic liquids following photo-excitation near the band-gap may prevent subsequent reactive damage associated with anions.

Ultrafast transient absorption spectrum of the room temperature Ionic liquid 1-hexyl-3-methylimidazolium bromide: Confounding effects of photo-degradation

The photochemistry of the charge transfer (CT) band of the room temperature ionic liquid (RTIL) 1-hexyl-3-methylimidazolium bromide (HMIm+/Br−) is investigated using near-IR to vis ultrafast transient absorption (TA) and steady-state UV absorption spectroscopies. Continuous irradiation of the CT band at 266 nm results in the formation of photo-products that absorb strongly at 266 nm. It is shown that these photo-products, which are apparently very stable, adversely affect ultrafast TA measurements. Elimination of these effects reveals at least two transient species that exist within the TA detection window of 100 fs to 3 ns and 500–1250 nm. One of the components is a short-lived (<1 ps) species that absorbs at 1080 nm. The second band exhibits a multicomponent spectrum that is very broad with an absorption maximum around 600 nm and a lifetime that is longer than the 3 ns window of our TA spectrometer. Within the signal to noise ratio of the TA spectrometer little to no solvated electron is generated by the CT mechanism.

Transport of secondary electrons and reactive species in ion tracks

The transport of reactive species brought about by ions traversing tissue-like medium is analysed analytically. Secondary electrons ejected by ions are capable of ionizing other molecules; the transport of these generations of electrons is studied using the random walk approximation until these electrons remain ballistic. Then, the distribution of solvated electrons produced as a result of interaction of low-energy electrons with water molecules is obtained. The radial distribution of energy loss by ions and secondary electrons to the medium yields the initial radial dose distribution, which can be used as initial conditions for the predicted shock waves. The formation, diffusion, and chemical evolution of hydroxyl radicals in liquid water are studied as well.

Radiosensitization of DNA by Cisplatin Adducts Results from an Increase in the Rate Constant for the Reaction with Hydrated Electrons and Formation of Pt(I).

Pulse radiolysis measurements of the decay of hydrated electrons in solutions containing different concentrations of the oligonucleotide GTG with and without a cisplatin adduct show that the presence of a cisplatin moiety accelerates the reaction between hydrated electrons and the oligonucleotide. The rate constant of the reaction is found to be 2.23 × 10(10) mol(-1) L s(-1), which indicates that it is diffusion controlled. In addition, we show for the first time the formation of a Pt(I) intermediate as a result of the reaction of hydrated electrons with GTG-cisplatin. A putative reaction mechanism is proposed, which may form the basis of the radiosensitization of cancer cells in concomitant chemoradiation therapy with cisplatin.

The solvation of electrons by an atmospheric-pressure plasma.

Solvated electrons are typically generated by radiolysis or photoionization of solutes. While plasmas containing free electrons have been brought into contact with liquids in studies dating back centuries, there has been little evidence that electrons are solvated by this approach. Here we report direct measurements of solvated electrons generated by an atmospheric-pressure plasma in contact with the surface of an aqueous solution. The electrons are measured by their optical absorbance using a total internal reflection geometry. The measured absorption spectrum is unexpectedly blue shifted, which is potentially due to the intense electric field in the interfacial Debye layer. We estimate an average penetration depth of 2.5±1.0 nm, indicating that the electrons fully solvate before reacting through second-order recombination. Reactions with various electron scavengers including H+, NO2−, NO3− and H2O2 show that the kinetics are similar, but not identical, to those for solvated electrons formed in bulk water by radiolysis.

Dynamics of electron solvation in methanol: Excited state relaxation and generation by charge-transfer-to-solvent.

The charge-transfer-to-solvent dynamics (CTTS) and excited state relaxation mechanism of the solvated electron in methanol are studied by time-resolved photoelectron spectroscopy on a liquid methanol microjet by means of two-pulse and three-pulse experiments. In the two-pulse experiment, CTTS excitation is followed by a probe photoejection pulse. The resulting time-evolving photoelectron spectrum reveals multiple time scales characteristic of relaxation and geminate recombination of the initially generated electron which are consistent with prior results from transient absorption. In the three-pulse experiment, the relaxation dynamics of the solvated electron following electronic excitation are measured. The internal conversion lifetime of the excited electron is found to be 130 ± 40 fs, in agreement with extrapolated results from clusters and the non-adiabatic relaxation mechanism.

Enhanced Photoreduction of Nitro-aromatic Compounds by Hydrated Electrons Derived from Indole on Natural Montmorillonite.

A new photoreduction pathway for nitro-aromatic compounds (NACs) and the underlying degradation mechanism are described. 1,3-Dinitrobenzene was reduced to 3-nitroaniline by the widely distributed aromatic molecule indole; the reaction is facilitated by montmorillonite clay mineral under both simulated and natural sunlight irradiation. The novel chemical reaction is strongly affected by the type of exchangeable cation present on montmorillonite. The photoreduction reaction is initiated by the adsorption of 1,3-dinitrobenzene and indole in clay interlayers. Under light irradiation, the excited indole molecule generates a hydrated electron and the indole radical cation. The structural negative charge of montmorillonite plausibly stabilizes the radical cation hence preventing charge recombination. This promotes the release of reactive hydrated electrons for further reductive reactions. Similar results were observed for the photoreduction of nitrobenzene. In situ irradiation time-resolved electron paramagnetic resonance and Fourier transform infrared spectroscopies provided direct evidence for the generation of hydrated electrons and the indole radical cations, which supported the proposed degradation mechanism. In the photoreduction process, the role of clay mineral is to both enhance the generation of hydrated electrons and to provide a constrained reaction environment in the galley regions, which increases the probability of contact between NACs and hydrated electrons.

Multiphoton Ionization of Liquid-to-Supercritical Methanol. Ultrafast Dynamics of Electron Localization and Geminate Recombination

Abstract Solvated electrons have been generated by 150 fs, 266 nm multiphoton ionization of methanol and their primary dynamics have been recorded through near-infrared transient absorption over a wide temperature (294 K ≤ T ≤ 523 K) and pressure (90 bar ≤ p ≤ 523 bar) region corresponding to the liquid and the supercritical phases of the solvent. On ultrashort time scales well below a few tens of picoseconds, the dynamics of electron localization, solvation, and thermalization reveal themselves as a delayed rise of the spectroscopic signal originating from the fully equilibrated solvated electron. The temperature-dependence of these dynamics is in line with a thermally-activated process involving hydrogen-bond breakage and formation that is induced by the librational/torsional dynamics of the methanol molecules. Longer time scales of up to 1 ns are governed by the dynamics of geminate recombination whose temperature dependence is indicative of multiple competing recombination pathways involving charged and neutral species, including radicals, of yet to be determined nature.

Angle-Resolved Photoemission of Solvated Electrons in Sodium-Doped Clusters.

Angle-resolved photoelectron spectroscopy of the unpaired electron in sodium-doped water, methanol, ammonia, and dimethyl ether clusters is presented. The experimental observations and the complementary calculations are consistent with surface electrons for the cluster size range studied. Evidence against internally solvated electrons is provided by the photoelectron angular distribution. The trends in the ionization energies seem to be mainly determined by the degree of hydrogen bonding in the solvent and the solvation of the ion core. The onset ionization energies of water and methanol clusters do not level off at small cluster sizes but decrease slightly with increasing cluster size.

Temperature effect on vertical detachment energy of bulk hydrated electron by dielectric continuum theory

In this work, vertical detachment energy (VDE), continuum slope (CS) and solvent reorganization energy (SRE) of solvated electrons in bulk liquids were calculated by the formulas derived from electron cavity model within the framework of dielectric continuum (DC) theory. The predicted VDE was in good agreement with the reported experimental data, which indicated that DC theory was a good approach to describe the energy level of solvated electrons in vertical detachment. Moreover, the effects of temperature on VDE, CS and SRE of bulk hydrated electron were discussed from 25 to $$500\,^{\circ }\hbox {C}$$ at an extreme pressure of 25 MPa. As temperature increases, VDE and CS were found to shift to lower energies (spectral red-shift), while SRE exhibited a maximum under supercritical condition and decreased significantly to about 0.53 eV at $$500\, ^{\circ }\hbox {C}$$ . Those significant energy changes near $$375\, ^{\circ }\hbox {C}$$ may be caused by extraordinary water properties at the critical point.

Tracing electron solvation in [formula omitted] clusters with K-shell photodetachment spectroscopy

We demonstrate that K-shell photoelectron spectroscopy is sensitive to binding motifs of cationic cores with excess electrons in solvated metals. Hereby, the satellites in core-level spectra and not the main line provide the important insight into ion-pairing effects. We found that contact ion-pairs in Li−(NH3)1–3 and a solvent-separated ion-pair in Li−(NH3)4 result in remarkably different Li1s spectra. The former spectra have numerous intense satellites of a charge-transfer character while that of Li−(NH3)4 contains only one strong satellite describing excitation of a solvated electron. We also explored K-shell photodetachment of the bare Li− and found that three-electron processes are essential here.

The alkali metals: 200 years of surprises

Alkali metal compounds have been known since antiquity. In 1807, Sir Humphry Davy surprised everyone by electrolytically preparing (and naming) potassium and sodium metals. In 1808, he noted their interaction with ammonia, which, 100 years later, was attributed to solvated electrons. After 1960, pulse radiolysis of nearly any solvent produced solvated electrons, which became one of the most studied species in chemistry. In 1968, alkali metal solutions in amines and ethers were shown to contain alkali metal anions in addition to solvated electrons. The advent of crown ethers and cryptands as complexants for alkali cations greatly enhanced alkali metal solubilities. This permitted us to prepare a crystalline salt of Na(-) in 1974, followed by 30 other alkalides with Na(-), K(-), Rb(-) and Cs(-) anions. This firmly established the -1 oxidation state of alkali metals. The synthesis of alkalides led to the crystallization of electrides, with trapped electrons as the anions. Electrides have a variety of electronic and magnetic properties, depending on the geometries and connectivities of the trapping sites. In 2009, the final surprise was the experimental demonstration that alkali metals under high pressure lose their metallic character as the electrons are localized in voids between the alkali cations to become high-pressure electrides!

Hydrogen Atom Transfer from Water or Alcohols Activated by Presolvated Electrons.

High-energy irradiation of protic solvents can transiently introduce excess electrons that are implicated in a diverse range of reductive processes. Here we report the evolution of electron solvation in water and in alcohols following photodetachment from aqueous hydroxide or the corresponding alkoxides studied by two- and three-pulse femtosecond spectroscopy and ab initio molecular dynamic simulations. The experiments reveal an ultrafast recombination channel of the excess electrons. Through the calculations this channel emerges as an H-atom transfer process to the hydroxyl or alkoxy radical species from neighboring solvent molecules, which are activated as the presolvated electron occupies their antibonding orbitals. The initially low activation barrier in the early stages of electron solvation was found to increase (from 12 to 44 kJ/mol in water) as full solvation proceeded.

Real-time measurement of the vertical binding energy during the birth of a solvated electron.

Using femtosecond time-resolved two-photon photoelectron spectroscopy, we determine (i) the vertical binding energy (VBE = 0.8 eV) of electrons in the conduction band in supported amorphous solid water (ASW) layers, (ii) the time scale of ultrafast trapping at pre-existing sites (22 fs), and (iii) the initial VBE (1.4 eV) of solvated electrons before significant molecular reorganization sets in. Our results suggest that the excess electron dynamics prior to solvation are representative for bulk ASW.

Solvation of an excess electron in pyrrolidinium dicyanamide based ionic liquids.

In a recent article [J. Am. Chem. Soc. 2011, 133, 20186], we described the nature of the "dry" excess electron in a variety of different ionic liquids. We found that this could delocalize over cations or anions depending on the nature of the ions involved. A second article [J. Am. Chem. Soc. 2013, 135, 17528] explored the nature of the "dry to trapped" excess electron transition, the early localization dynamics, and associated spectroscopic signatures in alkylamonium and pyrrolidinium bis(trifluoromethylsulfonyl)amide based ionic liquids. In this study we predicted that the trapped electron localizes on an anion, resulting in fragmentation that is undesirable for photochemical, electrochemical, and radiation chemistry applications. The current work focuses instead on an ionic liquid based on the dicyanamide anion that on a time scale relevant to electron transfer and solvation dynamics does not appear to undergo facile fragmentation. Although electrochemical cathodic and anodic limits were correctly predicted by our recent study, it is unclear whether the reaction channels explored are necessarily those responsible for the observed near-infrared (NIR) band typical of excess electrons at long time. Could it be possible that the electrochemically relevant reaction channel is not necessarily the one giving rise to the NIR signal? This work attempts to approach such structural and dynamical aspects relevant to photodegradation, radiation chemistry, and electrochemistry in the case of pyrrolidinium dicyanamide based ionic liquids.