The AHA Moment - ACS Publications - American Chemical Society

Article pubs.acs.org/JPCB

The AHA Moment: Assessment of the Redox Stability of Ionic Liquids Based on Aromatic Heterocyclic Anions (AHAs) for Nuclear Separations and Electric Energy Storage Ilya A. Shkrob*,† and Timothy W. Marin†,‡ †

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ Chemistry Department, Benedictine University, 5700 College Road, Lisle, Illinois 60532, United States S Supporting Information *

ABSTRACT: Because of their extended conjugated bond network, aromatic compounds generally have higher redox stability than less saturated compounds. We conjectured that ionic liquids (ILs) consisting of aromatic heterocyclic anions (AHAs) may exhibit improved radiation and electrochemical stability. Such properties are important in applications of these ILs as diluents in radionuclide separations and electrolytes in the electric energy storage devices. In this study, we systematically examine the redox chemistry of the AHAs. Three classes of these anions have been studied: (i) simple 5-atom ring AHAs, such as the pyrazolide and triazolides, (ii) AHAs containing an adjacent benzene ring, and (iii) AHAs containing electron-withdrawing groups that were introduced to reduce their basicity and interaction with metal ions. It is shown that fragmentation in the reduced and oxidized states of these AHAs does not generally occur, and the two main products, respectively, are the H atom adduct and the imidyl radical. The latter species occurs either as an N σ-radical or as an N π-radical, depending on the length of the N−N bond, and the state that is stabilized in the solid matrix is frequently different from that having the lowest energy in the gas phase. In some instances, the formation of the sandwich π-stack dimer radical anions has been observed. For trifluoromethylated anions, H adduct formation did not occur; instead, there was facile loss of fluoride from their fluorinated groups. The latter can be problematic in nuclear separations, but beneficial in batteries. Overall, our study suggests that AHA-based ILs are viable candidates for use as radiation-exposed diluents and electrolytes.

1. INTRODUCTION Ionic liquids (ILs) represent the largest body of singlecomponent organic liquids. Some estimates put their total number over 1010−12,1 whereas all other organic liquids add to less than a thousand. Whatever might be desired in a solvent, one can be reasonably assured that in this vast multitude there is a subset of ILs having the requisite properties, or any combination of these properties. However, finding this subset might be a daunting task. Trial and error, this venerable approach used in the optimization of molecular solvents, is less suitable for ILs, as the number of possibilities to explore can be enormous. The space of choice needs to be reduced and tightly constrained. The desired IL property that is examined in this study is radiation stability.2−6 This property is important in a diluent that is used for processing of spent nuclear fuel, as the cumulative dose per life cycle of the solvent used in liquid− liquid extractions of radionuclide cations can exceed 100 Mrad (100 rad = 1 J/kg). Ionizing radiation excites the constituent ions, causing them to fragment and/or ionize. The resulting species (see below) can also fragment, yielding secondary products. It is impossible to prevent energy deposition into the IL solvent bulk, as the radiation is emitted by radionuclide ions © 2015 American Chemical Society

that are already in this solvent. The typical excitation/ionization event induced by 1−3 MeV particles (such as α, β, and γ rays) deposits a few electronvolts of the energy into the solvent; this energy is sufficient to break any chemical bond. Thus, the “radiation stability” means, in practice, good reversibility of the radiation-induced reactions; all other means to protect the solvent (e.g., by using radical scavenging) are temporary. This implies relative stability of the reaction intermediates (radicals, excited states) to bond fission, so the excess energy is mainly dispensed through charge separation. Similar concerns exist in electrochemistry, as the redox active states generated during solvent breakdown are identical to the states that are generated radiolytically,7−9 and there is a deep connection between the two fields.10 Radiolytically and electrochemically stable ILs are solvents in which the radiation-induced neutralization of the constituent cations (CH+) and anions (A−):

A− → e−• + A•

(1)

Received: September 16, 2015 Revised: October 20, 2015 Published: October 27, 2015 14766

DOI: 10.1021/acs.jpcb.5b09057 J. Phys. Chem. B 2015, 119, 14766−14779

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The Journal of Physical Chemistry B CH+ + e−• → CH•

(2)

Scheme 1. Structural Formulas, Abbreviations, and Numbering Schemes for the Selected Simple AHAs

or their further charging: A− + e−• → A2 −•

(3)

CH+ → CH2 +• + e−•

(4)

yields species that do not easily fragment. Naturally, with stronger chemical bonds, the conjugated bond network over which the excess charge can delocalize is more extensive, making fragmentation reactions less likely. In most applications of ILs as solvents, the only chemical properties of the IL ions of interest are those in their normal charged state, and only such properties become optimized. Unfortunately, many of these properties inversely correlate with the properties of radical species that are generated in reactions 1−4. For example, while the parent anions can be exceptionally stable, their oxidized states may be highly unstable.11 For this reason, some of the compositions for “radiation-hard” ILs look exotic even to the experts in this field. On the other hand, the IL diluents that are currently being assessed for nuclear separations are quite often poorly suitable for this application, regardless of how well such systems perform in the absence of radiation, because of the extensive radiation damage.5 Finding the sweet spot, where the ILs diluents exhibit both separations efficacy and radiation stability, is a challenging task. The Achilles’ heel of the ILs is reaction 1.4,5,11 As the electron detaches from the electronically excited R−B− anion, the residual radical promptly dissociates: (R−B−)* → e−• + R• + B

Scheme 2. Structural Formulas, Abbreviations, and Numbering Schemes for the Benzene Ring Containing AHAs

(5)

−

In reaction 5, B denotes a polar headgroup (e.g., CO2−, −SO3−, −OSO2−, etc.). The majority of the ILs solvents cannot be used in high radiation fields, as they consist of anions for which reaction 5 is facile. This leaves just a few classes of hydrophobic anions, such as CxNy−12 and imide anions11,13 that do not fragment after oxidations (there are isolated examples of radiolytically stable anions of other classes). By far the largest class of such anions encompasses aromatic heterocyclic anions (AHAs). These are familiar to IL researchers in a different context, as they are used as reversible absorbers of CO2 and SO2 gases.14−17 With carbon dioxide, these AHAs react by forming the corresponding carbamate anions:

A− + CO2 ⇌ ACO2−

(6)

conditions (such as Sr2+ and Cs+). However, the avoidance of reaction 5 is such a prized property that overlooking this largest class of (potentially) radiation-resistant ILs can be ill afforded: separations and such ILs need to be adapted to each other. This task is less prohibitive than it may seem. Some of the recently developed processes do not involve aqueous solutions at all, relying on ILs that are immiscible with organic solvents, including other ILs.22,23 In such systems, hydrolysis of the AHAs may not be a concern. Still, overly strong coordination of d- and f- block metal ions by AHAs can interfere with the extraction chemistry. The basicity, however, can be tuned through the appropriate choice of AHAs. In particular, strategic placement of electron-withdrawing groups makes it possible to increase pKa values into an acceptable range (as is also the case for the imide12 and polycyanide13 anions). For example, the pKa value for unmodified tetrazole (N1234 in Scheme 1) is 4.8; for 5-halosubstituted tetrazole, it is 2−2.1, and for 5trifluoromethyl and 5-nitro tetrazoles, it further decreases to 1.0 and −0.8, respectively.24

Some of the AHAs used in CO2 capture are shown in Schemes 1 and 2, and the abbreviations used below are given in this scheme and Tables 1 and 2. Scheme 1S indicates the nitrogen atoms involved in the carbamate formation (see ref 17 and references therein). Naturally, there is a strong correlation between the basicity of the AHAs and the enthalpy of reaction 6; see Table 1S and Figure 1S. Such expandable, switchable IL solvents currently attract a lot of attention (e.g., see refs 18−21). Most of these AHAs and their respective carbamates are basic, undergoing hydrolysis on contact with water: A− + H 2O ⇌ HA + HO−

(7)

ACO2− + H 2O ⇌ HA + HCO3−

(8)

Reactions 7 and 8 preclude the use of such AHA-based ILs in noncaustic metal ion extractions by limiting their use to just a few metal ions that do not undergo hydrolysis under basic 14767


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The Journal of Physical Chemistry B Table 1. Abbreviations Used for the AHAs Examined in This Study AHA

full chemical name

Scheme

Oxa N1 2CNPyr N12 N13 N123 N124 N1234 Indo Isndo Cbz BnIm Inda BnTr DCTA DCTR TDI TCI DTCI TFI Phth TPA DTPA Sac BnDIm Ace

oxazolidinone pyrrolide 2-cyanopyrrolide pyrazolide imidazolide 1,2,3-triazolide 1,2,4-triazolide tetrazolide indolide isoindolide carbazolide benzimidazolide indazolide benzotriazolide 4,5-dicyano-1,2,3-triazolide 3,5-dicyano-1,2,4-triazolide 2-trifluoromethyl-4,5-dicyanoimidazolide 2,4,5-tricyanoimidazolide 2-cyano-4,5-di(trifluoromethyl)imidazolide 2,4,5-tri(trifluoromethyl)imidazolide phthalimide 3-trifluoromethylpyrazolide 3,5-di(trifluoromethyl)pyrazolide saccharinate 1,2-benzenedisulfonimide acesulfamate

1 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 2S 3 3 2S 2S 2S

Scheme 3. AHAs Containing Electron-Withdrawing Groups

and 5f electrons, enabling ion selectivity. In fact, many of the selective extraction agents for separations of minor actinides from lanthanides (which rely on “soft” atom interactions)34 include the protonated AHAs shown in Schemes 1 and 2, for example, 6-methyl-2-(2-pyridyl)benzimidazole,35 and the closely related triazinylpyridine N-donor ligands (see ref 36 and references therein). Another consideration is that carbamates and sulfamates are considerably less basic than the parent AHAs. While such anions undergo radiolytic decomposition similar to reaction 5:

Henderson’s group25,26 and others27,28 provided examples of this approach by synthesizing ILs composed of 2-trifluoro-4,5dicyanoimidazolide (TDI, Scheme 3) and 4,5-dicyano-1,2,3triazolide (DCTA, Scheme 3) that were used as electrolytes in Li ion batteries (see also ref 29). Such AHAs are as weakly coordinating as the more familiar imide anions (e.g., bistriflimide and dicyanamide) that are widely used to compose IL diluents. Furthermore, this coordination can be exploited rather than avoided (e.g., refs 30−32; see also ref 33), as AHAs have “soft” nitrogen atoms that can covalently interact with 4f

ACO2− → e−• + A• + CO2

(9) •

the subsequent charge transfer to A yields the parent anion that once again becomes a carbamate in the CO2 expanded IL solvent via reaction 6, reversing the radiolytic damage. That is, the radiolytic damage can be repaired by the addition of more CO2 to the IL solution.

Table 2. Selected Energetics for AHAs Shown in Schemes 1 and 2 and Related Species (Gas-Phase DFT Calculations) AHA

PA,a eV

EDE,b eV

D,c eV

DH,d eV

D,e eV

D,f eV

D,g eV

−ΔH0,h eV

DH,i eV

N1 N12 N13 N123 N124 N1234 Oxa 2CNPyr Indo Isndo Cbz BnIm Inda BnTr

15.91 15.72 15.47 15.34 15.22 14.68 15.75 14.99 15.50 15.36 15.30 15.10 15.49 14.90

2.07 3.00 2.57 3.52 3.81

4.37 5.11 4.43 5.26 5.43

0.60 0.79 0.62 0.87 1.39

1.19 0.77 1.26 1.70

2.04 3.52 2.04 3.62 4.02

3.33 4.06 3.60 4.39 4.86

2.63 3.08 2.31 1.92 2.40 3.23 2.71 3.97

4.78 4.46 4.21 3.67 4.10 4.72 4.53 4.80

1.23 1.06 1.15 0.92 1.08 0.96 1.22 1.35 1.43 1.56 1.55 1.40 1.56 1.52

2.84 1.95 1.85 1.69 1.55 2.66 2.44 2.82

3.51 3.64 3.29 2.82 3.17 4.01 3.46 3.50

1.45 1.33 1.47 1.27 1.33 1.18 0.21 1.45 1.41 1.99

1.18 0.79 0.41

0.59 0.66 0.66

1.42 1.55 1.55

Adiabatic proton affinity for A−. bAdiabatic electron detachment energy for A−. cThe energy for the hemolytic N−H bond dissociation in HA. dThe energy of H atom addition to A−. eBinding energy for the lowest energy sandwich dimer radical anion A2−•. fBinding energy for the lowest energy N−C bound dimer radical anion A2−•. gBinding energy for the neutral dimer A2. hStandard heat for the oxidative fragmentation reaction 9. iBinding energy for H atom addition to ACO2− anion. a

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spectra, we studied mainly the AHAs shown in Scheme 1 (see section 3.2). The calculations of the hyperfine coupling constants (hfcc’s) and radical structures in section 3.1 were carried out using a density functional theory (DFT) method with the B3LYP functional37,38 and 6-31+G(d,p) basis set from Gaussian 09W.39 In the following, aiso denotes the isotropic hfcc corresponding to the hfc tensor A (with anisotropic part denoted as B). For convenience, the principal values of the gtensor are reported as δgνν = (gνν − 2·1) × 104, where ν = x,y,z are the principal axes. The powder EPR spectra were simulated using second-order perturbation theory assuming arrested rotation, and the hfc tensors were optimized using a genetic algorithm.

Given these considerations, with this study we initiate a systematic examination of AHA-based ILs. As the main justification for using such ILs would be their purported radiation resistance, matrix isolation electron paramagnetic resonance (EPR) spectroscopy was used to identify radical products of radiolysis. To save space, supporting sections, tables, schemes, and figures have been placed in the Supporting Information. When referenced in the text, these materials have the designator “S”, as in Figure 1S. Our examination proceeds as follows. First, in section 3.1, a brief tutorial on the general radiation chemistry of the ILs consisting of aromatic anions (from a few known examples of such systems) is given. In section 3.2, computational results on the tentative radical (ion) species derived from the AHAs are examined. Our chief aim in this examination is to recognize the features of these radicals that can be used to identify them using EPR spectroscopy and estimate their energetics. The possibility of the N σ- versus N π-tautomerism in oxidized AHAs is recognized, and the elimination of N2 from certain of such radicals is shown to be thermodynamically favorable, suggesting their instability. We then examine the irradiated alkali salts and ILs consisting of AHAs, which are considered class by class, progressing from the simplest AHAs shown in Scheme 1 to the benzene conjugated AHAs shown in Scheme 2 and AHAs containing the electron-withdrawing groups (Scheme 3). In this survey, our prime purpose is to establish the common threads in the redox chemistry of the AHAs. We demonstrate that the product of their oxidation is a tautomer of the imidyl radical and establish that such radicals are, actually, sufficiently stable to decay by H abstraction from the IL cation as opposed to (thermodynamically favored) N2 elimination. We further demonstrate that AHA reduction yields an H atom adduct that is also stable to dissociation, except when the AHAs are derivatized by trifluoromethyl groups; in this latter case, there is defluorination. The ramifications of these observations for the IL-based radionuclide separations and the use of such ILs as Liion battery electrolytes are considered in section 5.

3. RESULTS 3.1. Tutorial on the General Radiation Chemistry of ILs. Certain aromatic imides that we studied13 (shown in Scheme 2S) are similar to the AHAs, and we speculated that the radiation chemistry of these AHAs somewhat resembles these systems. Reactions 1−4 yield the primary species. Some of these are unstable, undergoing prompt (de)protonation: CH2 +• + A− → C+• + H δ +Aδ −

(10)

A2 −• + H δ +Aδ − → HA−• + A−

(11)

For the IL cations, deprotonation reaction 10 typically involves their aliphatic arms, yielding the corresponding carboncentered H loss alkyl radicals. Shortly after the ionization event, the radicals present in the radiolytic spurs are A•, C+•, CH•, and HA−•, and their spectroscopic identification is complicated by crowding. The CH• radicals are observed only for aromatic IL cations,40−44 whereas for the aliphatic ones the only species observed are C+• radicals generated in reaction 10.5,40,42,45−47 To minimize interference, we complemented the measurements on ILs with their corresponding alkali salts, in which the radical species derived from the IL cations are not present. For some of the neutral radicals CH• and A•, the formation of the dimer radical ions is observed:

2. EXPERIMENTAL AND COMPUTATIONAL METHODS The synthetic methods and nuclear magnetic resonance (NMR) spectra for alkali salts of AHAs and ILs based on tri(n-hexyl)-n-tetradecylphosphonium (P666,14+) cation are given in Section 1S and Table 2S. This cation was chosen as it synthetically easily yields ILs with all of the AHAs examined; also reaction 2 does not occur for this cation. The samples were placed in sealed Suprasil tubes, evacuated, and irradiated by 3 MeV electrons from Argonne’s van de Graaff accelerator to a total dose of 3−10 kGy. Radiolytically generated radicals were observed using a 9.44 GHz Bruker ESP300E spectrometer, with the sample placed in a flow 4He cryostat (Oxford Instruments CF935). The magnetic field and the hyperfine coupling constants (hfcc’s) are given in the units of gauss (1 G = 10−4 T). If not stated otherwise, the first derivative EPR spectra were obtained using 0.02 mW of microwave power and 2 G modulation at 100 kHz at 50 K. The radiation-induced EPR signal from the E′ (Si dangling bond) center and H atoms in the Suprasil sample tubes is shadowed white in some of the EPR spectra. Weak resonance lines from Cl2−• centers trapped in Suprasil glass were numerically subtracted where possible. Because radiolysis of the P666,14+ cation yields H loss radicals in the aliphatic arms of the cation (section 3.1), the resonance lines of these C+• radicals tend to obscure the resonance lines from the AHA-derived radicals. To obtain better resolved EPR

CH• + CH+ ⇌ (CH)2+•

(12)

A• + A− ⇌ A 2−•

(13)

This reaction involves either the formation of two center−three electron σ2σ*1 bonds between the C and N atoms in the monomers2,4,11,12,41−43 or their π-stacking.44 In a matrix, the equilibrium reaction 13 is shifted one way or the other, depending on the specific electrostatic interactions, as the formation of the dimer radical anion carries the penalty of reduced polarization energy (pairing with a larger counterion favors shifting of the equilibrium to the right).42 The occurrence of reaction 12 prevents the protonation of aromatic CH• radicals that occurs otherwise:5,42,44 CH• + H δ +Aδ − → CH 2+• + A−

(14)

+•

The resulting CH2 radicals have EPR spectra that are similar to those of HA−• radicals generated in reaction 11, so unambiguous spectroscopic identification of the latter species becomes difficult. For this reason, the ILs consisting of aliphatic cations were used despite the strong interference from C+• radicals. With these general trends in mind, we turn to the structural characterization of the tentative reaction intermediates, aiming 14769


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indicate that the •N124 and •N12 radicals are, in fact, very stable in the solid matrix. Still, the inherent instability of such radicals suggests that the AHAs shown in Scheme 2 and the simple AHAs of the N1 and N13 families could make better choices for radiation hardy IL ions. Schemes 4S−6S summarize the hyperfine constants estimated for these A• radicals in their lowest energy conformations in the gas phase. The only AHA-related imidyl radicals for which hfcc’s have been reported in the literature51,52 are •N1 and •N13 (Scheme 4S), and these data are in excellent agreement with our DFT estimates, instilling hope in the validity of the approach. For the AHAs in Scheme 2, the spin densities in the 1H and 14N nuclei are relatively low, and their EPR spectra correspond to the poorly resolved singlets (BnTr), doublets (Inda, BnIm, Indo), or triplets (Isndo, Cbz), as shown in Figure 2S, making it difficult to fingerprint these radicals. For the imidyl radicals of the N σ-type, hfcc’s in the nitrogens can be as large as 40−60 G (Schemes 5S and 6S), and the corresponding EPR spectra are well resolved, as illustrated in Figure 3S for N12. For the imidyl radicals derived from the AHAs shown in Scheme 3 (Figure 4S), there is a progressive increase in the number of resonance lines, as more fluorine-19 nuclei become coupled to the electron spin (Scheme 5S); the spin density in the cyanide nitrogens is negligible. For most of these imidyl radicals, there is a single minimum on the potential surface that corresponds either to an N σradical (in which the spin density resides in the nitrogen dangling bonds) or to an N π-radical (in which the spin density resides in N 2p orbitals perpendicular to the plane of the ring). However, for some of the C2v symmetrical species, a closer examination reveals that there are two local minima corresponding to an N σ-radical with a short N−N bond or an N π-radical with an extended N−N bond (Scheme 6S and Figures 5S−7S). For example, for the •N12 radical (Figure 5S), there is a long-distance (1.47 Å) 2B1 conformer and a shortdistance (1.25 Å) 2B2 conformer that has ca. 0.1 eV higher energy. The former is an N π-radical with small hfcc’s in the two 14N nitrogens (∼5 G, see Scheme 6S) and large hfcc in the H4 proton (−16.5 G), whereas the latter is an N σ-radical with large hfcc’s in the two nitrogens (∼58 G) and small hfcc in the H4 proton (−2.3 G). Given the small difference in the energies, either one of these conformers can be expected to exist in the solid matrix. The two conformers also exist for the •N123 radical (Figure 6S): the short-bond (1.27 Å) 2A1 conformer, which is an N σ-radical, is 0.21 eV higher in energy than the long-bond (1.38 Å) 2B1 conformer, which is an N π-radical. For the •N124 radical (Figure 7S), this ordering is reversed: the short-bond (1.35 Å) 2B2 conformer is 0.28 eV lower in energy than the long-bond (1.44 Å) 2A1 conformer. As shown in section 4.1, the experimentally observed conformers of the imidyl radicals trapped in solid matrices are not necessarily the ones that exhibit the lowest energy in the gas phase, and vice versa. For many of these radicals, one can reasonably expect the occurrence of reaction 13, as can be seen from Table 2. According to our DFT calculations, all of these reactions are weakly exergonic in the gas phase. The most common geometry of the resulting A2−• anions is the C2h symmetrical π-stack sandwich (see Figure 1 for N124 and Figures 8S and 9S), but there are examples of Ci, Cs, and C2 symmetrical structures. The strongest interaction between the two subunits is obtained for the •N123 and •N124 radicals (1.0 and 1.4 eV, respectively), and there is general anticorrelation between the

to recognize their reactions and magnetic resonance signatures that facilitate their spectroscopic observation by matrix isolation EPR spectroscopy. 3.2. Scholium: Computational Characterization of AHAs. Here, we review the energetics and EPR properties of radicals and radical ions postulated as reaction intermediates in section 3.1. In addition to the anions shown in Scheme 1, previously studied13 aromatic imide anions phthalimide (Phth), saccharinate (Sac), acesulfamate (Ace), and 1,2-benzendisulfonimide (BnDIm) anions are shown in Scheme 2S. Except for phthalimide, the conjugate acids of these imides are relatively strong acids.13 The abbreviations used in the text are given in Table 1. These AHAs vastly differ in their adiabatic gas-phase adiabatic proton affinity, PA (Tables 2 and 2S), which strongly correlates with their CO2 affinity (that is, the heat of reaction 6, see Table 2S and Figure 1S); a similar correlation exists between the PA and the heat of reaction 15 (see Table 1S and Figure 1S): HA + CO2 ⇌ ACO2 H

(15)

For the least basic AHAs with PA < 14.3 eV, absorption of CO2 is thermodynamically prohibited. According to our DFT calculations, all of the AHAs shown in Scheme 3 have PA less than 13.7 eV, which is comparable to the proton affinity of the BnDIm anion (pKa = −2). The TCI anion has the lowest PA of 13.07 eV, suggesting that it is a base form of a superacid. The gas-phase adiabatic electron detachment energy (EDE) also varies significantly across the set (see Tables 2 and 3S), from 1.9 eV for N1 to 5.66 eV for DCTA. This EDE generally anticorrelates with PA, so the most basic AHAs exhibit the lowest EDE, and vice versa. Among the AHAs shown in Schemes 1 and 2, N124 has the highest EDE of 4.1 eV, followed by BnTr (ca. 4.0 eV, which is close to Phth), and N123 (3.7 eV). In the condensed phase, the solvation/ polarization energy needs to be taken into account, so the EDEs change significantly. However, given that the AHAs shown in Scheme 1 all have similar sizes, the relative ordering is expected to be retained. Therefore, N123 and N124 are exceptional in that their anions are most costly to ionize; this cost can be further increased through the use of electronwithdrawing groups. It is known that ultraviolet photoexcitation of N1234 anion causes the concerted elimination of two N2 molecules and formation of the carbene.48 Nitrogen elimination with the formation of H2CCN− anions is also known to occur in vibronically activated N123 anions in the gas phase.49,50 Thus, while the ground state of the N-rich AHA can be very stable, the excited states may not be. The same applies to the A• radicals that can potentially eliminate small fragments, such as N2 and HCN, as shown in Scheme 3S. Our DFT calculations indicate that for •N1234 the elimination of N2 is exergonic by 2.13 eV, while the elimination of HCN is exergonic by 0.65 eV. For •N123 and •N124 radicals, the estimates are 2.16 and 0.38 eV and 1.79 and 0.02 eV, respectively, and for •N12 we obtained 0.36 and 0.12 eV. These energetics suggest that the electron detachment from such AHAs can, in principle, lead to their fragmentation, which is the prime concern for their radiation stability. For other AHAs shown in Schemes 1 and 2, such fragmentation is energetically prohibitive. We note that while the elimination of N2 is thermodynamically favored, this reaction can involve high reaction barriers, and the experiments discussed in section 4.1 14770


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cation (ca. 4.2 eV). For AHAs shown in Schemes 1 and 2, reaction 16 is unlikely for •Cbz and •Isndo (Table 2), but for all other species it is strongly exergonic, so in an IL, the most likely reaction of a radiolytically generated A• radical is reaction 16. Turning to the reduction of AHAs, the protonated HA−• radicals generated in reaction 11 can be formally considered as H atom adducts of the parent anion. The energy of such H atom addition varies from 1.0 to 1.45 eV (Tables 2 and 3S). Scheme 7S shows the preferred sites and the relative energy for the H atom addition. For N1, N12, and Oxa, the preferred addition is to the nitrogen atom, although the addition in the Cα-position in the aromatic ring is close energetically. For all other AHAs shown in Scheme 1, the H atom addition is to the carbons in the ring. For the AHAs shown in Scheme 2, the preferred addition is to carbon-4 or carbon-7 in the benzene ring, while for the cyano derivatives in Scheme 3, the preferred addition is to the cyanide carbon, as is typical for such compounds:13 (RCN)− + H• → (R−CHN•)−

In the resulting H adduct radicals (HA−•), the two protons at the carbon atom in the ring (Scheme 8S) have large hfcc’s of 40−60 G (for 5-ring H atom addition) or 30−40 G (for 6-ring H atom addition), and the resulting triplets are easy to recognize even when many other radicals are present in the system (section 4). These HA−• radicals are the primary reduction products provided that the progenitor (A2−• radical dianion) becomes protonated in reaction 11 before it fragments. Judging from previously studied systems,13 such a situation may not be the case when the AHAs include electronwithdrawing (pseudo)halide groups (−X), such as −CF3 and −CN. Such RX− anions can undergo dissociative electron attachment releasing X−:

Figure 1. Optimized geometries calculated for (a,b) π-stack sandwich and (c,d) N−C bound dimer radical anions, A2−•, (e) neutral dimer, A2, and (f) H atom adduct, HA−•, for the 1,2,4-triazolide anion (N124).

EDE and this binding energy (cf., Table 2). Because of the even spreading of the spin density between the two equivalent subunits and the concomitant decrease (“halving”) in the hfcc’s, the EPR spectra of the dimer radical anions correspond to an unresolved singlet line, as illustrated for (N12)2−• in Figure 3S. Neutralization of such species can yield N−N bound dimers, as shown in Figure 1e. The formation of such sandwich dimer anions is not the only possibility, however, as the imidyl radical can add to the carbon atoms of the parent anion. For the •N1 radical, the sandwich geometry is preferred energetically; for other AHAs, the species in which there is a bond between the carbons of one unit and the nitrogens of another is lower in electronic energy. For N12 and N124, the preferred bond is N1−C3 (Figure 1); for N13, it is N1−C2; and for N123, it is N1−C4. The strongest N−C bond is obtained for N124 (1.7 eV). The typical EPR spectra for such N−C bound adducts are demonstrated in Figures 10S and 11S; such dimer radical anions, had they been generated in radiolysis, would be readily recognizable due to the large isotropic hfcc’s in the 14N and 1H involved in the N−C−H bond formation. We examined other types of bonding between the two units and concluded that such N−C bound species always have the lowest energy. The formation of either the πstack sandwich or the N−C bound A2−• species, however, is not a given in an ionic medium, as the bonding energy between the two units is comparable to the increase in the polarization energy of the medium due to the increased anion size. Our experiments suggest that in most systems, reaction 13 is not favored despite the exothermicity of this reaction in the gas phase. For the imidyl radicals,13 especially those of the N σ-type, the H atom abstraction reaction 16 A• + CH+ → H δ +Aδ − + C+•

(17)

RX− + e−• → R•− + X−

(18)

The resulting radical anions R•− can be very basic and become rapidly protonated: R•− + H δ +Aδ − → RH• + A−

(19)

Another possible route to these RH radicals is through reaction 20: HRX•− → RH• + X−

(20)

For polycyanide anions of the CxNy− type,13 reactions 19 and 20 were indeed observed, and like reactions can occur for the cyanide containing AHAs shown in Scheme 3. Our DFT calculations, however, indicate that in all cases defluorination is greatly preferred to the loss of cyanide. Scheme 9S lists estimates for the hfcc’s in the relevant radical fragments. We conclude this survey with a few notes on the carbamate anions ACO2− (Scheme 1S) of the AHAs shown in Scheme 1. Our DFT calculations indicate that in all cases reaction 9 is strongly exergonic and leads to the elimination of CO2 from the oxidized anion. Protonation of the carbamate anions by radiolytically generated acids (reaction 10) also results in their dissociation: H+ + ACO2− → H δ +Aδ − + CO2

(16)

is facile, as the N−H bond (4.4−5.4 eV, see Table 2) is stronger than the Cα−H bond for aliphatic arms in the IL

(21)

With these preliminary insights, we turn to specific examples, which are addressed in the order of increasing complexity. 14771


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4. RESULTS 4.1. Simple AHAs (Scheme 1). 4.1.1. 2-Cyanopyrrolyde (2CNPyr). The EPR spectrum for irradiated K 2CNPyr consists of a narrow singlet and the superimposed doublet separated by 87.5 G and centered at g = 2.0044 (Figures 2 and 12S). The

the protons in the former radical anion is residual moisture and/or the HA molecules. When the frozen P666,14 2CNPyr is irradiated, the EPR spectrum observed at 50 K is given by trace i in Figure 3. The

Figure 3. First-derivative EPR spectrum from frozen vitreous P666,14 2CNPyr irradiated at 77 K and observed at 50 K (trace i). Trace ii is the scaled EPR spectrum of irradiated P666,14 Br, which originates entirely from C+• radicals. Trace iii is a difference spectrum that is compared to trace iv, which is the EPR spectrum from irradiated K 2CNPyr at 50 K. The open rectangles indicate the outer lines of the H adduct radical.

Figure 2. First-derivative EPR spectra from crystalline powder of K 2CNPyr irradiated at 77 K after (i) 200 K anneal (observed at 200 K) and (ii, iii) 300 K anneal (at 200 and 50 K, respectively). In trace iii, the outer lines from HA−• radical anion are indicated with open rectangles, and the magnified EPR signals in the wings are shown at the top of the plot. In trace ii, the dashed line indicates the Lorentzian fit for the central singlet line that is attributed to a π-stack sandwich A2−• dimer radical anion.

main contribution arises from the C+• radical cations (H loss alkyl radical derived from the cations in reaction 10), as can be seen from the comparison with the scaled EPR spectrum (trace ii in Figure 3) observed in the irradiated P666,14 Br (the Br2−• radical does not overlap with the EPR signal of these C+• radicals). When the EPR signal of the alkyl radicals is (imperfectly) subtracted (trace iii in Figure 3), one observes the same features as in the irradiated K 2CNPyr in Figure 2, that is, poorly resolved outer lines of the HA−• radical and a narrow singlet at the center of the EPR spectrum that we attributed to the A2−• radical. As the irradiated frozen IL is warmed to 175 K, the C+• radicals decay to a greater extent than the H adduct radicals, and the well-resolved resonance lines of the HA−• radical with the same superhyperfine structure as that observed in irradiated K 2CNPyr become apparent, as shown in Figure 16S. Thus, the same trapped-hole and trapped-electron centers are generated in both of these 2CNPyr compounds. We shall see that this is a general feature for such ILs. 4.1.2. Pyrazolide (N12). Per discussion in section 3, pyrazolide has two possible A• conformers with drastically different EPR spectra (Scheme 6S and Figures 3S and 5S). The EPR spectrum observed at 50 K after radiolysis of K N12 is complex (Figure 17S). Some simplification can be brought out by warming of the sample over 200 K (Figures 4 and 17S). In Figure 18S, we demonstrate that this “simplified” EPR spectrum can be accounted for by a radical species having two coupled 14N nuclei with aiso(14N) ≈ 35.7 G (see the caption to Figure 18S for the set of simulated parameters), suggesting that it is an N σ-radical. The resonance lines of this radical are indicated with arrows in Figure 4. Our DFT calculations for the gas-phase C2v symmetrical imidyl radicals (section 3) suggest that this N σ-radical (2B2) has ca. 0.1 eV higher energy than a π-radical (2B1); the calculated hfcc

latter originates from the outer lines of the H atom adduct (HA−• radical anion) generated via reaction 11, with aiso(1H) ≈ 44 G (Figure 13S). The H atom adducts at carbon-2 and the cyanide group can be excluded, as they cannot account for these resonance lines, whereas the carbon-4 and carbon-5 adducts would have aiso(1H) > 50 G (Figure 13S). This HA−• radical is stable to 300 K (Figures 2, trace ii, and Figure 14S). At this high temperature, each resonance line in the doublet is resolved into an anisotropic sextet (Figure 14S), and the comparison with the simulated EPR spectra (Figure 13S) indicates that only a carbon-3 adduct is consistent with the observed pattern. This H atom adduct is the lowest energy HA−• radical anion in the gas phase. There is no evidence for cyanide elimination or H atom addition to the cyanide group. After integration there is approximate parity between the HA−• radical and the singlet structureless line at the center of the EPR spectrum in Figure 2. As the former radical is the trapped electron center, this singlet is likely to be from a trapped hole center. This line is nearly Lorentzian (see Figure 2, trace iii) with a peak-to-peak separation ΔBpp of 8.3−9.3 G, depending on the temperature. As the corresponding A• radical would have a large hfcc in the H5 proton (−11.4 G), the simulated EPR spectrum for this species (trace i in Figure 15S) is a doublet rather than a singlet. On the other hand, a π-stack sandwich A2−• radical anion (reaction 13) would yield a singlet line with a ΔBpp value that is close to that experimentally observed (trace ii in Figure 15S), and we tentatively attribute this feature to the Ci symmetrical dimer radical anion shown in panel b of Figure 8S. Thus, the radiolysis of K 2CNPyr yields two trapped charge centers, the HA−• and A2−• radical anions. The likely source of 14772


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Figure 4. First-derivative EPR spectra from crystalline K N12 irradiated at 77 K and observed after 200 and 300 K anneal (see the legend). In the 300 K trace, the vertical arrows and the “○” indicate the resonance lines attributed to the imidyl N σ- and N π-radicals, respectively.

Figure 5. First-derivative EPR spectra from polycrystalline K N123 irradiated at 77 K, annealed at 200 K, and observed at 50, 100, and 200 K, as indicated in the plot. Solid and dashed traces were obtained with microwave power of 0.02 and 2 mW, respectively.

from the •CH2CN radical that is formed by elimination of N2 (Scheme 3S), suggesting that the imidyl radical •N123 becomes thermally unstable. In frozen P666,14 N123, the predominant contribution to the EPR spectrum observed at 50 K is from the C+• radical (trace i in Figure 24S). Subtracting this contribution (trace ii in Figure 24S), we obtained trace iii in Figure 24S. The characteristic EPR spectrum of the imidyl radical is recognizable in this difference trace; in addition, there are resonance lines from the HA−• radical. For N124, our DFT calculation yields the N σ-radical (2B2) as the lowest state of the imidyl radical in the gas phase (section 3 and Figures 7S and 11S), whereas the N π-radical (2A1) has 0.28 eV higher energy. Contrary to these results, the species observed in irradiated Na N124 (Figure 6) is undoubtedly the N π-radical, as seen from comparison with the simulated EPR spectra obtained using the hfc tensors estimated from the DFT calculations in Figure 11S. Using these estimates, we refined the

parameters for these two radicals are shown in Scheme 6S. Only the N σ-radical can account for the sextet of lines observed in Figures 4 and 18S. However, when the simulated EPR spectrum for the N πradical is compared to the experimental one (Figure 18S), it is seen that the resonance lines of this radical would account for the features indicated by the “○” in Figure 4. It appears therefore that both of the C2v conformers coexist in the irradiated sample. Further scrutiny indicates that the outer lines of the sextet of the N σ-radical do not accurately reproduce the experimental trace (Figure 18S), suggesting that they overlap with the outer resonance lines of the H adduct radical. As only the carbon-3 adduct would have the required spacing between the resonance lines and exhibits unresolved structure in each component of the triplet (Figure 19S), it appears that as in the case of 2CNPyr, preference is given to the H adduct that has the lowest energy in the gas phase. 4.1.3. Triazolides (N123 and N124). For N123, our DFT calculation indicates the N π-radical (2B1) as the lowest state of the imidyl radical (see section 3 and Figure 6S). It also suggests that the imidyl radical is inherently unstable, as it can eliminate N2 and yield the cyanomethyl radical, •CH2CN (Scheme 3S); alternatively it can form an N−C bound dimer radical anion that can also eliminate one or two N2 molecules. The resulting fragment radicals can add to the aromatic ring of the adjacent anions, etc. Contrary to these expectations, the EPR spectrum of irradiated K N123 is surprisingly “clean” (Figure 20S). There are two outer resonance lines (a doublet of doublets with the aiso(1H) ≈ 59 G and an additional 14 G splitting, indicated by arrows in Figure 20S) that correspond to 57 G estimated for the unique HA−• radical anion (trace i in Figure 21S). As seen from the simulated EPR spectrum, the additional 14 G splitting is accounted for by hfcc’s in the 1H5 and 14N3 nuclei in this H atom adduct, assuming fast averaging of the anisotropies. There is also a group of resonance lines that originates from a single species, as the EPR spectrum undergoes only minor changes when the temperature is increased to 200 K (Figures 5 and 20S). The low-temperature EPR spectrum can be reasonably accounted for by an N π-radical (see Figure 22S and the caption). When the sample is warmed to 300 K, this radical decays and the predominant feature is a broad triplet, as shown in Figure 23S. Just such an EPR spectrum would be expected

Figure 6. Observed variations of the first-derivative EPR spectra obtained in the irradiated polycrystalline Na N124 as the temperature is increased from 50 to 175 K, as indicated in the plot. Trace i is the EPR spectrum of the same sample observed at 200 K after annealing at 300 K. The two resonance lines indicated with arrows in trace i originate from the HA−• radical, while the rest is from the imidyl N πradical. 14773


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as indicated with the vertical arrows in Figure 27S. Warming of the sample reveals that these lines form a triplet of triplets corresponding to aiso(1H) ≈ 44 G with an additional splitting of 11 G. The progenitor of these features is undoubtedly one of the H adducts of the BnIm anion (Figure 28S). All of the resonance lines of this species are resolved at 200−300 K (Figure 29S). In Figure 28S, we simulated the EPR spectra for three types of the H atom adduct (at carbons-2, -4, and -5). It is seen that the radical species observed in Figures 27S and 28S is specifically the carbon-4 adduct, which has the lowest energy of the three in the gas phase. The 11 G splitting originates through the nearly degenerate hfcc’s in the 1H5 and 1H7 protons in this adduct (see the top of Figure 28S). As for the broad singlet line that is observed at low temperatures (Figure 27S), there is also a narrow line (indicated with the arrow in Figure 27S) that becomes more visible as the sample temperature increases. Only part of this EPR signal originates from the central component of the EPR spectrum for the H atom adduct (Figure 28S). Turning to the IL, in Figure 8 we used the “subtraction approach” described in section 4.1 to remove the interfering

magnetic parameters in Figure 25S; the set of the optimum parameters is given in the caption to this figure. In addition to this imidyl radical, there are prominent signals from the HA−• radical (corresponding to aiso(1H) of 57.5 G vs the estimated 51.3 G; see Figure 21S). The latter radical is the only species that persists above 175 K (Figure 6). For frozen irradiated P666,14 N124, subtraction of the C+• radical reveals the “hidden” HA−• radical and the poorly resolved imidyl radical (Figure 7). In both of these frozen ILs,

Figure 7. As Figure 3, for irradiated (i) frozen P666,14 N124 liquid and (iv) polycrystalline Na N124 at 50 K. The outer resonance lines of the HA−• radical anion are indicated with the filled squares, and the multiplet of the imidyl N π-radical is indicated by the vertical lines connected with a horizontal bar.

P666,14 N123 and P666,14 N124, the yield of the imidyl radicals is relatively low (cf., Figure 26S), and the EPR signals from these radicals disappear at 125−150 K (which is of significantly lower temperature when compared to the thermal annealing of the same EPR signals in the irradiated alkali salts). This suggests that as the matrix become softer, these imidyl radicals migrate and abstract H from the cations (reaction 16) as suggested by our DFT calculations (section 3). To summarize our results, the reduced anion becomes protonated and the oxidized anion typically exists as a neutral imidyl radical. The gas-phase energetics accurately predict the preferential site for H atom addition in the HA−• radicals, whereas such calculations fail to predict the conformer of the imidyl radical prevailing in a solid matrix. 4.2. AHAs Containing 6-Atom Aromatic Rings (Scheme 2). In these AHAs, the spin and charge density in the reduced and/or oxidized anions spread over the entire πsystem, yielding more stable radicals than the simple AHAs considered in section 4.1. The benzene ring also reduces the energy barrier to the radical addition that preferentially occurs in this ring. The ensuing spin delocalization in the radicals decreases the hfcc’s, reducing the spectral resolution, which makes it difficult to identify these radicals. Below we consider the two AHAs of this type, benzimidazolide (BnIm) and indazolide (Inda). The former anion has the C2v symmetry and is easier to analyze. The low-temperature EPR spectrum of irradiated K BnIm indicates a broad singlet line and also two groups of side lines,

Figure 8. Like Figure 3 for irradiated (i) vitreous P666,14 BnIm and (iv) polycrystalline K BnIm at 50 K. In trace iii, there are discernible resonance lines from the HA−• radical anion (indicated with vertical dashed lines and ○). The two vertical arrows indicate the outer lines of the A• radical. The “■” indicates the interfering EPR signal from the Cl2−• center in the irradiated glass tube.

resonance lines from the C+• radicals. EPR signals from the HA−• radical are observed in this way (with a much higher relative yield than in the ILs consisting of the AHAs examined in section 4.1). In addition, there is a broad singlet line that is similar in appearance to the signal observed in K BnIm. As the temperature increases to 175 K, there is relatively little variation in the normalized EPR spectra (Figure 30S), suggesting that the relative yield of the C+• radicals is indeed quite small. The C+• radicals fully decay at 200 K, and the residual EPR spectrum is qualitatively similar to that observed in the irradiated K BnIm at the same temperature (Figure 9). Thus, the same species accounts for the features observed in K BnIm and P666,14 BnIm. In Figure 31S, we compare the difference trace from Figure 8 at 50 K with the residual EPR signal at 200 K (Figure 9), in which the main contribution is from the HA−• radical anion. Subtracting one EPR spectrum from another, one obtains the doublet indicated with the vertical arrows with a separation of 14774


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for (i) the lowest energy Cs symmetrical rotamer and (ii) the set of the rotamers with arrested motion. It is seen that the doublet structure in trace i collapses to a singlet in trace ii. Similar behavior is observed in the EPR spectra observed in irradiated K TPA (Figure 10). At 50 K, the

Figure 9. First-derivative EPR spectrum of frozen vitreous P666,14 BnIm irradiated at 77 K and annealed and observed at 200 K (trace i). Trace ii is the appropriately scaled EPR spectrum of irradiated K BnIm at 300 K. The resonance lines of the HA−• radical anion are indicated with “○”.

Figure 10. First-derivative EPR spectra for polycrystalline K TPA irradiated at 77 K as the sample temperature is increased from 50 to 200 K. The transformation of the doublet at the center into a singlet is fully reversible.

18 G. This EPR spectrum closely resembles that simulated for the planar A• radical shown in trace i, Figure 32S. The same simulation indicates that this EPR spectrum should yield a central component, which is the “narrow” line seen in Figure 27S; this line overlaps with another narrow line originating from the HA−• radical anion. The formation of a sandwich dimer radical anion (Figure 9S and trace ii in Figure 32S) is excluded by our results. For indazolide (Inda), given the results obtained for BnIm, we simulated the EPR spectra for the H atom adducts, as shown in Figure 33S. In the gas phase, carbon-4 and carbon-7 adducts have similar energies, while carbon-5 and carbon-6 adducts have 0.35 eV higher energies. The EPR spectrum of the corresponding imidyl radical is a doublet, as shown in Figure 34S; the HA−• and A• radicals have overlapping EPR spectra, making it difficult to disentangle their contributions. Nevertheless, comparison between these simulated EPR spectra and the ones observed for irradiated K Inda suggests that the combination of these two species would account for the spectral features (Figure 35S). To conclude this section, despite the presence of the benzene ring, the two AHAs containing six-member aromatic rings exhibit radiation chemistry similar to that of the simple AHAs: the two main radical products are the imidyl radical and the H atom adduct (which involves the benzene ring as opposed to the 5-atom ring). The main difference appears to be that in the corresponding IL, the relative yield of these two radicals is significantly higher (with respect to the alkyl radicals) than in the ILs formulated using simple AHAs. This is due to the lower ionization potential and the higher electron affinity. 4.3. AHAs Containing Trifluoromethyl Groups (Scheme 3). In section 3.1, we noted that fluoride loss is a typical outcome of dissociative electron attachment involving an RCF3− anion; for the AHAs, this reaction can be especially facile as the residual RCF2−• radical is stabilized due to the conjugation between the π-system of the aromatic ring and the spin bearing C σ-orbital. This carbon is pyramidal, and the C−F bond makes a ca. 30° angle with the plane (Figure 36S). The −CF2• group has a ∼0.03 eV inversion barrier and a ∼0.2 eV rotation barrier. Figure 36S exhibits the angular dependence for the two fluorine-19 nuclei in the F loss radical for TPA anion, and Figure 37S shows the simulated EPR spectra of this radical

EPR spectrum can be accurately described by the C s symmetrical F loss radical (see the simulation and model parameters in Figure 38S). As the temperature increases to 200 K, the doublet “collapses” into a singlet (Figure 10) like the one shown in Figure 37S. This transformation is fully reversible (Figures 39S and 40S). In the EPR spectrum of K TPA obtained at 300 K (Figure 11), the outermost resonance lines

Figure 11. First-derivative EPR spectra for polycrystalline K TPA and K DTPA irradiated at 77 K and observed at 300 K. The outermost resonance lines for the F loss radical for TPA anion exhibit the same multiplet structure as that observed in our simulated EPR spectra.

are completely resolved, exhibiting additional structure from the two nitrogen-14 nuclei. This structure is also observed in the simulations shown in Figure 37S. Almost identical EPR spectra were also observed for K DTPA (Figures 11 and 41S). Our results indicate that fluoride loss is the predominant pathway for anion fragmentation. 14775


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for TPA and DTPA, more than one radical contributed to the overall EPR spectrum. At 300 K, only one of the two centers survives, whose EPR spectrum is a Lorentzian line (Figure 13). This species is likely to be a trapped hole center (see below). In frozen P666,14 TDI, the EPR spectrum shown in Figure 14 was

Trace i in Figure 12 indicates the EPR spectrum obtained in irradiated frozen P666,14 TPA at 50 K. When the EPR spectrum

Figure 12. First-derivative EPR spectra for P666,14 TPA and K TPA irradiated at 77 K and observed at 50 K (traces i and ii, respectively). These traces were overlapped at the resonance lines indicated with the vertical arrows. The difference trace iii is compared to the scaled EPR spectrum of irradiated P666,14 Br (trace iv).

Figure 14. Like Figure 3, for P666,14 TDI at 50 K. Trace iii is for Li TDI at 50 K. The resonance lines in the difference trace iii indicated with “○” are also observed at 200 K (Figure 15) and are attributed to the TDA imidyl radical. The resonance lines indicated with “□” are artifacts of the “subtraction” routine.

of the F loss radical in K TPA trace ii is subtracted from trace i, using the resonance lines indicated with the arrows for scaling, the residual trace iii is almost identical to the EPR spectrum of the C+• radicals. The same is observed for P666,14 DTPA, as illustrated in Figure 42S. It follows that the radiolysis of these ILs yields two main products (by the oxidation and reduction of the IL ions, respectively) that are the C+• radicals derived from the parent cations and the F loss radicals derived from the TPA or DTPA anions. As seen from Figure 43S, the latter radicals are stable to 175 K, which is the softening point of the matrix; in the alkali salts, these radicals are stable at room temperature. We turn to the TDI compounds (Scheme 3). It is reasonable to expect that the F loss radical is also formed in this case. However, the yield of this species in irradiated Li TDI (Figures 13 and 44S) is relatively low, and the broad resonance lines suggest significant site variation. Furthermore, unlike the case

observed (trace i therein). As the resonance lines of the corresponding F loss radical (trace iv in Figure 14) strongly overlap with the lines of the C+• radicals, the subtraction routine was less successful than in other systems, and the resonance lines marked with “□” in Figure 14, trace iii, are artifacts of this procedure. The residual EPR spectrum corresponds to a triplet. When the temperature is increased to 200 K (trace i in Figure 15), the F loss and C+• radicals decay, and at 220 K (trace ii) this triplet also decays, so the difference between the 200 and 220 K traces (trace iii) is mainly from the progenitor of the triplet. Just such a triplet would be observed from the imidyl radical (the inset and trace

Figure 15. Traces i and ii are the EPR spectra of the irradiated P666,14 TDI obtained at 200 and 220 K, respectively, and trace iii is the difference trace. The EPR signal in the rectangular frame is from the Cl2−• center in the sample tube. Trace iv is the simulated EPR spectrum for the ensemble of rotamers comprising the •TDI radicals. The “○” indicate the resonance lines attributed to the outermost lines of this imidyl radical.

Figure 13. First-derivative EPR spectra for Li TDI irradiated at 77 K and observed at 200 and 300 K, as indicated in the plot. The “○” indicate the resonance lines of the F loss radical derived from TDI, which are not observed in the 300 K trace. The latter spectrum is a Lorentzian line with the peak-to-peak separation of 12.5 G. 14776


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reaction 18. It is likely that fluoride loss (as is the case for other ILs)11,54 results in the formation of HF and ensuing corrosive damage. It may be necessary to use other types of the electronwithdrawing groups (such as the nitro- and carbonitrilederivatives) to eliminate this kind of damage. Taking this approach to the logical end through exhaustive cyano substitution of the AHAs would yield yet another class of the CxNy− type obtained. In this respect, the present study is closely related to our previous study on such anions.12 However, what may be a problem in one application can be beneficial in another one. ILs find increasing use as electrolytes in Li ion batteries,55,56 and lithium salts of the corresponding ions (such as Li TDI) are used both in such IL electrolytes and in more common molecular electrolytes.26 For bistriflimide11,40 and bis(fluorosulfonyl)imide anions,9 there is a reaction:

iv in Figure 15), assuming that it arises from an ensemble of the frozen rotamers. Our DFT calculations indicate that the rotation barrier in the corresponding A• radicals is negligible (Figure 45S), so one can expect free rotation of this group even at low temperatures. Indeed, the unresolved singlet line in the 300 K trace in Figure 13 may arise from a radical undergoing such rapid rotation in room-temperature Li TDI. However, if this rotation becomes arrested in the frozen IL, the resulting EPR spectrum would be the triplet shown in Figure 15. Thus, for TDI (in contrast to TPA and DTPA), in addition to the C+• radicals there is also another trapped hole center, the imidyl N π-radical.

5. CONCLUSION We have examined three classes of AHAs using their alkali salts and phosphonium based ILs. In addition to simple 5-atom heterocycle anions (Scheme 1), we considered AHAs condensed with a benzene ring (Scheme 2) and derivatives containing trifluoromethyl and cyanide groups (Scheme 3). The latter were used to modify these anions into less strong bases. For AHAs belonging to the former two classes, the electron is trapped as an H atom adduct and the hole is trapped either as an imidyl radical or as a dimer radical anion (observed for 2CNPyr). In the presence of phosphonium cations, there are also H loss radicals derived from the aliphatic arms of the cation. Close scrutiny of the EPR spectra and DFT models indicates that both the N σ and the N π types of the imidyl radical can be formed in radiolysis, depending on the N−N bond distance. The short N−N bonds correspond to N σradicals, and the extended bonds correspond to N π-radicals. These two forms can be very close in energy. The state that is supported by the solid matrix can be different from the minimum energy conformer in the gas phase; in one case (for N12) both states coexist in the solid. This π-radical to σ-radical tautomerization is not an isolated example; quite similar behavior has been very recently reported by Adhikary et al.53 for 6-atom ring heterocycle radicals. For AHAs containing trifluoromethyl groups, the elimination of fluoride (reaction 18) occurs in preference to the protonation of the intermediate radical dianion (reaction 11). It appears that in these anions, reaction 18 is more facile than in aliphatic anions such as bistriflimide. The complementary trapped hole center, the imidyl radical, was observed for TDI anion (Scheme 3), but not for TPA and DTPA anions (Scheme 3). With the exception of the imidyl radical for 1,2,3-triazolide and the trifluoromethyl AHA derivatives, we did not observe radiolytically induced fragmentation in the radicals derived from the AHA anion. We surmise that the elimination of N2 requires considerable activation. It appears that it occurs on a longer time scale than the decay of the imidyl radicals via recombination and H abstraction. The AHAs are radiolytically resilient anions. Given the high basicity of AHAs, this property may not be of immediate import for nuclear separations, as the metal ions form strongly bound coordination compounds with the constituent anions; that is, such ILs cannot be used as diluents. However, these anions can serve as task-specific IL agents in other ILs. Furthermore, through derivatization of AHAs with electron-withdrawing groups, this basicity can be greatly reduced, so that the AHA-based ILs become weakly coordinating compounds. We have examined such modification with trifluoromethyl groups. The radiation stability of these anions turned out to be relatively poor due to the occurrence of

Li0 + RF− → R•− + LiF

(22)

which is analogous to reaction 18. Our DFT calculations suggest that reaction 22 would be equally facile for trifluoromethylated AHAs, especially TPA and DTPA. The formation of the LiF layer at the electrode surface is beneficial, as it precludes further consumption of the electrolyte by contributing to the formation of the so-called solid electrolyte interface (SEI), while it does not preclude Li+ cation migration.57 In general, the fragmentation of the electrolyte molecules and ions due to their electrochemical reduction yields radical species that can initiate polymerization in the outer SEI layer, thereby forming a polymer coat over the inner mineral layer. For Li metal batteries,58 this overcoat can present a problem, as the deposited Li metal can build up pressure behind this elastic coating, which is released in a sudden squirting of metal that sustains dendrite growth (see ref 9 and references therein). In our previous study,9 we linked the ability of the bis(fluorosulfonyl)imide IL to inhibit dendrite growth with the low reactivity of the •SO2NSO2F− radical released in reaction 22, which cannot abstract H from the aliphatic arms in the IL cations, so the initiation of their polymerization is impeded. Following this line of reasoning, the protective action largely depends on the energetics of reaction 23: R•CF2− + CH+ → RCF2H− + C+•

(23)

The energy of the C−H bond in the aliphatic arm of an IL cation is lowest in the α-position to the heteroatom. For the alkyl imidazolium, ammonium, and phosphonium cations, the DFT estimate for this bond energy is 4.36, 4.56, and 4.48 eV, respectively. The energy of the C−H bond in the RCF2H− anion is 4.22 eV for TPA, 4.25 eV for DTPA, and 4.39 eV for TDI. This can be compared to 4.56 eV for the bistriflimide and 3.81 eV for bis(fluorosulfonyl)imide. Therefore, for bistriflimide (but not for bis(fluorosulfonyl)imide), the F loss radical can readily abstract H from common IL cations via reaction 23, initiating the subsequent polymerization of C+• radicals. In contrast, the F loss radicals derived from TPA, DTPA, and TDI cannot readily abstract from aliphatic cations, and (in the case of TPA and DTPA) from imidazolium cations; that is, these AHAs will behave more like bis(fluorosulfonyl)imide, and the corresponding ILs can potentially arrest the growth of Li metal dendrites, too. We urge the battery scientists to examine such as a possibility. 14777


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b09057. Synthetic methods in section 1S, Schemes 1S−9S, computational results in Tables 1S−3S, and Figures 1S−45S with captions, including experimental and simulated EPR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: (630) 252-9516. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank S. Chemerisov, D. Quigley, and R. Lowers of Argonne for operation of the accelerator and S. Dai of Oak Ridge and J. Brennecke of the University of Notre Dame and their associates for providing us with some of the ionic liquids used in this study. I.A.S. thanks D. Abraham for many fruitful discussions of the battery chemistry. We are also grateful to him for providing the Li TDI salt that is being considered for use in lithium-ion cells. This material was obtained from Argonne’s Materials Engineering Research Facility (MERF), which is supported within the core funding of the Applied Battery Research (ABR) for Transportation Program. This work was supported by the US-DOE Office of Science, Division of Chemical Sciences, Geosciences and Biosciences, under contract no. DE-AC02-06CH11357. Programmatic support via a DOE SISGR grant “An Integrated Basic Research Program for Advanced Nuclear Energy Separations Systems Based on Ionic Liquids” is gratefully acknowledged. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under contract DE-AC02-06CH11357.

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