Dose Rate Effects in Fluorescence Chemical Dosimeters Exposed to Picosecond Electron Pulses: An Accurate Measurement of Low Doses at High Dose Rates.

The development of ultra-intense electron pulse for applications needs to be accompanied by the implementation of a practical dosimetry system. In this study four different systems were investigated as dosimeters for low doses with a very high-dose-rate source. First, the effects of ultra-short pulses were investigated for the yields of the Fricke dosimeter based on acidic solutions of ferrous sulfate; it was established that the yields were not significantly affected by the high dose rates, so the Fricke dosimeter system was used as a reference. Then, aqueous solutions of three compounds as fluorescence chemical dosimeters were utilized, each operated at a different solution pH: terephthalic acid - basic, trimesic acid - acidic, and coumarin-3-carboxylic acid (C3CA) - neutral. Fluorescence chemical dosimeters offer an attractive alternative to chemical dosimeters based on optical absorption for measuring biologically relevant low doses because of their higher sensitivity. The effects of very intense dose rate (TGy/ s) from pulses of fast electrons generated by a picosecond linear accelerator on the chemical yields of fluorescence chemical dosimeters were investigated at low peak doses (<20 Gy) and compared with yields determined under low-dose-rate irradiation from a 60 Co gamma-ray source (mGy/s). For the terephthalate and the trimesic acid dosimeters changes in the yields were not detected within the estimated (∼10%) precision of the experiments, but, due to the complexity of the mechanism of the hydroxyl radical initiated reactions in solutions of the relevant aromatic compounds, significant reductions of the chemical yield (-60%) were observed when the C3CA dosimeter was irradiated with the ultra-short pulses.

Time-dependent yield of the hydrated electron and the hydroxyl radical in D2O: a picosecond pulse radiolysis study.

Picosecond pulse radiolysis measurements were performed in neat D2O and H2O in order to study the isotopic effect on the time-resolved yield of the hydrated electron and hydroxyl radical. First, the absorption band of the hydrated electron in D2O, eD2O-, is measured between 250 and 1500 nm. The molar absorption coefficient of the solvated electron spectrum in D2O was determined using the isosbestic point method by scavenging the solvated electron using methyl viologen. The amplitude and shape of the absorption spectrum of the hydrated electron in D2O are different from those previously reported in the literature. The maximum of the hydrated electron in the D2O absorption band is ca. 704 nm with a molar absorption coefficient of (22 900 ± 500) L mol-1 cm-1. Based on this extinction coefficient, the radiolytic yield of eD2O- just after the 7 ps electron pulse was determined to be (4.4 ± 0.2) × 10-7 mol J-1, which coincides with the one for eH2O- in H2O. The time-dependent radiolytic yield of eD2O- was determined from a few ps to 8 ns. To determine the OD˙ radical yield, the contribution of the solvated electron and of the transient species produced by the electron pulse in the windows of the fused silica optical cell was taken into account for the analysis of the transient absorption measurements at 260 nm. Therefore, an appropriate experimental methodology is used for measuring low absorbance at two different wavelengths in ps pulse radiolysis. The yield of the OD˙ radical just after the 7 ps electron pulse was found to be (5.0 ± 0.2) × 10-7 mol J-1. In the spurs of ionization, the decay rate of eD2O- is slower than eH2O-, whereas the decay rate of OD˙ is similar to the one of OH˙. Here, the established time-dependent yield of the solvated electron and the hydroxyl radical provide the foundation for improving the models used for spur reaction simulations in heavy water mainly for the chemistry of CANDU reactors.

Effect of the solvation state of electron in dissociative electron attachment reaction in aqueous solutions.

It is generally considered that the pre-solvated electron and the solvated electron reacting with a solute yield the same product. Silver cyanide complex, Ag(CN)2-, is used as a simple probe to demonstrate unambiguously the existence of a different reduction mechanism for pre-hydrated electrons. Using systematic multichannel transient absorption measurements at different solute concentrations from millimolar to decimolar, global data analysis and theoretical calculations, we present the dissociative electron attachment on Ag(CN)2-. The short-lived silver complex, Ag0(CN)22-, formed by hydrated electron with nanosecond pulse radiolysis, can be observed at room temperature. However, at higher temperatures only the free silver atom, Ag0, is detected, suggesting that Ag0(CN)22- dissociation is fast. Surprisingly, pulse radiolysis measurements on Ag(CN)2- reduction, performed by a 7 ps electron pulse at room temperature, show clearly that a new reduced form of silver complex, AgCN-, is produced within the pulse. This species, absorbing at 560 nm, is not formed by the hydrated electron but exclusively by its precursor. DFT calculations show that the different reactivity of the hydrated and pre-hydrated electrons can be due to the formation of different electronic states of Ag0(CN)22-: the prehydrated electron can form an excited state of this complex, which mainly dissociates into Ag0CN- + CN-.

Observation and Simulation of Transient Anion Oligomers (LiClO4)n- (n = 1-4) in Diethyl Carbonate LiClO4 Solutions.

NMR measurements show that diethyl carbonate (DEC, a solvent with a low dielectric constant) solutions of LiClO4 contain (LiClO4)n oligomers. The reduction of these species by solvated and presolvated electrons is followed by picosecond pulse radiolysis measurements. The data analysis shows that several anions absorbing in the near-infrared (NIR) and visible range are formed after the 7 ps electron pulse. In contrast with tetrahydrofuran (THF) solutions of LiClO4, the anionic monomer (LiClO4)- is not observed in DEC solutions. This is due to the fact that DEC is a nonpolar solvent favoring the clustering of monomers in the nonirradiated solution, as shown by NMR results, and also due to the instability of the anionic monomer. The absorption spectra of the anionic dimer (LiClO4)2-, trimer (LiClO4)3-, and tetramer (LiClO4)4- are clearly observed in NIR and visible ranges. Compared to the results obtained for the same system in THF and in agreement with simulated absorption spectra, the experimental results show that the absorption bands are shifted to the blue end of the spectrum when n increases. The kinetics recorded for the molar LiClO4 solution indicates that the solute is only in the form of oligomers (LiClO4)n with a large n value and that the reduced species absorb weakly in the visible region. Lastly, and contrary to what is known for well-separated ions in polar solvents, it is shown that the (LiClO4)n- anions are not stable with respect to self-reduction, leading to the decomposition of perchlorate anions. In this reaction, the perchlorate anion ClO4- is reduced by the Li atom into a chlorate anion ClO3-. This is proved by the presence of ClO3- and chlorinated species detected by mass spectrometry measurements in irradiated DEC solutions containing LiClO4.

Ultra-fast charge migration competes with proton transfer in the early chemistry of H2O˙.

Oxidation by the ultra-short lived radical cation of water, H2O˙+, can potentially take place at the interface of water and numerous heterogeneous systems involved in radiation therapy, energy and environmental industries. The oxidation processes induced by H2O˙+ can be mimicked in highly concentrated solutions where the nearest neighbors of H2O˙+ may be molecules other than water. The reactivity of H2O˙+ and D2O˙+ is probed in hydrogenated and deuterated sulfuric acid solutions of various concentrations. The oxidized solute, sulfate radical, is observed at 7 ps and remarkably higher yields are found in deuterated solutions. The isotopic effects reveal the competition between two ultrafast reactions: proton transfer toward H2O (D2O) and electron transfer from HSO4- to H2O˙+ (D2O˙+). Density functional theory simulations decipher the electron transfer mechanism: it proceeds via sub-femtosecond charge migration and is not affected by isotopic substitution. This work definitively demonstrates why direct oxidation triggered by H2O˙+ can be competitive with proton transfer.

Ultrafast Scavenging of the Precursor of H(•) Atom, (e(-), H3O(+)), in Aqueous Solutions.

Picosecond pulse radiolysis measurements have been performed in several highly concentrated HClO4 and H3PO4 aqueous solutions containing silver ions at different concentrations. Silver ion reduction is used to unravel the ultrafast reduction reactions observed at the end of a 7 ps electron pulse. Solvated electrons and silver atoms are observed by the pulse (electron beam)-probe (supercontinuum light) method. In highly acidic solutions, ultrafast reduction of silver ions is observed, a finding that is not compatible with a reaction between the H(•) atom and silver ions, which is known to be thermally activated. In addition, silver ion reduction is found to be even more efficient in phosphoric acid solution than that in neutral solution. In the acidic solutions investigated here, the species responsible for the reduction of silver atoms is considered to be the precursor of the H(•) atom. This precursor, denoted (e(-), H3O(+)), is a pair constituting an electron (not fully solvated) and H3O(+). Its structure differs from that of the pair of a solvated electron and a hydronium ion (es(-), H3O(+)), which absorbs in the visible region. The (e(-), H3O(+)) pair , called the pre-H(•) atom here, undergoes ultrafast electron transfer and can, like the presolvated electron, reduce silver ions much faster than the H(•) atom. Moreover, it is found that with the same concentration of H3O(+) the reduction reaction is favored in the phosphoric acid solution compared to that in the perchloric acid solution because of the less-efficient electron solvation process. The kinetics show that among the three reducing species, (e(-), H3O(+)), (es(-), H3O(+)), and H(•) atom, the first one is the most efficient.

Picosecond Pulse Radiolysis of Highly Concentrated Carbonate Solutions.

Highly concentrated potassium carbonate aqueous solutions are studied by picosecond pulse radiolysis with the purpose of exploring the formation processes of carbonate radical CO3(•-). The transient absorption band of solvated electron produced by ionizing is markedly shifted from 715 to 600 nm when the solute concentration of K2CO3 is 5 mol L(-1). This spectral shift is even more important than that observed for the solvated electron in 10 mol L(-1) KOH solutions. The broad absorption band of solvated electron in K2CO3 solutions overlaps with that of carbonate radical CO3(•-) formed at ultrashort time. Nitrate ion is used to scavenge the solvated electron and to observe the contribution of carbonate radical CO3(•-). The analysis of the amplitude and the kinetics of carbonate radical formation in highly concentrated solutions shows that CO3(•-) is formed within the electron pulse (7 ps) by two parallel mechanisms: a direct effect on the solute and the oxidation of the solute by water radical hole H2O(•+). These two mechanisms are followed by an additional one, by reaction between the solute and OH(•) radical especially in lower concentration. The radiolytic yield of each process is discussed.

Identification of Transient Radical Anions (LiClO4)(n)(-) (n = 1-3) in THF Solutions: Experimental and Theoretical Investigation on Electron Localization in Oligomers.

Picosecond pulse radiolysis measurements of tetrahydrofuran (THF) solutions containing LiClO4 over a wide range of concentration are performed to investigate the formation of transient species. The (35)Cl NMR measurements of these solutions prior to irradiation show that the salt is in the form of (LiClO4)n oligomers. Kinetics and transient absorption spectra of intermediates in each solution are obtained on the time scale from 10 to 3800 ps. A global spectro-kinetic matrix of the data is analyzed by the multicurve resolution alternated least-squares (MCR-ALS) method. It shows the presence of 3 transient species induced by electron pulse, in addition to the solvated electron. A hybrid Monte Carlo/DFT molecular simulation method is elaborated, using the MPW1K functional for the configuration sampling and B3LYP for the spectra calculations. The maximum of the absorption band of the monomer (LiClO4)(-), dimer (LiClO4)2(-), trimer (LiClO4)3(-), and tetramer (LiClO4)4(-) anions are deduced from the simulations. They enable one to label the MCR-ALS spectra (differences are below 0.1 eV) and to interpret the kinetic data. The simulations show also that Li(I) ion catalyzes the reduction of perchlorate by excess electrons. Only the dimer anion, due to its unique structure with a stable Li2(+) core and two nonbridging perchlorates, presents higher stability toward ClO4(-) reduction into ClO3(-). It corresponds to the long-lived species observed in the experiments.

Ultrafast Decay of the Solvated Electron in a Neat Polar Solvent: The Unusual Case of Propylene Carbonate.

The behavior of carbonates is critical for a detailed understanding of aging phenomena in Li-ion batteries. Here we study the first reaction stages of propylene carbonate (PC), a cyclical carbonate, by picosecond pulse radiolysis. An absorption band with a maximum around 1360 nm is observed at 20 ps after the electron pulse and is shifted to 1310 nm after 50 ps. This band presents the features of a solvated electron absorption band, the solvation lasting up to 50 ps. Surprisingly, in this polar solvent, the solvated electron follows an ultrafast decay and disappears with a half time of 360 ps. This is attributed to the formation of a radical anion PC(-•). The yield of the solvated electron is low, suggesting that the radical anions are mainly directly produced from presolvated electrons. These results demonstrate that the initial electron transfers mechanisms are strongly different in linear compared with cyclical carbonates.

Electrolytes Ageing in Lithium-ion Batteries: A Mechanistic Study from Picosecond to Long Timescales.

The ageing phenomena occurring in various diethyl carbonate/LiPF6 solutions are studied using gamma and pulse radiolysis as a tool to generate similar species as the ones occurring in electrolysis of Li-ion batteries (LIBs). According to picosecond pulse radiolysis experiments, the reaction of the electron with (Li(+), PF6(-)) is ultrafast, leading to the formation of fluoride anions that can then precipitate into LiF(s). Moreover, direct radiation-matter interaction with the salt produces reactive fluorine atoms forming HF(g) and C2H5F(g). The strong Lewis acid PF5 is also formed. This species then forms various R(1)R(2)R(3) P=O molecules, where R is mainly -F, -OH, and -OC2H5. Substitution reactions take place and oligomers are slowly formed. Similar results were obtained in the ageing of an electrochemical cell filled with the same model solution. This study demonstrates that radiolysis enables a description of the reactivity in LIBs from the picosecond timescale until a few days.

Unexpected Ultrafast Silver Ion Reduction: Dynamics Driven by the Solvent Structure.

Picosecond pulse radiolysis measurements have been performed in neutral and highly acidic aqueous solutions containing silver ions at different concentrations. Silver ion reduction is used to understand the ultrafast chemistry of irradiated water and aqueous solutions. The absorption band measured at the end of the 7-ps electron pulses has an intense band with a maximum at 360 nm due to the formation of silver atoms. Kinetics shows that the amount of silver atom formed at the end of the electron pulse in phosphoric acid solutions is greater than that in neutral water. This unexpectedly high yield of silver atom formation cannot be explained solely by the reaction between silver ions and solvated electrons in neutral solutions nor by the reaction with hydrogen atoms in phosphoric acid solutions. To explain the observed ultrafast reduction of silver ions, the presolvated electron, be it free or paired to the hydronium cation, must react very quickly with a silver ion, potentially competing with geminate recombination of the electron and its sibling radical cation.

Picosecond Pulse Radiolysis of Highly Concentrated Phosphoric Acid Solutions: Mechanism of Phosphate Radical Formation.

Eight solutions containing phosphoric acid with concentrations ranging from 2 mol L(-1) to neat acid have been studied by picosecond pulse radiolysis. The absorbance of the secondary radical H2PO4(•) formed within 7 ps of the electron pulse is observed using pulse-probe method in the visible. Kinetic analysis shows that the radicals of phosphoric acid are formed via two mechanisms: direct electron detachment and oxidation by the radical cation of water, H2O(•+). On the basis of molar extinction coefficient value of 1850 L mol(-1) cm(-1), at 15 ps the radiolytic yield of H2PO4(•) formation by direct energy absorption is 3.7 ± 0.1 × 10(-7) mol J(-1) in neat phosphoric acid. In highly concentrated phosphoric acid solutions, the total yield of phosphate radical at 15 ps exhibits an additional contribution that can be explained by electron transfer from phosphoric acid to H2O(•+). The efficiency of the electron transfer to this strongly oxidizing species in phosphoric acid solutions is lower compared with the one in sulfuric acid solutions. Two explanations are given to account for a relatively low efficiency of H2O(•+) scavenging in concentrated phosphoric acid solutions.

A broadband ultrafast transient absorption spectrometer covering the range from near-infrared (NIR) down to green.

We present a new development for pump-probe absorption spectroscopy that allows the simultaneous measurement from the green part of the visible spectrum (510 nm) over the whole near-infrared range to >1600 nm, corresponding to 0.77-2.40 eV. The system is based on a sub-picosecond supercontinuum generated in bulk material used as a broadband probe that is dispersed with a custom-made prism spectrometer and detected by an InGaAs array with extended sensitivity to the visible. Two versions, with and without probe referencing, are implemented for operation at laser repetition rates of a few hertz and kilohertz, respectively. After presentation of the optical configuration of the spectrometer, its performance is characterized and further illustrated on two time scales, with the ultrafast radiolysis of isopropanol induced by a picosecond electron pulse and with the instantaneous response of a BK7 plate to a femtosecond light pulse. The photophysics of the dye IR-140 is resolved from the femto- to picosecond regime. Stable and easy day-to-day routine use of the spectrometer also can be achieved in non-optical laboratory surroundings. For operation in a hazardous environment, the optical probe beams can be transported to the detector unit by optical fibers.

Picosecond pulse radiolysis of highly concentrated sulfuric acid solutions: evidence for the oxidation reactivity of radical cation H2O(•+).

Aqueous solution of sulfuric acid is used as a suitable system to investigate the reactivity of the short-lived radical cation H2O(•+) which is generated by radiation in water. Ten aqueous solutions containing sulfuric acid with concentration from 1 to 18 mol L(-1) are studied by picosecond pulse radiolysis. The absorbance of the secondary radical SO4(•-) (or HSO4(•)) formed within the 10 ps electron pulse is measured by a pulse-probe method in the visible range. The analysis of the kinetics show that the radicals of sulfuric acid are formed within the picosecond electron pulse via two parallel mechanisms: direct electron detachment by the electron pulse and oxidation by the radical cation of water H2O(•+). In highly concentrated solution when SO4(2-) is in contact with H2O(•+), the electron transfer becomes competitive against proton transfer with another water molecule. Therefore, H2O(•+) may act as an extremely strong oxidant. The maximum radiolytic yield of scavenged H2O(•+) is estimated to be 5.3 ± 0.1 × 10(-7) mol J(-1).

Reactivity of the Strongest Oxidizing Species in Aqueous Solutions: The Short-Lived Radical Cation H2O(•.).

The radical cation H2O(•+) formed under irradiation of liquid water undergoes an ultrafast proton transfer reaction and consequently exhibits an extremely short lifetime. The proton transfer yields an oxidizing OH(•) radical whose reactivity has been extensively studied. By contrast, H2O(•+) reactivity with molecules other than water has not been established experimentally and was subject to controversy. The direct oxidation by H2O(•+) can take place in various situations. In highly concentrated solutions, the radical cation H2O(•+) may also be involved in ultrafast electron transfer reactions. We have applied picosecond pulse radiolysis conducted at the electron accelerator ELYSE on solutions with various H2SO4 concentrations to determine the scavenging yield of H2O(•+). The yield of H2O(•+) at a few tens of femtoseconds is estimated to be around 5.3 × 10(-7) mol J(-1), and its reactivity is quantitatively determined. Moreover, a simple estimation of the reduction potential of this short-lived radical cation shows that it is the most powerful oxidizing species.

Direct Evidence for Transient Pair Formation between a Solvated Electron and H3O(+) Observed by Picosecond Pulse Radiolysis.

The reaction between the solvated electron and hydronium cation H3O(+) in water constitutes a fundamental reaction in chemistry. Due to significant rearrangement of solvent molecules around both the electron and H3O(+), the reaction rate of this process is not controlled by diffusion. The presence of a reaction barrier suggests the formation of an intermediate that has so far not been observed. Here, the time-resolved visible absorption spectra in three concentrated acid solutions, perchloric, sulfuric, and phosphoric, at various concentrations are recorded by the picosecond pulse radiolysis method. In contrast to previous reports, a strong blue shift of the absorption band of the solvated electron in acidic solutions compared to neat water is clearly observed, consistent with formation of a pair between the solvated electron and hydronium cation.

Reduction of earth alkaline metal salts in THF solution studied by picosecond pulse radiolysis.

Picosecond pulse radiolysis of tetrahydrofuran (THF) solutions containing earth alkaline metal salt, M(II)(ClO4)2, at different concentrations are performed using two different supercontinua as probe pulse, one covering the visible and another the near-infrared (NIR) down to the visible. Two types of line scan detectors are used to record the absorption spectra in the range from 400 to 1500 nm. Because of the strong overlap between the spectra of the absorbing species in the present wavelength range, global matrices were built for each M(II) system, by delay-wise binding the matrix for pure THF with the available matrices for this cation. The number of absorbers was assessed by Singular Value Decomposition of the global matrix, and a MCR-ALS analysis with the corresponding number of species was performed. The analysis of the results show clearly that solvated electron reacts with the earth alkaline metal molecule and the product has an optical absorption band very different than that of solvated electron in pure THF. So, contrarily to the case of solution containing free Na(+), in the presence of Mg(II), Ca(II) and Sr(II) the observed absorption band is not only blueshifted, but its shape is also drastically changed. In fact with Na(+) solvated electron forms a tight-contact pair but with earth alkaline metal cation solvated electron is scavenged by the undissociated molecule M(II)(ClO4)2. In order to determine the structure of the absorbing species observed after the electron pulse, Monte Carlo/DFT simulations were performed in the case of Mg(II), based on a classical Monte Carlo code and DFT/PCM calculation of the solute. The UV-visible spectrum of the solute is calculated with the help of the TDDFT method. The calculated spectrum is close to the experimental one. It is due to two species, a contact pair and an anion.

Picosecond pulse radiolysis of the liquid diethyl carbonate.

The diethyl carbonate, DEC, is an ester that is used as a solvent in Li-ion batteries, but its behavior under ionizing radiation was unknown. The transient optical absorption spectra, the decay kinetics, and the influence of various scavengers have been studied by using the picosecond laser-triggered electron accelerator ELYSE. In neat DEC, the intense near-IR (NIR) absorption spectrum is assigned to the solvated electron. It is overlapped in the visible range by another transient but longer-lived and less intense band that is assigned to the oxidized radical DEC(-H). The solvated electron molar absorption coefficients and radiolytic yield evolution from 25 ps, the geminate recombination kinetics, and the rate constants of electron transfer reactions to scavengers are determined. The radiolytic mechanism, indicating a certain radioresistance of DEC, is compared with that for other solvents.

Spur reactions observed by picosecond pulse radiolysis in highly concentrated bromide aqueous solutions.

The formation of the well-known product Br3(-), observed in the steady-state radiolysis of highly concentrated Br(-) aqueous solutions, has now been directly observed at ultrashort times corresponding to the relaxation of the spur. The transient absorption induced by picosecond pulse radiolysis of 6 M Br(-) aqueous solution was probed simultaneously at 260 nm with the third harmonic laser wave and from 350 to 750 nm with a supercontinuum generated by the fundamental laser wave. This approach allows several transient radiolytic species to be followed in parallel, particularly the solvated electron, BrOH(-•), Br2(-•), and Br3(-). The kinetics measured within 4 ns at 260 and 370 nm clearly exhibit that the decay of Br2(•-) is correlated with the formation of Br3(-). In highly concentrated Br(-) solutions, the OH(•) radical is fully replaced by Br2(•-), and the spur kinetics of OH(•) radical in pure water is comparable with that of Br2(-•). Model calculations indicate that the main OH(•) radical combination product H2O2 in pure water has formation kinetics similar to that of Br3(-) in 6 M Br(-) solutions. Moreover, they point out that oxidation of Br(-) occurs within the electron pulse both by direct energy absorption and by scavenging of the water radical cation, H2O(•+).

Picosecond pulse radiolysis study on the distance dependent reaction of the solvated electron with organic molecules in ethylene glycol.

The decay of solvated electron e(s)(-) is observed by nanosecond and picosecond pulsed radiolysis, in diluted and highly concentrated solutions of dichloromethane, CH(2)Cl(2), trichloromethane, CHCl(3), tribromomethane, CHBr(3), acetone, CH(3)COCH(3), and nitromethane, CH(3)NO(2), prepared in ethylene glycol. First, second-order rate constants for the reactions between e(-)(s) and the organic scavengers have been determined. The ratio between the highest rate constant that was found for CH(3)NO(2) and the lowest one that was found for acetone is 3. This difference in reactivity cannot be explained by the change of viscosity or the size of the molecules. Then, from the analysis of decay kinetics obtained using ultrafast pulse-probe method, the distance dependent first-order rate constant of electron transfer for each scavenger has been determined. The amplitude of the transient effect observed on the picosecond time scale differs strongly between these solvated electron scavengers. For an identical scavenger concentration, the transient effect lasts ≈650 ps for CH(3)NO(2) compared to ~200 ps for acetone. For acetone, the distance dependent first-order rate constant of electron transfer is decreasing very rapidly with increasing distance, whereas for nitromethane and tribromomethane the rate constant is decreasing gradually with the distance and its value remains non-negligible even at ~10 Å. This rate constant is controlled mostly by the free energy of the reaction. For nitromethane and tribromomethane, the driving force is great, and the reaction can occur even at long distance, whereas for acetone the driving force is small and the reaction occurs almost at the contact distance. For nitromethane and acetone, the one-electron reduction reaction needs less internal reorganization energy than for alkyl halide compounds for which the reaction occurs in concert with bond breaking and geometric adjustment.