Picosecond Pulse Radiolysis of the Liquid Diethyl Carbonate - The

Article pubs.acs.org/JPCA

Picosecond Pulse Radiolysis of the Liquid Diethyl Carbonate Fayçal Torche, Abdel Karim El Omar, Philippe Babilotte, Sébastien Sorgues, Uli Schmidhammer, Jean-Louis Marignier, Mehran Mostafavi, and Jacqueline Belloni* Laboratoire de Chimie Physique, ELYSE, UMR 8000 CNRS-UPS, Université Paris-Sud, Bât. 349, 91405 Orsay, France S Supporting Information *

ABSTRACT: 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.

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INTRODUCTION The esters alkyl carbonates are aprotic solvents displaying very interesting properties, namely low fusion temperatures, biodegradability, and high solubility power for various molecules. These properties make them attractive for replacing other less eco-friendly solvents in an increasing number of applications.1,2 They are used in organic synthesis,3,4 such as alkylation and carbonylation and in the synthesis of polymers and carbamates.5 The most significant application is their use as solvents in rechargeable and low-temperature Li-ion batteries, generally as a mixture in variable proportions of cyclic ethylene and propylene carbonates, diethyl and dimethyl carbonate, ethyl acetate, and vinylidene carbonate.6 However, very few studies have been devoted to the chemical effects induced by high energy radiation on the alkyl carbonates,7−9 whereas the radio-resistance of the systems is of major concern for their long-lived stability in agressive radiation environments, such as space, nuclear plants, or radioactive waste repositories. Moreover, these previous studies have essentially focused on the formation yield of the stable radiolytic products, but none on the primary effects of radiation on these liquids. In fact, the primary effects of radiation on a liquid are the ionization into an electron and a cation and the excitation of the molecules.10 However, depending on the chemical structure of the liquid, the further fate of the electron may be very different.11 If the molecule displays a high electron affinity (such as aromatics, ketones, halogenocompounds, liquid hydrogen sulfide), an electron attachment onto the molecule gives rise to an anion radical. In addition, for unsaturated compounds, the radical anion may initiate a chain reaction of polymerization. In liquids without electron affinity (such as water,12,13 alcohols, amines,14 ethers,14 ionic liquids15), the thermalized electron polarizes the surrounding molecules and is © 2013 American Chemical Society

converted into a solvated electron. In rare cases, as in dimethyl sulfide, both processes are in competition and a solvated electron is formed concomitantly with an anion issued from the electron attachment.11,16,17 The aim of this work is to observe, using nano- and picosecond pulse radiolysis and synchronized time-resolved optical detection, the fate of the electron and other radiolytic species and their time-dependent yield as a probe of the primary radiation effects on a carbonate ester liquid, specifically on the diethyl carbonate (H5C2O)2CO, and to compare the results with the radiolysis of other well-known solvents. In addition, electron transfer processes to some solutes will be studied to assign and calibrate the different optical absorption bands directly observed and the time-dependent solvated electron yield. However, in view of the very low polarity of diethyl carbonate, the reactions, in particular the geminate recombination between electrons and parent cations, are expected to be very fast and observable essentially at the picosecond scale.

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EXPERIMENTAL SECTION All the reagents, diethyl carbonate (DEC), methanol, ethanol, acetone, biphenyl, perchloric acid (70% in water), were pure chemicals and purchased from Sigma-Aldrich. They were used without further purification. Diethyl carbonate is a transparent liquid in the range −45 to +126 °C at normal pressure. The molecular weight is 118.13 g mol−1, and the mass density at 20 °C is 0.9750 g mL−1 (neat DEC is 8 mol L−1). The refraction index is 1.383 ± 0.001, the Received: July 11, 2013 Revised: September 26, 2013 Published: September 27, 2013 10801

dx.doi.org/10.1021/jp406856u | J. Phys. Chem. A 2013, 117, 10801−10810

The Journal of Physical Chemistry A

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Figure 1. Left: transient optical absorption spectra in neat diethyl carbonate after the picosecond electron pulse (25 ps to 7.5 ns) or the nanosecond electron pulse (dots ≥10 ns). Right: enlargement of the UV−visible spectra.

dipolar moment is 1.10 D, the viscosity is η = 0.748 mPa s, and the static dielectric constant at room temperature is εs = 2.82. DEC is slightly soluble in water but soluble in organic solvents such as alcohols.18 During the irradation, the DEC samples were saturated with very pure Ar and changed through a circulation system. Pulse radiolysis was performed using two irradiation facilities. At the nanosecond scale, electron pulses of 3 ns, 600 keV were delivered by a Febetron 706 accelerator19 to the samples contained in a quartz suprasil cell through a 0.2 mm entrance window. The phenomena were probed by transient absorption using the light beam of a Xe lamp at 90° to the electron beam. Five to seven measurements of the transient absorbance are obtained for each wavelength to average for pulse-to-pulse dose variation. The dose per pulse is determined from the absorbance at 450 nm of the hydrated electron in pure water. Assuming a radiolytic yield G(e−aq)3ns = 3.45 × 10−7 mol J−1,20 and a molar absorption coefficient at 450 nm ε450(e−aq) = 3.89 × 103 L mol−1 cm−1,21 the dose per pulse was DW = 520 Gy = 520 J L−1 (energy absorbed per liter). The intensity of the signals in DEC was of a few tenths of absorbance unit in a cell with an optical path of 1 cm. At the picosecond scale, the study was performed using the ELYSE laser-induced electron accelerator. The pulses were 4−6 nC, 10 ps, with an electron energy of 5−9 MeV, at a repetition frequency of 10 Hz.22,23 The transient absorbance was measured at two exits of the electron beam, one equipped with a broad-band pump−probe detection system, whose principle was already described,24 the second equipped with a homemade repetitive flash-lamp25 and a streak camera.26 Both probe beams are collinear with the electron beam. A short-lived absorbance centered at 590 nm due to the fused silica cell windows was detected in the empty cell when irradiated27 and was subtracted from the absorbance data of the cell containing the DEC sample. At the first electron beam exit, the wavelength range of the pump−probe system is from the visible to the near-infrared

(700−1400 nm) as described elsewhere.28 The optical path was 0.5 cm. The time resolution is 20 ps. The dose per pulse was deduced from the absorbance of the hydrated electron e−aq in water, measured just before a series of experiments in DEC. The dose was then derived from the yield at 20 ps:29 G(e−aq)20ps = 4.25 × 10−7 mol J−1 (corresponding to G(e−aq)3ns = 3.45 × 10−7 mol J−1 at 3 ns) and from the molar absorption coefficient at 700 nm ε700 = 1.91 × 104 L mol−1 cm−1.21 The dose in water was thus DW = 28.4 Gy per pulse. The values of the absorbance changes in DEC were on the level of a few percent at most and were the average of a few hundreds of single measurements per delay step. At the second electron beam exit, the wavelength range of the streak camera was 300−750 nm and a time scale from 20 ps to 1 ms. The optical path was 1 cm and the time resolution was 40 ps. The dose per pulse was derived from the absorbance of the hydrated electron at 660 nm with ε660 (e−aq) = 1.81 × 104 L mol−1 cm−1,21 and G(e−aq)3 ns = 3.45 × 10−7 mol J−1. The dose in water DW was currently 40−50 Gy (or J L−1). The absorbance values were an average of measurements over 1000 pulses. According to the lower electronic density of DEC compared to that for water, the dose per pulse in DEC, DDEC in J L−1, is 4% lower than DW under the same irradiation conditions. To compare the results obtained at the picosecond and nanosecond scales with different doses, the data are given in Gt(M) × ελ(M) units, calculated from the measured total absorbance Aλ,t at the wavelength λ and time t, as Gt(M) × ελ(M) = Aλ,t/ (DDEC × l), where Gt(M) is the radiolytic yield in 10−7 mol J−1, ελ(M) is the molar absorption coefficient in L mol−1 cm−1 of the molecules M absorbing at λ, DDEC is the dose in J L−1, and l is the optical path in cm.

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RESULTS AND DISCUSSION Diethyl Carbonate in Methanol Solutions. After the primary ionization of the solvent diethyl carbonate by the irradiation, the electron may solvate into a solvated electron

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e−DEC or attach onto a molecule into the anion radical DEC−•. For instance, electrons generally attach readily on >CO groups such as in acetone, whereas in contrast the ion carbonate CO32− reacts very slowly with the hydrated electron (rate constant ≤3.9 × 105 mol L−1).30 Considering that the possibility of the formation of a solvated electron in a solvent may be correlated with the absence of reactivity of the solvent molecules with other solvated electrons,11 we first studied the possible reaction of the DEC molecules with electrons solvated in methanol. The decay of electrons solvated in neat methanol was compared with their decay in methanolic solutions of DEC at various concentrations. Actually, the decay of e−MeOH was unchanged and no reaction with DEC was detected even at concentrations as high as 1 mol L−1 of DEC in methanol (Figure S1, Supporting Information). The absence of reaction between DEC and e−MeOH even at 1 mol L−1 DEC in methanol solutions supports indeed that the solvation of electrons in DEC is more probable than an attachment of the electrons onto a DEC molecule. Neat Diethyl Carbonate. The transient absorbance spectrum detected in neat DEC after the irradiation pulse extends from the UV to the visible and the near-infrared. Figure 1 presents the evolution with time of the product G × ε in L J−1 cm−1. In the wavelength range from 400 to 1400 nm, the absorbance initially increases immediately within the pulse. At the end-of-pulse, the shape of the spectrum is constituted of a very broad absorbance increasing monotoneously from 400 nm where G40ps × ε400nm = 0.3 × 10−3 L J−1 cm−1 up to the near-IR (NIR) detection limit at 1400 nm where G25ps × ε1400nm = 3.6 × 10−3 L J−1 cm−1. After the pulse, the time evolution of the absorption spectrum in the visible and the NIR is a decay from 25 ps to 200 ns but the spectrum shape changes, as shown in Figure 1, left. In the NIR from 1100 to 1400 nm, the absorbance decreases rapidly, by more than two-thirds within 3 ns from G25ps × ε1400nm = 3.6 × 10−3 L J−1 cm−1 at 25 ps to G3ns × ε1400nm = 1.0 × 10−3 L J−1 cm−1 and vanishes at about 150 ns (Figure 1, left). Instead, in the visible, the absorbance at 700 nm decreases by only one-half within 3 ns. Actually, the decay is constituted of two components, the first one being as fast as in the NIR (from G25ps × ε700nm = 0.85 × 10−3 L J−1 cm−1 at 25 ps to G3ns × ε700nm = 0.45 × 10−3 L J−1 cm−1 at 3 ns), and the second one being much slower and lasting up to more than 400 ns (Figure 2, and Figure S2, Supporting Information). A weak band with a maximum at around 600−650 nm is now observed in the 40−400 ns range (Figure 1, right). In addition, a UV band of very low intensity is also observed (Figure 1, right). The absorbance first increases rapidly just after the pulse up to a maximum value at 1 ns of G1ns × ε330nm = 3.5 × 10−4 L J−1 cm−1 and then decays very slowly to 10−4 L J−1 cm−1 at 40 ns and still 0.6 × 10−4 L J−1 cm−1 at 400 ns (Figure 1, right and Figure S2, Supporting Information). The results on the time evolution of the transient spectra between 300 and 1400 nm suggest that they are constituted of at least three overlapping bands, which should be assigned to three different radiolytic species, one short-lived species in the NIR, and two long-lived species in the visible and in the UV. Because the decay time scales are distinct, the NIR species do not disappear by mutual reaction with the others. As a general mechanism, the primary effects of radiation on a liquid are to ionize and excite the molecules.10 Therefore, we assume that the same processes occur in DEC:

Figure 2. Decay of the product G × ε at various wavelengths in neat DEC. The abscissae scale is in logarithmic units. 330 nm: streak camera data (40 ps to 300 ns). 700 nm: Streak camera data (40 ps to 300 ns) and pump−probe data (25 ps to 3 ns). 700−1200 nm: pump−probe data (25 ps to 3 ns) and Febetron data (≥7 ns).

DECvvvv → DEC*, DEC+•, e−

(1)

The excited states DEC* readily deactivate by collisions:

DEC* → DEC

(2)

Hence, we neglect the formation of transients issued from the dissociation of DEC*. The shape of the transient spectra in neat DEC with an absorbance increasing to the NIR suggests an assignment to an electron solvated in DEC: e− + DEC → e−DEC

(3)

The fast initial increase of the NIR absorbance within the picosecond pulse indicates that the solvation time τsolv of the process in eq 3 is much shorter than the detection time, that is, τsolv < 20 ps. Before solvation, part of the electrons may have been consumed by recombination with the cation DEC+•. The second long-lived species absorbing around 650 nm is also formed very early. It should be issued from the primary cation DEC+•, produced in reaction 1. However, it is excluded that the 650 nm band observed at picosecond scale would be assigned to the primary cation DEC+• itself, because, if so, it would disappear rapidly by geminate recombination with e−DEC at the same rate, in contrast with the experimental results. The primary cation DEC+• may undergo a fast ion−molecule reaction with DEC, homologue of the process occurring in water31 and alcohols.32 This would result, via a proton transfer, in the DEC protonated form DECH+ and the neutral radical DEC(−H)•: DEC+• + DEC → DEC( −H)• + DECH+

(4)

According to the very low dielectric constant of DEC (εs = 2.83), the decay of e−DEC, which is mainly due to the geminate recombination with the protonated form DECH+, should be indeed very fast and it is in fact observed as almost over at 100 ns (Figure 2). 10803


The Journal of Physical Chemistry A DECH+ + e−DEC → DEC(+ H)•

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these opposite charged species in the slightly polar solvent DEC of low dielectric constant. Ethanol in DEC Solvent. To examine the part of the oxidizing radicals in the transient spectra of irradiated neat DEC, ethanol was added at 0.5 and 1 mol L−1 to the solvent DEC. The radiation dose directly absorbed by ethanol is only 2 or 4%, of the dose in DEC (8 mol L−1), respectively, and was neglected. As shown in Figure 4, right, the decay of the NIR absorbance at 1200 nm is unchanged in the presence of ethanol that does not react indeed with electrons solvated in DEC.

(5)

The critical Onsager radius rc in this solvent is very large (rc(DEC) = e2/εs k T = 200 Å), where e is the electron charge, k is the Boltzmann constant, and T is the temperature). Hence, the escape probability of the electrons from the recombination and the free electron yield are expected to be very low. Also, due to the low dielectric constant, the maximum of the absorption band of e−DEC is expected, as in ethers,14 to be located at a wavelength much higher than our upper NIR detection limit of 1400 nm. To identify and calibrate in molar absorption coefficients and yields the different components of the neat solvent transient spectra, we studied the influence of various solutes used as selective radical scavengers. Perchloric Acid Solutions in DEC. In concentrated solutions of perchloric acid (70% HClO4 in water) in the low dielectric constant solvent DEC, the HClO4 molecules are probably only partly dissociated into the hydronium ion H3O+ and the perchlorate anion. Nevertheless, the pairs (ClO4−,H3O+) are also expected to react with the solvated electrons. In both cases, the expected resulting radical is DEC(+H)•. However, the neutral pairs should react with e−DEC more slowly than the hydronium H3O+. At 5 × 10−2 mol L−1 HClO4, the decay of the NIR absorbance as well as of the first component of the decay in the visible are both markedly accelerated, compared to that for neat DEC, so confirming their assignment to e−DEC (Figure 3). The

Figure 4. Effect of the addition of 0.5 mol L−1 ethanol, compared to that in neat DEC, on the kinetics of the absorbances at 700 nm (left) and 1200 nm (right).

In contrast, when the solvent contains ethanol at 0.5 mol L−1, the initial absorbance at 700 nm and 25 ps is markedly decreased within the pulse and also the further decay is accelerated (Figure 4, left). The reaction seems to be over within 1 ns. The kinetics at 700 nm after that time is now correlated with the 1200 nm decay and should be also assigned to the absorbance of the solvated electron e−DEC at 700 nm. Thus the absorbance depletion induced by ethanol at 0.5 mol L−1 corresponds to the scavenging of the second species absorbing at 700 nm that was detected in the neat DEC spectrum. Ethanol is known to act as a scavenger of oxidizing radicals via electron tranfer or H-atom abstraction and to produce radicals CH3C·HOH with strong reducing properties.33 Acetone in DEC Solvent. Acetone, which is an efficient electron scavenger, has been added to DEC at different concentrations (10−2, 0.5, and 1 mol L−1). In contrast with neat DEC or ethanol solutions, the NIR absorbance now decays very rapidly in acetone solutions, hence confirming the assignment of this band to e−DEC (Figure 5 bottom, left and right).

Figure 3. Decay at picosecond scale of the product G × ε in 5 × 10−2 mol L−1 HClO4 solutions in DEC compared to that in neat DEC at 700 nm (left) and 1200 nm (right).

NIR absorbance at 1200 nm vanishes almost totally within 1 ns. The absorbance at 700 nm disappears in acidic medium with two components (Figure 3). The first one corresponds to the visible part of the solvated electron spectrum and is correlated with the NIR decay within 1 ns. Within 1 and 3 ns, the second component of the absorbance that is distinct from e−DEC (G1ns × ε700nm = 0.25 × 10−3 L J−1 cm−1) remains almost constant. Simultaneously, there is no increase in the visible and NIR that could be assigned to the product DEC(+H)• of the reaction of e−DEC. From the pseudo-first-order rate constant of the NIR absorbance decay and the HClO4 concentration, the overall rate constant of the reaction is close to k(e−DEC+HClO4) = (8 ± 3) × 1010 L mol−1 s−1 (Figure S3, Supporting Information) and is a lower limit for the reaction of e−DEC with free H3O+ between

e−DEC + (CH3)2 CO → (CH3)2 CO−•

(6)

High concentrations of the scavenger are required to compete efficiently with the geminate recombination (5). The initial NIR absorbance at 25 ps in 10−2 mol L−1 acetone solutions is the same as in neat DEC, but it is less than half at 0.5 mol L−1 (Figure 5, bottom right). The absorbance in the region 1100−1400 nm is suppressed after 4 ns in 10−2 mol L−1 solutions and after 150 ps only in 0.5 mol L−1 solutions. The rate constant of reaction 6 can be evaluated as k6 = 4 × 1010 L mol−1 s−1 (Figure S4, Supporting Information) corresponding to the value C37(acetone) = 0.5 mol L−1 (where 37% of 10804



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DEC(− H)• + CH3CH 2OH → DEC + CH3CH•OH (8)

with the rate constant k8 = (1 ± 0.3) × 1010 L mol−1 s−1 DEC(− H)• + CH3COCH3 → DEC + CH3COCH 2• (9) −1 −1

with the rate constant k9 = (1.5 ± 0.5) × 10 L mol s , to be compared with the homologue rate constant of H abstraction from acetone by the •OH radical in water (k = 1.1 × 108 L mol−1 s−1).30 Because H atoms are not efficiently scavenged by acetone in aqueous solution, we assume that it is the same for the radicals DEC(+H)•. Thus the transient spectrum at 150 ps in 0.5 mol L−1 acetone solutions may also contain in the UV (330−400 nm), a component assigned to DEC(+H)•. Taking into account the spectrum of the radical DEC(−H)• from the results in 0.5 mol L−1 acetone solutions at 150 ps (Figure 5, top), we conclude that the initial value of the G × ε product for this species at 25 ps and 700 nm is G25ps × ε700nm (DEC(−H)•) = 0.45 × 10−3 L J−1 cm−1. Hence, from the difference with the total absorbance at 25 ps in the neat solvent, one obtains for the solvated electron: G25ps × ε700nm(e−DEC) = 0.47 × 10−3 L J−1 cm−1. We obtain separately the decay kinetics of e−DEC at 700 nm in neat DEC (Figure 6) after normalizing with the ratio 9

Figure 5. Top: transient absorption spectra in 10−2, 0.5, and 1 mol L−1 acetone solutions in DEC at 25 ps, 1 ns, and 150 ps. Bottom: decay kinetics of the absorbance at 700 nm (left) and 1200 nm (right).

electrons survive at 25 ps). The homologue rate constant of hydrated electron scavenging by acetone in water is kw = 6.5 × 109 L mol−1 s−1.30 The radical (CH3)2CO−• that is produced is expected not to absorb in the visible but in the UV as in water,34,35 and to display strong reducing properties. It may be converted to the neutral form (CH3)2C•OH by subsequent reaction with DECH+ at longer time: (CH3)2 CO−• + DECH+ → (CH3)2 C•OH + DEC

(7)

Figure 6. Two kinetics components of e−DEC and DEC(−H)• at 700 nm in the experimental signal in neat solvent. The respective end-ofpulse absorbances of e−DEC and DEC(−H)• are derived from the data in 0.5 mol L−1 acetone solution. The decay kinetics of e−DEC at 700 nm is then derived by normalization of the signal at 1200 nm in neat solvent. The decay kinetics of DEC(−H)• is obtained by subtracting the e−DEC kinetics from the experimental decay in neat DEC at 700 nm (see text).

Simultaneously, at 700 nm the initial fast decay within 150 ps corresponds to the same solvated electron scavenging as at 1200 nm (Figure 5, bottom right). Hence, the absorbance signal at 700 nm beyond that time in 0.5 mol L−1 solutions is assigned to the sole second long-lived species and the transient spectrum at 150 ps corresponds to this second species (Figure 5, top). The species with an absorption band at 600−700 nm then decays in 0.5 mol L−1 acetone solutions within 3 ns (Figure 5, bottom left), which is much faster than in neat DEC, but markedly slower than the solvated electron (Figure 5, bottom right). The species absorbing at 700 nm is scavenged in both ethanol and acetone solutions. We excluded the cation DECH+ for the assignment because its decay would be at the same rate as e−DEC. The DEC(+H)• radical was not observed in HClO4 solutions within the domain investigated and was supposed to absorb in the UV. Hence, we assign the 700 nm component to the radical DEC(−H)• that would abstract H atoms from ethanol and acetone:

G25ps × ε700nm(e−DEC)/G25ps × ε1200nm(e−DEC) the corresponding kinetics for e−DEC at 1200 nm in neat solvent (Figure 5, bottom right). Then, by subtracting this signal from the experimental kinetics, we obtain the component of DEC(−H)• kinetics in neat solvent (Figure 6). The comparison, shown in Figure S2 (Supporting Information), between the simultaneous decays at long time in neat DEC of the absorbances at 600 nm assigned to DEC(−H)• and at 330 nm assigned to DEC(+H)• (arising from the geminate recombination (5) of the solvated electrons) suggests that both radicals DEC(−H)• and DEC(+H)• are consumed by the same reaction 10: 10805


The Journal of Physical Chemistry A DEC( −H)• + DEC( +H)• → 2DEC

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H3CCH 2OCOO•CHCH3

(10)

Note that in 5 × 10−2 mol L−1 HClO4 solutions (Figure 3, left) the absorbance at 700 nm after the solvated electron scavenging was, at a given time, lower than in neat DEC and in acetone solutions (Figure 5, left). Actually, not only the recombination (5) was markedly accelerated, but also the faster formation of DEC(+H)• via (5) induced a faster decay of DEC(−H)• by (10). Another hypothesis is that reaction 4 could be reversible and that the primary cation radical DEC+• does not absorb or less than DEC(−H)• in the visible. The shape of the absorption spectrum of the oxidized diethyl carbonate radical DEC(−H)• is given by the absorption spectrum obtained in 0.5 mol L−1 acetone after 150 ps when the solvated electrons are scavenged (Figure 5, top). It looks like the broad spectrum at long time in neat DEC with a maximum at 600−650 nm (Figure 1, right). As noticed above, a component due to DEC(+H)• may be superimposed at 330− 400 nm. Due to the very slow decay of the radical DEC(−H)•, the spectrum at 25 ps is assumed to be the same as at 150 ps (Figure 7).

→ H3CCH 2OC•OHOCHCH 2

(11)

The absorption spectrum of the carbonate C-centered radical H3CCH2−OC•OHOCHCH2, as it is observed with a maximum at 600−650 nm (Figures 1, right, and 7), compares probably better than that of the methylene Ccentered radical H3CCH2OCOO•CHCH3 with the spectrum of the inorganic carbonate CO3H• that absorbs in the visible.36 Biphenyl in DEC Solvent. To determine the timedependent yield of the solvated electron, the value of its molar extinction coefficient is required. Therefore, we studied the electron scavenging by the solute biphenyl, Ph2, and the formation yield of the anion radical Ph2−•. The transient spectra and the kinetics signals of solutions of biphenyl in DEC at increasing concentrations were observed at biphenyl concentrations from 5 × 10−4 to 1.0 mol L−1. The decay of the NIR absorption band of e−DEC is accelerated in these solutions compared to that for neat DEC (Figure 8, bottom). Biphenyl is indeed known to react rapidly with solvated electrons in producing radical anions Ph2−•: e−DEC + Ph 2 → Ph 2−•

(12)

The anion radical Ph2−• absorbs strongly at 405 nm (ε410(Ph2−•) = 3.6 × 104 L mol−1 cm−1) and 640 nm (ε640(Ph2−•) = 1.2 × 104 L mol−1 cm−1).38 We assume that the molar absorption coefficients are the same in DEC as in other solvents. In DEC solutions containing biphenyl, the absorption spectrum changes rapidly from the initial value as in neat DEC to the absorption spectrum of the radical anion Ph2−• as it is shown in Figure S5 (Supporting Information). No other contribution of any absorbance is observed with a maximum or shoulder close to 690 nm that could be assigned to the radical cation Ph2+• 39 (ε690(Ph2+•) = 8 × 103 L mol−1 cm−1)40 as formed by the scavenging of the primary cation DEC+•. The Ph2−• absorbance, as measured at 410 nm, increases with the biphenyl concentration, and its formation is over for instance within 1 ns at 10−2 mol L−1 (Figure 8, top right). However, as pointed out in a previous work on THF,41 the Ph2−• buildup is a two-step process, the first step being extremely fast within the pulse, and the second one during the first hundreds of picoseconds, but both are much faster than in THF (Figure 8, top). According to the discussion on THF results,41 biphenyl probably scavenges not only e−DEC but in the first fast step also part of electrons prior to their solvation because their mobility is much larger and their scavenging rate constant should be still higher than for e−DEC. The first step rate of the presolvated electrons scavenging reaction is already over soon after 25 ps. It is too fast to enable us to determine a rate constant. Regarding the second step of the solvated electrons scavenging, the bimolecular rate constant k12 is derived from the dependence on the biphenyl concentration of the observed pseudo-first-order rate constant kobs of the second-step Ph2−• formation at 630 nm, as k12 = 2.7 × 1011 L mol−1 s−1 (Figure S6, Supporting Information) (that is 7 times faster than for the scavenging rate by acetone or C37(Ph2) = 0.07 mol L−1). The reaction is faster than in THF or diethyl ether (DEE) (Table 1). We derive42 the value of the solvated electron mobility μ(e−s ) from the rate constant k(e−s +Ph2) via the Smoluchowski

Figure 7. Absorption spectra of e−DEC and DEC(−H)• as components of the transient optical absorption spectra in neat diethyl carbonate at 25 ps. The spectrum of DEC(−H)• (λmax = 600−650 nm) is derived from the spectrum in 0.5 mol L−1 acetone at 150 ps, and the spectrum assigned to e−DEC (λmax > 1400 nm) is obtained by subtracting this spectrum from the experimental spectrum at 25 ps in neat DEC.

We obtain the whole spectrum of e−DEC at 25 ps in the neat solvent in G × ε units (Figure 7) by subtracting this radical DEC(−H)• spectrum obtained in the 0.5 mol L−1 acetone solution at 150 ps (Figure 5, top) from the total spectrum at 25 ps in the neat solvent (Figure 1). The absorption maximum of e−DEC is beyond 1400 nm, in agreement with other solvents of low dielectric constants as ethers.14 The radical DEC(−H)• spectrum may be compared with that of the oxidized radical of the inorganic carbonate CO3H• (λmax = 600 nm, ε = 1800 L mol−1 cm−1).36 The structure of the radical DEC(−H)• (reaction 4) after the H abstraction from the methylene group (similar to the H-abstraction mechanism by •OH in water with k•OH+DEC = (7.9 ± 3.2) × 108 L mol−1 s−1)37 is presumably H3CCH2OCOO•CHCH3, which is assumed to absorb in the UV as alkyl radicals. However, its structure may be also rapidly rearranged by intramolecular migration or intermolecular transfer of an H atom from the terminal methyl to the carbonyl oxygen and the creation of a double −CC− bond: 10806



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Figure 8. Top: dependence of the absorbance kinetics on the biphenyl concentration in DEC at 410 nm (pulse 5 ps, dose 50 J L−1). Left: 10−3 and 2.5 × 10−3 mol L−1. For clarity, the signals are shifted by 0.2 units on the ordinates scale. Right: 10−2 mol L−1. Bottom: dependence of the absorbance kinetics of e−DEC on the biphenyl concentration at 1075 nm (pulse 5 ns, dose 500 J L−1).

Assuming that the sum of reactant radii is 5 Å,32 the mobility of e−DEC in DEC is μ(e−DEC) = 2.5 × 10−2 cm2 s−1 V−1, which is higher than μ(e−THF) = (0.4 − 1.1) × 10−2 cm2 V−1 s−1 in THF,11,44,45 and than μ(e−DEE) = 1.2 × 10−2 cm2 V−1 s−1 in diethyl ether42 (Table 1). Then, the Ph2−• absorbance decays rapidly (within 150 ns at [Ph2] = 10−2 mol L−1), via probably a reaction with the protons DECH+ (Figures S7 and S8, Supporting Information). In spite of this fast decay, an important feature in the time evolution of the absorption spectrum during the second step of the reaction formation 12 of Ph2−•, as observed for either 500 ps at low (10−2 mol L−1) (Figures S9 and S10, Supporting Information) or 50 ps at high concentration (1 mol L−1) (Figure S11, Supporting Information), is that, at a given wavelength, the time-depending absorbance increases between 540 and 720 nm and decreases below 540 nm and beyond 720 nm. This is a signature of two isosbestic wavelengths at 540 and 720 nm. Hence, the molar absorption coefficients at these wavelengths of the solvated electron and the stoichiometrically resulting Ph2−• (taken from the spectrum in ref 38) are equal during the scavenging time:

Table 1. Comparison in Some Solvents and DEC of the Reaction Rate Constants of Solvated Electrons with Biphenyl and of the Calculated Solvated Electron Mobilities solvent H2O43 H2O40 THF41 THF44 THF45 DEE42 DEC (this work)

dielectric constant εs 78

viscosity η (mPa) 0.89

7.4

0.46

4.4 2.83

0.25 0.748

reaction rate constant k(e−s + Ph2) (L mol−1 s−1)

solvated electron mobility μ(e−s )42 (cm2 V−1 s−1)

7 × 109 1.2 × 1010 3.9 × 1010 5.8 × 1010 1.1 × 1011 1.2 × 1011 2.7 × 1011

8 × 10−4 1.2 × 10−3 4 × 10−3 6 × 10−3 1.1 × 10−2 1.2 × 10−2 2.5 × 10−2

equation for diffusion-controlled reactions and the Nernst− Einstein equation: k(e−s +Ph 2) = 4πNAkBTR reactμ( ‐s)/103e

with NA the Avogadro number, kB the Boltzmann constant, T the absolute temperature in K, Rreact the reaction distance equal to the sum of reactant radii in Å, and e the electron charge in C. In Table 1 are compared, in DEC, water, and some ethers, the solvent physical properties, the corresponding reaction rate constants between the solvated electron and the biphenyl and the solvated electron mobilities.

ε540(e−DEC) = ε540(Ph 2−•) = 2.5 × 103 ε720(e−DEC)

−•

and

= ε720(Ph 2 ) = 3.6 × 10 L mol−1 cm−1 3

Note that the absorbance, while decreasing at 520 nm (Figure S9, Supporting Information), increases also over 500 ps close to the second sharp band maximum of Ph2−• around 410 nm. 10807



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However, the uncertainty is high on the intermediate isosbestic wavelength value around 440 nm. − Considering the values of ε(eDEC ) at the isosbestic wavelengths 540 and 720 nm (determined from the spectrum in ref 38), we are now enabled to calibrate the whole e−DEC spectrum of Figure 7 (that was expressed in G × ε units) in molar absorption coefficients (Figure 9). The molar absorption

Figure 10. Time evolution of the solvated electron yield in neat DEC. The data of the curve are obtained from the absorbance decay of e−DEC in the NIR and from the extinction coefficients derived from the isosbestic points with the Ph2−• absorption spectrum.

at 550 ps and 635 nm, G500ps × ε635nm = 1.2 × 10−3 L J−1 cm−1, measured in 5 × 10−2 mol L−1 biphenyl solutions (Figure S5, Supporting Information), is the sum of the absorbances of DEC(−H)• (0.3 × 10−3 L J−1 cm−1) and of the Ph2−• anion radical (0.9 × 10−3 L J−1 cm−1). Because ε635nm(Ph2−•) = 12000 L mL−1 cm−1,38 G550ps(Ph2−•) = 0.75 × 10−7 mol J−1. Therefore, G550ps(e−DEC) > 0.75 × 10−7 mol J−1. At the much higher concentration of 1 mol L−1 of biphenyl, the absorbance value at 635 nm, G25ps × ε635nm = 4.2 × 10−3 L J−1 cm−1, is also the sum of the absorbances of DEC(−H)• (0.3 × 10−3 L J−1 cm−1) and of the Ph2−• anion radical (3.9 × 10−3 L J−1 cm−1). Therefore, G25ps(Ph2−•) = 3.25 × 10−7 mol J−1. Indeed, as in THF,41 the scavenging of presolvated electrons already in 0.1 mol L−1 biphenyl solutions is still more efficient than at lower concentrations (Figure 8) in the competition with their recombination with the primary cations DEC+• in the spurs (Figure 11). Hence, though the solvation time is not yet determined, the value G25ps(Ph2−•) = 3.25 × 10−7 mol J−1 constitutes the scavenging yield at 1 mol L−1 Ph2 of presolvated electrons and possibly solvated electrons, which is a lower limit for the total initial yield of the negative charges G0(ion) ≥ 3.25

Figure 9. Optical absorption spectrum of e−DEC calibrated in molar absorption coefficient and compared with solvated electron spectra in some other solvents: water,12,13 methanol,14 1,2-propanediol,47 liquid ammonia (9 bar),48,49 hydrazine,50,51 ionic liquid R4NNTF2,15 THF,14 diethyl amine,14 diethyl ether,14 liquid n-propane-d8.52

coefficients of e−DEC in the NIR are for example ε1200nm(e−DEC) = 2.0 × 104 L mol−1 cm−1 at 1200 nm, and ε1400nm(e−DEC) = 2.8 × 104 L mol−1 cm−1 at 1400 nm. Hence, we can compare the e−DEC spectrum, at least in the range up to our NIR detection limit of 1400 nm, with those of solvated electrons in other solvents (Figure 9). Though the wavelength of the maximum for e−DEC could not be determined, its absorption spectrum appears to be close to the spectra in ethers,14 where also the dielectric constant is very low, and thus the interaction electron−solvent is weak.46 Radiolytic Yield of the Solvated Electron in DEC. From the values of the molar absorption coefficients ε(e−DEC) in the NIR determined above (Figure 9) and from the decay kinetics of the G × ε values in this region where only e−DEC absorbs (Figure 2), the time evolution of the radiolytic yield of the solvated electron e−DEC in neat DEC may be now derived (Figure 10). The yield of e−DEC in neat DEC decreases rapidly from the value G25ps(e−DEC) = 1.4 × 10−7 mol J−1 at 25 ps. The G25ps(e−DEC) value is much lower than in other polar solvents, such as THF41 because of the lower dielectric constant and the faster geminate recombination (5). In addition to the information derived from the isosbestic wavelengths, lower limits of scavenging yields may be obtained from the 635 nm absorbance values (where one of the maxima of the Ph2−• spectrum is located) that are observed at concentrations of 5 × 10−2 and 1 mol L−1 of biphenyl at the end of the reaction (Figure S5, Supporting Information). Actually, as mentionned above, a supplementary process causes the Ph2−• anion radical absorbance to decay during the reaction of electron scavenging (Figure 8, top and Figure S7, Supporting Information) and the measured Ph2−• yield value is just a lower limit for the solvated electron yield. The total absorbance value

Figure 11. Tentative mechanism of the radiolysis of the neat diethyl carbonate: time evolution of reactions of e−DEC, DEC(−H)•, and DEC(+H)• (numbers refer to reactions in the text). 10808



Article

× 10−7 mol J−1. It is accepted that the initial ionization yield in solvents is around (4−5) × 10−7 mol J−1.10 However, it is worthy to note that, because ε(e−DEC) was determined from ε(Ph2−•) at isosbestic wavelengths for e−DEC and Ph2−•, the yield values of Figure 10 concern exclusively solvated electrons. The radical DEC(−H)• decays in neat DEC much more slowly than e−DEC, only over hundreds of nanoseconds (Figure S2, Supporting Information). These radicals are issued from reaction 4 of primary cations having escaped to the recombination with presolvated electrons (the excited molecules DEC* are assumed to deactivate without dissociation) (Figure 11). The yield of the long-lived radical DEC(−H)• at 25 ps should be equal, from the material balance, to the initial yield of solvated electrons at the solvation time but this is as yet undetermined. The long-lived UV band at 320 nm in neat DEC is tentatively assigned to DEC(+H)• radicals (produced by reaction 5). This is specifically supported by the fast supplementary formation of the absorbance at 330 nm between the end-of-pulse and 1 ns, which may be correlated with the fast decay in the NIR of e−DEC assigned to the recombination reaction 5. Then, after a maximum value, the DEC(+H)• absorbance decays at long time (>150 ns) at the same slow rate as DEC(−H)• (Figure S2, Supporting Information), suggesting that the species DEC(+H)• and DEC(−H)• in neat DEC are both consumed in the same reaction 10 to form again DEC and are the only ones left, hence with the same yield. Because, within this time scale, the absorbance of DEC(+H)• at 330 nm is higher than that of DEC(−H)• at 600 nm (Figures 2 and S2, Supporting Information), we conclude that ε 330 nm (DEC(+H)• ) > ε600 nm(DEC(−H)•). Eventually, the mechanism of main reactions 1−5 and 10 in neat DEC, which is summarized in Figure 11, results in the complete recombination into DEC molecules that were initially ionized or excited in (1). Therefore, the final degradation yield would be close to zero, a significant property for its use in any radiation environment (space or nuclear plants). This conclusion is in agreement with former γ-radiolysis results indicating yields of final gaseous products (such as H2, CH4, CO) lower than the detectable limit,7−9 that is, a radioresistance of neat DEC. These results also imply either that the initial excited DEC molecules deactivate (reactions 1 and 3) or that their dissociation products recombine into DEC. Our hypothesis on a minor role of radiolytic species other than DEC(−H)•, DEC(+H)•, DECH+, and e−DEC is thus justified.

Electron transfer processes to solutes have been studied and rate constants determined. These data are applicable not only in alkyl carbonate radiation chemistry but also in free radical chemistry and electrochemistry such as in Li-ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S11 of absorbance decays, absorption spectrum, rate constant variation, formation and decay kinetics, and time evolution of the absorbance. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*J. Belloni: e-mail, [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally to results, to the discussion and to the writing of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge J.-P. Larbre, P. Jeunesse, F. Gobert, A. Demarque, and Ma Jun for their technical assistance, and the Réseau Thématique de Recherches Avancées “Triangle de la Physique”’ for the financial support.

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REFERENCES

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CONCLUSION In spite of the low dielectric constant of the ester diethyl carbonate DEC that favors the very fast reactions between oppositely charged species, the kinetics and spectral properties of the radiolytic species in this solvent were studied by using the picosecond pulse radiolysis facility ELYSE. From scavenging studies by various solutes, the optical absorption spectrum of the solvated electron has been identified. The absorption maximum is in the near-infrared, above 1400 nm where ε1400nm (e−DEC) = 2.8 × 104 L mol−1 cm−1. The radiolytic yield (1.4 × 10−7 mol J−1 at 25 ps) and mobility (2.5 × 10−2 cm2 V−1 s−1) of the solvated electron in DEC have been compared with those for homologues in other solvents. The mechanism of DEC radiolysis has been discussed. It supports a marked radio-resistance of the solvent. 10809



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