Identification of Transient Radical Anions (LiClO4)nâ (n = 1â3) in THF

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Identification of Transient Radical Anions (LiClO) (N= 1- 3) in THF Solutions, Experimental and Theoretical Investigation on Electron Localization in Oligomers Jun Ma, Pierre Archirel, Pascal Pernot, Uli Schmidhammer, Sophie Le Caer, and Mehran Mostafavi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b11315 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016

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The Journal of Physical Chemistry

Identification of Transient Radical Anions (LiClO4)n- (n = 1-3) in THF Solutions, Experimental and Theoretical Investigation on Electron Localization in Oligomers Jun Maa, Pierre Archirela*, Pascal Pernota, Uli Schmidhammera, Sophie Le Caërb and Mehran Mostafavia* a b

Laboratoire de Chimie Physique/ELYSE, UMR 8000, CNRS/Univ. Paris-Sud, Bât. 349, 91405 Orsay Cedex, France. CEA/Saclay, DSM/IRAMIS/NIMBE UMR 3685/LIONS, Bât. 546, F-91191 Gif-sur-Yvette Cedex, France

ABSTRACT: Picosecond pulse radiolysis measurements of Tetrahydrofuran (THF) solutions containing LiClO4 over a wide range of concentration are performed to investigate the formation of transient species. The 35Cl NMR measurements of these solutions prior to irradiation show that the salt is in the form of (LiClO4)n oligomers. Kinetics and transient absorption spectra of intermediates in each solution are obtained on the timescale from 10 ps to 3800 ps. A global spectrokinetic matrix of the data is analyzed by the multi curve resolution alternated least squares (MCR-ALS) method. It shows the presence of 3 transient species induced by electron pulse, in addition to the solvated electron. A hybrid MonteCarlo/DFT molecular simulation method is elaborated, using the MPW1K functional for the configuration sampling and B3LYP for the spectra calculations. The maximum of the

absorption band of the monomer (LiClO4)-, dimer (LiClO4)2-, trimer (LiClO4)3- and tetramer (LiClO4)4- anions are deduced from the simulations. They enable to label the MCR-ALS spectra (differences are below 0.1 eV) and to interpret the kinetic data. The simulations show also that LiI ion catalyzes the reduction of perchlorate by excess electron. Only the dimer anion, due to its unique structure with a stable Li2+ core and two non-bridging perchlorates, present higher stability towards ClO4- reduction into ClO3-. It corresponds to the long lived species observed in the experiments.

KEYWORDS: Anion dissociation, solvated electron pairing, electron localization, picosecond pulse radiolysis, solvated electron, alkaline metal ions reduction, MCR-ALS, Monte Carlo/DFT/PCM simulations, MPW1K functional.

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Lithium has been frequently used in organic chemistry, either as a reducing agent or in preparation of organolithium agents. For example, aromatic rings are reduced by dissolved lithium powder in liquid ammonia and such reduction constitutes one of the most fundamental reaction in organic chemistry, named the Birch reduction. Atomized lithium in ether or THF is also used for the elimination of epoxides by reduction to olefins with excellent yields.1 The alkali metal in these processes is eventually oxidized to monovalent Li+ (LiI species), but it involves several steps (solvation, spontaneous ionization, electron transfer, etc). The mechanism of this oxidation process and the possible role and nature of transient Li+ ion in the presence of excess charges is not well established. Li+ ions are also considered to play a catalytic role in promoting the organic Diels-Alder reaction, acting as the charge transfer transient lowering the activation energy.2,3 In addition, it is considered that in Lithium ion batteries, in which LiPF6 is in carbonate mixture solutions at molar concentration, the charge is transported by Li+. The reductive stability of lithium electrolytes is a major concern during the charge-discharge cycle, particularly when a lithium metal electrode is used.4,5 Therefore, it is of great interest to know the dynamics and intermediates of the reduction of lithium salts in low polar solvents. In fact, for the reduced form of the Li+ ion, in the simplest cases, two possible states can be considered: the electron-Li+ pair or the radical anion, where Li+ is bound to a counter ion. These two species do not have the same reactivity and the same decomposition fate in solutions. The focus of this work is to investigate this point and to understand the localization of the excess electron in moderate and concentrated solution such as in electrolytes. The use of pulse radiolysis of concentrated LiI, typically lithium perchlorate THF solutions, and the theoretical investigation of the structure of the reduced species is our strategy to unravel this issue. The solvated electron, as the simplest reducing agent, can be formed in a variety of solvents with an intense optical signature. This absorption band, in turn, enables us to follow its chemical reactivity towards many compounds in liquids, which is extensively experimentally studied.6-20 Depending on the solute, several types of redox reactions were observed. Some solutes react with the solvated electron at a rate controlled by diffusion. In some cases, the reaction can even take place at large distances, and is faster than diffusion controlled reactions.21 Other solutes react with the solvated electron with an activation barrier. In this last case, it is possible to observe an encounter pair formed between the solute and the solvated electron.22 Another possibility is ion pairing with some unreactive metal ion, without any substantial chemical reaction, only accompanied by a spectral blue shift of the solvated electron. For instance, in aqueous solutions containing metal cations, such as alkaline and earth alkaline metal cations, a simple blue shift of the absorption band of the hydrated electron is observed. This shift is attributed to the formation of a pair between the metal cation and the hydrated electron. For a given concentration, the amplitude of the band shift depends on the cation charge (Tb3+ > Mg2+ > Na+).23 Due to the very low redox potential of the couples Tb3+/Tb2+, Mg2+/Mg+, Na+/Na0, and Li+/Li0, the reduction of these metal cations in water by hydrated electron is unlikely to take place. But the question is still remaining of what would happen in low polar solvents when the solvated electron is produced in the presence of these metal cations.

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In ethers, such as THF, it is known that the maximum of the absorption band of es-, located at 2200 nm, shifts significantly towards 890 nm in presence of sodium salts. This phenomenon was attributed to the formation of a cation-electron pair (M+, es-) (M = Na, K, Cs).24 However, the electron localization in lithium salts in a weakly polar environment has been a controversial issue. In the case of LiAlH4 the absorption band of the solvated electron in THF is not affected.25 Shortly after, it was observed that when LiClO4 is used as a solute, a pair is formed in the presence of 0.028 mol L-1 LiI. 26 The shift of the maximum of the absorption band of the solvated electron in THF from 2200 to 1180 nm was assigned to a spectral feature of the (Li+, es-) pair. In addition, in other low polar solvents (dimethoxyethane, diglyme and triglyme) in the presence of various alkaline metal cations, it was reported that the band was split into two overlapping peaks. The results were explained through the formation of a (M+, es-) pair. Loose pairs and contact ions pairs on the basis of solvent coordination effect and temperature were evoked to explain this change of the absorption band.27 In these early studies, the kinetics of the pair formation initiated by a nanosecond or microsecond electron pulse were not studied in detail and the spectral resolutions were not high enough to point out any subtle structure of the absorption band. The formation of (M+, es-) pairs, observed by pulse radiolysis, was supported by ESR measurements, showing that a “monomeric” neutral species is formed between es- and M+.28 Later, Miller et al.29 reported that in 2,5-dimethytetrahydrofuran the absorption spectrum of the pair (Na+, es-)s presents three distinct bands. However, the nature and interpretation of these bands was not clear.29 More recently a new method was used to observe the formation of the (M+, es-) pair. In that case, an ultrafast laser beam excites the anion, Na-, from the CTTS (charge-transfer-tosolvent) band in THF. The formation of the solvated atom observed by pump-probe method was followed by that of the pair within 10 ps.30 These results suggested that the species observed around 900 nm in THF is due the (Na+, es-) contact pair instead of the reduced atom. In the present work, the kinetics of reaction between the solvated electron and LiClO4 in THF is investigated. The decay of the solvated electron is followed by picosecond pulse radiolysis with a pulse-probe method, in a large spectral window from 700 to 1500 nm, and the effect of the concentration of the salt is investigated. The spectral resolution in the first work reporting the transient absorption of THF solution containing LiClO4 was not high enough to identify the structure of the absorption band.26 Here, various transient absorption bands at different times and different concentrations are observed with high time and spectral resolutions. Theoretical calculations are also performed, to identify the species responsible of these transient absorption spectra. The simulated absorption spectra of the products are compared with experimental data and the details of electronic transitions are explained. The possible role of the Li+ cation in the reduction of the perchlorate anion is also discussed. Experimental and theoretical methods Chemical. The purity of LiClO4 was greater than 99.9% and used as purchased from Sigma-Aldrich. THF solvent purity was 99.8% and purchased from ACS reagent and it was used as received.

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NMR measurements. In order to describe the nature of species present in THF/LiClO4 solutions, 35Cl nuclear magnetic resonance experiments (NMR) were recorded on a Bruker Avance 1 400, with a 5 mm BBO probe, at 39 MHz. The NMR parameters were a spectral width of 40 kHz, 32 K data points for the time domain, a 0.4 s acquisition time, a recycle time of 0.1 s and an accumulation of 300 scans. Chemical shifts were reported with reference to 35Cl in a solution of NaCl in D2O. Picosecond pulse radiolysis set-up. The experimental study was performed using the ELYSE laser-induced electron accelerator. The pulses are of 3-5 nC, 5-10 ps, with an electron energy of 6-8 MeV, at a repetition frequency of 10 Hz.31,32 The transient absorbance was measured with a broadband pump-probe detection device, that was recently described in detail.33 Briefly, the continuum probe pulse generation with the Ti:Sapphire amplifier laser source was optimized on the NIR using a single crystal of yttrium aluminum garnet (YAG). Probe and reference beam were each coupled into an optical fiber and transmitted to a prism based broadband polychromator coupled to an InGaAs photodiode array with extended sensitivity to the visible. The optical path of the flow cell is 0.5 cm. The time resolution is 15 ps (the pulse width of electron pulse is around 7 ps). The radiation dose per pulse is deduced from the absorbance of the hydrated electron e-aq in water as a reference, measured before and after a series of experiments in THF. The dose was then derived from the yield at 15 ps: G(e-aq)15ps = 4.2 × 10-7 mol J-1 and from the molar absorption coefficient ελ=800 nm = 1.53 × 104 L mol1 cm-1.34 The dose per pulse measured in water was around D = 50 Gy per pulse. For each solution spectro-kinetics matrices data (wavelength × time) are obtained by averaging about 50 scans, and corrected for the baseline that exhibits a wavelength dependency mainly due to Cherenkov light induced by the electron pump. Data analysis method. The data matrices are analyzed by a multivariate curve resolution alternating least squares (MCRALS) approach.35,36 As in this type of system the spectra of the absorbing species strongly overlap, causing the difficulty to unmix and sort out individual spectra, a global data analysis approach was used. Global matrices were built by delay-wise binding matrices for neat THF with the data matrices for different time resolutions and concentrations. In this configuration, the components spectra are common to all the experiments in the global dataset, while each experiment has its own set of kinetic traces. The presence of a neat THF matrix enables to constrain the solvated electron spectrum. The number of distinguishable absorbers in the global matrix was assessed by Singular Value Decomposition (SVD)37. MCR-ALS analysis with the corresponding number of species was then performed. Positivity constraints were imposed for all spectra and kinetics, and in the neat THF matrix the kinetics of all species except solvated electron were constrained to be null. Based on quantum mechanical information, unimodality constraints can also be imposed to the spectra. Residuals maps were inspected to assess the absence of model inadequacy.

All the calculations have been made with codes developed inhouse in the R environment.38 Our MCR-ALS code was adapted from the ALS package by K.M. Mullen.39 Molecular simulations of the transients. We have performed molecular simulations of the different species of interest with the Monte-Carlo/DFT method.40 This method may be outlined in three steps: 1- The solute configurations are sampled by the classical MC (Monte-Carlo) method. 41 2- The solute energy is calculated by the DFT method. In this work we have used the B3LYP42 and MPW1K43 functionals, available in the Gaussian 09 package. 44 3- The solvent is taken into account implicitly, with the help of the PCM (Polarized Continuum) method.45 In this work we have used the SMD (Solvation Model using Density) formalism,46 which we have already used with success for absorption spectra.40 The number of MC steps has been taken between 10 000 and 30 000, according to the size of the system and to the computation time available. This results in computation times ranging from a few days (for the monomer) to several weeks (for the trimer and tetramer) on the 8-core machines of our computation platform. We have used a sampling strategy which emphasizes Li atoms and flexible ClO4 fragments. In this way the sampling distinguishes the translation of the Li atoms, the global translation and rotation of the ClO4 fragments and the translations of the individual Cl and O atoms. The three moves: global translation and rotation of the ClO4 fragments and translation of the individual atoms have been given the same probability 1/3. When the simulation is performed, the equilibration period is discarded and the electronic absorption spectrum can be calculated with a series of TDDFT calculations on a small list of configurations, extracted from the full list by taking one configuration out of 50. The absorption spectrum is then given by the convolution of the absorption lines of each individual configuration with a Gaussian function with full width at half maximum (fwhm) set to 0.1 eV.47 For the monomer (LiClO4)- we have used the ccpvdz basis set for the simulation and the aug-cc-pvdz for the absorption spectrum. For the oligomers (LiClO4)n- (n = 2 - 4) we have used a smaller basis set for the simulation: 6-311g(d) for the Li atoms and the SDD basis and core pseudopotentials for the O and Cl atoms, complemented with one d Gaussian (exponent 0.65, optimized in free ClO4-). For the dimer (n = 2) we have added two diffuse sp shells (exp. 0.01 and 0.005) on Li atoms. For the spectra we have complemented the basis with diffuse s (exp. 0.08, 0.04) and p Gaussians (exp. 0.04) on O atoms. Results NMR characterization of the solutions. It has been shown that the contact ion pair formation depends not only on the dielectric constant of the solvent (7.58 for THF, which is a very low value), but also on the donor ability of the molecule.48 This donor number corresponds to the enthalpy of formation of a covalent bond between the solvent and SbCl5.48 THF has a donor number of 20.0 which is a medium value.49 Therefore, THF has a low dielectric constant and a medium donor number.

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and 0.1 mol L-1, we can expect n to be equal to 1. Different values for n (1 and 2 for example) are expected for the 0.5 and 1 mol L-1 concentration, but they are the same at these two concentrations. Again, larger values for n are expected at the highest concentration. Therefore, the measurements show clearly that, for high concentration of the salt, the solution contains a distribution of the different oligomers. The presence of different oligomers at different concentrations should be considered to understand the reduction reaction of solvated electron in pulse radiolysis measurements of these solutions.

Figure 1. 35Cl NMR spectra of THF/LiClO4 solutions with various concentrations of LiClO4. Conductance measurements evidenced that, in THF, Na+-anions ion pairs do not dissociate in a similar concentration range as the one used in the present study.48 Similar conductivity measurements were performed for solutions containing LiClO4 in THF.50 By using Fuoss-Krauss triple ion theory, it was deduced that the amount of free Li+ in solution is very low. For 0.01, 0.1, and 1 mol L-1 LiClO4 in THF only a maximum of 4 × 10-5, 1 × 10-4 and 3 × 10-4 mol L-1 free Li+ ion is present in solution, respectively. The major part of the solute is found to be under neutral form. But the conductivity measurements cannot give any information about the neutral species. Therefore, NMR experiments were performed to have more information about the presence of different species in these solutions.

Pulse radiolysis measurements. The kinetics observed in THF solutions containing LiClO4 salts with concentrations from 0.01 up to 1 mol L-1 are presented at four selected wavelengths, 730, 1000, 1200, and 1400 nm, probing the decay and formation of the absorbing species after the electron pulse (Figure 2). In neat THF, only an identical decay is observed at these wavelengths due to the recombination reaction. In contrast, in the presence of LiClO4, the kinetics are very different and depend on the concentration of salt. At 1400 nm the presence of lithium salts decelerates the decay kinetics, and at 730 and 1000 nm the formation of new absorbing species is clearly observed for the solutions containing 0.1 to 1 mol L-1 LiClO4. At the higher concentrations (0.5 and 1 mol L-1), the kinetics in the NIR part (1200 nm and 1400 nm) differ much from those in the visible (730 nm and 1000 nm). At 1400 nm a faster decay and at 1200 nm an increase of the absorbance is observed during the first 500 ps. A close-up of the transients signal within the initial 250 ps after the pulse is shown in Figure 3.

The 35Cl NMR results obtained for a series of THF/LiClO4 concentrations are displayed in Figure 1. At 0.05 and 0.1 mol L1 , the signals are identical and the resonance is narrow, as expected in the case of a symmetrical electronic density around the nucleus. It is clear from this Figure that the maximum of the NMR spectrum is slightly down-shifted (from 1008.1 to 1007.4 ppm) when the salt concentration increases from 0.05 to 2 mol L1 , whereas the band significantly broadens. The change in the chemical shift indicates that the environment close to the 35Cl nucleus changes with the concentration. The broadening means that the perchlorate ion undergoes interactions which break the symmetry of the ion.49 As the line broadening (and also the variation in chemical shift) are greatest for nearest-neighbour interactions, the strong concentration dependence of the 35Cl resonance (the width at half maximum increases from 44 Hz at 0.05 mol L-1 to 210 Hz at 2 mol L-1 concentration), in agreement with previous measurements,49 is a signature of contact ion pairs as shown in Reference 48. Very interestingly, the change in the NMR signals is stepwise: the signals are the same at 0.05 and 0.1 mol L-1 LiClO4 concentration; then they change, but the signals recorded for the 0.5 and 1 mol L-1 concentrations are very similar, and then the signal changes again at the 2 mol L-1 concentration. The evolution of the NMR spectra with the salt concentration can be interpreted as the signature of the presence of the contact ion pairs (Li+, ClO4-)n with increasing values of n. At 0.05

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0.1 mol L-1 0.5 0.05 0.01 Neat THF

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-1

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0.02 0.00 1000

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Time (ps) Figure 2. Kinetics at selected wavelengths (730 nm, 1000 nm, 1200 nm and 1400 nm) after picosecond pulse radiolysis of THF solutions containing lithium perchlorate (LiClO4) with different concentrations (0.01 – 1 mol L-1).

1 mol L-1

730 nm

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Absorbance / 0.5 cm

-50

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Figure 3. Formation of intermediates between solvated electron and lithium perchlorate observed in concentrated solutions of LiClO4 in THF. This observation is in disagreement with the previous work in which only one signal peak located at 1180 nm was found, and it was attributed to the formation of the pair, (esol-, Li+). It is quite reasonable that the reaction of solvated electron with lithium complexes or oligomers in THF results in the formation of several absorbing species. 0.10 1 mol L-1

Figure 4 presents the transient absorption spectra at a delay time of 3.5 ns in all solutions and in neat THF for comparison. An increase of absorption accompanied by a strong blue spectral shift with increasing salt concentration is observed. Simultaneously, two distinct bands appear: one around 1200 nm and the other located at 950 nm. The spectra as a function of delay time in three selected solutions, 0.01, 0.1 and 1 mol L-1 are reported in Figure 5.

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0.08 Abs / 0.5 cm

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0.1 mol L-1

0.06

0.05 mol L-1 0.04 0.01 mol L-1 LiClO4 0.02

With increasing delay time (30 ps up to 3.5 ns), the initial absorption spectrum shifts to lower wavelength and the band in the visible becomes more intense than that in the NIR for the solution with 1 mol L-1. The overall absorption band of the three cases is broad and could result from the contribution of several species.

Neat THF 0.00

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Figure 4. Transient absorption spectra of THF solutions containing LiClO4 salts with different concentrations observed at 3.5 ns. Data Analysis. The global analysis of 10 spectro-kinetic data matrices was performed, including all data available for neat THF, 0.1, 0.5 and 1 mol L-1 LiClO4 concentrations. For each concentration, a short- (250 ps) and long-time (4 ns) scan were used, with additional intermediate-time scans (1 ns) for 0.1 and 1 mol L-1.

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The Journal of Physical Chemistry 800

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therefore impossible to estimate the molar extinction coefficients of the transients (except for the solvated electron).

0.008 (LiClO4)2-

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In absence of concentration information, pseudo-kinetic traces were calculated by integrating over the wavelength the spectrokinetic contribution of each species to the data matrix (Figure 7). 0 20

λ (nm) Figure 5. Spectral evolution of transient absorption of intermediates observed in THF solutions containing 0.01, 0.1 and 1 mol L-1 LiClO4.

1000 2000 3000 4000 5000

0.1 mol L-1

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Total absorbance esol-

10

(LiClO4)2LiClO4(LiClO4)3-

5

The 4-species decomposition provides a marginal improvement of the fit quality (LOF: 4.27 %). The solvated electron and long lived species spectra are not significantly different from those obtained in the 3-species analysis, but the two transient spectra are oscillating in phase opposition (not shown). This means that there is not enough information in the data to fully discriminate these two transients. An additional constraint of unimodality (justified by molecular simulation results, see below) was then used to unmix the intermediate spectra, which results in two bands peaking at ~1100 and ~1280 nm, respectively (Figure 6). The presented spectra have been smoothed to remove steps-like artifacts due to the unimodality constraint. Considering the complexity of the solution before irradiation (see NMR results), one does not have access to the concentration of each solute. It is

es-

Figure 6. Normalized absorption spectra of transient radical anions and solvated electron obtained by global data (presented in Figures 2-5) analysis with a MCR-ALS method.

1600

A 700 - 1550 nm wavelength range was selected to reject areas with very low S/N ratios. The global matrix size is 3482 × 229 pixels. The SVD analysis of the global data matrix shows that there are 3 or 4 distinguishable components. The MCR-ALS analysis with 3 species results in a lack-of-fit (LOF) of 4.32 %. The recovered spectra contain the spectrum of the solvated electron (absorbing with a maximum at 2200 nm), a spectrum corresponding to the long-time absorbance (with a maximum at ~900 nm and a shoulder at ~1200 nm), and the spectrum of a transient component comprising apparently two bands at ~1000 and ~1200 nm. MCR-ALS analysis of the data at individual concentrations shows that the amplitude ratio of the bands at ~1000 and ~1200 nm depends on the LiClO4 concentration, whereas the spectrum of the long-lived species remains unchanged. This suggests that the transient spectrum is a combination involving 2 species or more, justifying a 4-species analysis.

(LiClO4)3-

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0 20

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Figure 7. Kinetic traces from the MCR-ALS analysis of global data presented in Figures 2-5.

These curves enable to appreciate the impact of the solution concentration on the time evolution of the 4 species and their contribution to the total absorbance. The latter is mainly due to two species: es- and a long lived species, identified by molecular simulation as (LiClO4)2- (see below). The decay of es- is accelerated by increasing the salt concentration. The traces of the two minor species, identified by molecular simulation as (LiClO4)and (LiClO4)3-, are always nearly parallel and their decay is also accelerated by increasing the salt concentration.

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Molecular simulations for metastable anions A new approach with a hybrid MPW1K/ B3LYP DFT method. In the present case, the Monte-Carlo/DFT method proved difficult, because the anions are metastable with respect to selfreduction. For the simplest anion this reaction reads:

( LiClO 4 )

−

→ OLiClO3 −

(1)

which is an intramolecular reduction of the perchlorate anion ClO4- into the chlorate anion ClO3- by the Li atom. We have calculated that the product of reaction (1) absorbs in the UV and cannot be considered a candidate for the observed spectra. The reasonable candidate is therefore the metastable anion but its molecular simulation, at least with B3LYP, is impossible because the simulation readily leads to self-reduction. We have already addressed a similar issue in the past, when we did simulations of the magnesium perchlorate anion [Mg(ClO4)2]-.51 In this last work, we could circumvent the issue thanks to the freezing of the ClO4- geometries during the simulations, and obtained in this way absorption bands in acceptable agreement with the measured ones, though much too narrow. This suggests that B3LYP may be used for the absorption spectra. Table 1. DFT values (in eV) of the reaction barrier and reaction energy for reaction 1 in the vacuum. ∆E≠

∆G≠

∆E

∆G

B3LYP

+0.064

-0.005

-2.45

-2.58

MPW1K

+0.634

+0.533

-2.10

-2.26

Coming now to the electronic absorption spectrum, we found out that the MPW1K spectra are poor, but that the geometries of the MPW1K configurations can be scaled, so as to enable the use of B3LYP for the calculation of the absorption spectra. We show this for the simple cases of two anions: ClO3-, because its absorption spectrum has been published,52 and ClO4-, more relevant for the present work. It is known that between 200 and 500 nm, ClO3- has an absorption onset at 240 nm,52 and we have measured that ClO4- has an onset at 230 nm. We have performed MC simulations of these anions in water, with 10 000 MC steps and with the B3LYP and MPW1K functionals and the cc-pvdz Gaussian basis set, and obtained the absorption spectra of Figure 9. B3LYP MPW1K MPW1K / B3LYP ClO3 anion

4000

Dimer anion (LiClO4)2-

0.5

In Figure 8, we show the energy fluctuations of the MPW1K simulation of (LiClO4)- in the vacuum. This is the longest simulation we could achieve on this system. It can be seen that the bond breaking occurs after 12 000 MC steps, leaving around 10 000 MC steps for the calculation of the absorption spectrum. The MPW1K simulation in THF undergoes bond breaking after 6 000 MC steps, the B3LYP simulation after 100 MC steps only (not shown). In Figure 8, we show also the energy fluctuation of the MPW1K simulation of the dimer anion (LiClO4)2- in the vacuum. It can be seen that a long simulation of 30 000 MC steps has been possible, without bond breaking.

6000

In the present work, we propose a new approach for the simulations of the anions without constraint on the geometries by using the MPW1K functional. MPW1K has been designed for the calculation of reaction barriers, and shown to be much better than B3LYP.43 In Table 1, we give the DFT values of the reaction energy and barrier for reaction (1) in the vacuum. It can be seen that B3LYP yields a very low barrier, explaining why the B3LYP simulations rapidly lead to self-reduction of the anions. Conversely MPW1K yields a large value of the reaction barrier (0.6 eV), making the simulation of the metastable anion feasible.

2000

0

B3LYP MPW1K MPW1K / B3LYP

6000

ClO4- anion

4000

2000

0.0 Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ε (L mol-1 cm-1)

Page 7 of 14

0

-0.5 -1.0

140

Monomer anion (LiClO )4

OLiClO30

10000 20000 Monte-Carlo step

180 200 λ (nm)

220

240

Figure 9. Absorption spectrum of the ClO3- and ClO4- anions in water, with the MPW1K and B3LYP functionals, and with the hybrid method.

-1.5 -2.0

160

30000

Figure 8. Monte-Carlo simulations with the MPW1K functional of the monomer anion (LiClO4)- and dimer anion (LiClO4)2-.The zero energy is arbitrary.

It can be seen that the B3LYP functional predicts an onset close to the experimental value for both ions, and that the MPW1K spectra are poor, with absorption onsets near 200 nm for ClO3and 180 nm for ClO4-. We now note that the two functionals yield different values of the ClO distance in ClO4-, with the B3LYP value larger by 2.6 % (Table 2). We then have taken the MC configurations of the MPW1K simulations and performed an increase of the four ClO distances by 2.6 %, and calculated the B3LYP spectrum with these scaled configurations. Figure 9

7


The Journal of Physical Chemistry shows that these “hybrid” MPW1K/B3LYP spectra are in excellent agreement with genuine B3LYP and experimental data spectra. We now may propose a hybrid MPW1K/B3LYP method to:

According to this Figure, the Li+ cation first sees O atoms (between 1.7 and 2.4 Å), then a Cl atom and then O atoms again (between 3.2 and 4.0 Å). Integration of the rdfs shows that two O atoms are close to Li+ and also two O atoms are remote from it, confirming the bidentate binding mode.

1. Perform the molecular simulations of the metastable anions with the MPW1K functional, in the vacuum.

dLiLi (Å)

dLiCl dLiO(Å)

2. Perform a scaling of the perchlorate geometries of the MPW1K configurations, through increasing all the ClO distances by some amount, to be defined. 3. Use the B3LYP functional for the calculation of the absorption spectra on these scaled configurations, using the SMD method for modeling the solvent. We now address the shortcomings of the PCM method, which uses no explicit solvent molecule, so that the solvent cage is absent. As a consequence the excited diffuse orbitals freely expand into the continuum and the corresponding orbital energies are too low. This can lead to spurious low lying excited states. There is no easy way to identify these spurious transitions. In this work we identify them by comparison of the simulated and measured spectra.

in the vacuum

Structure

in THF

B3LYP

MPW1K

B3LYP

MPW1K

+0.175

+0.229

+0.096

+0.222

Bridging dimer ClO4- Li2+ ClO4-

g(r) (arb. unit)

(a)

(b)

B3LYP MPW1K

LiCl n = 1 LiO n = 1 LiCl n = 2 LiO n = 2

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

n=1 n=2 n=3 n=4

(d)

n=2 n=3 n=4

(c)

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2

1.40

1.45

dLiLi (Å)

1.50

dClO (Å)

1.55

1.60

Figure 11. Radial distribution functions of the (LiClO4)n- oligomers. LiLi, LiCl and LiO rdfs for the B3LYP simulation of the dimer: (a) and (b), LiLi rdfs for the MPW1K simulations of n = 24: (c) and ClO rdfs for the MPW1K simulations of n = 1-4: (d). The distance step is 0.1 Å.

In Figure 12 (left) we show the simulated absorption spectrum of the monomer and in Figure 13 (left) we show the structure and the orbitals of a particular configuration. We call indifferently somo (singly occupied molecular orbital) or homo (highest occupied molecular orbital) the orbital accommodating the additional electron. The homo and lumo (lowest unoccupied molecular orbital) will be noted h and l hereafter.

Inserted dimer Li ClO4- Li+ ClO40

+0.032

+0.176

Linear dimer -

+

ClO4 Li2 ClO4

ε (L mol-1 cm-1)

0

0

monomer LiClO4-

10000

0 600

800

1000

-

-

-

Hybrid Simulations of the monomer. Geometry optimization at 0 K with the MPW1K functional and the aug-cc-pvdz basis yields a symmetrical C2v structure with a bidentate ClO4-, namely with two O atoms bond to Li+ (Figure 10). The MPW1K simulations spare this bidentate binding mode, as can be seen on Figure 11b showing the LiCl and LiO rdf (radial distribution functions).

1200

1400

2 parameters 1 parameters

400

10000

600

800

1000

1200

1400

B3LYP 2 parameters 1 parameters

(LiClO4)2-

(LiClO4) 5000

0

0

600

800

1000

λ (nm)

-

-

2 parameters

5000

400

Figure 10. Structures of the monomer anion (LiClO4)- and dimer anion (LiClO4)2-. The free energies at 300K for the two functionals and the aug-cc-pvtz basis are in eV. The structures are that of the B3LYP optimization. Li atoms are pink, Cl atoms green and O atoms red.

(LiClO4)2

20000

10000 0

15000

0

full h -> l, l+1

30000

20000

-

+0.045

(LiClO4) 2 parameters

40000

400

+0.056

40000

full h -> l

60000

ε (L mol-1 cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 14

1200

1400

400

600

800

1000

1200

1400

λ (nm)

Figure 12. Absorption spectra of the monomer (LiClO4)- and dimer (LiClO4)2- : extraction of the homo → lumo transition for the monomer (top left) and of the homo → lumo, lumo+1 for the dimer (bottom, left). Comparison of the hybrid methods with different parameterizations (top and bottom, right).

The full spectrum of the monomer, Figure 12 (top left) displays two low energy bands. We have checked that the intense band at 860 nm corresponds to the two 2pπ (l +1, l + 2) orbitals of lithium. This transition is absent from the experimental spectrum and is very probably an artifact of the PCM method. Since these orbitals have a marked diffuse character, we believe that a simu-

8


Page 9 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lation with explicit solvent molecules would shift the corresponding transition toward higher energies. Therefore, in the following we will consider only the h → l spectrum for the monomer.

simplest method yields a less regular spectrum nevertheless, slightly shifted toward the visible. It can be seen on Figure 13 (left) that: the somo orbital lies in the vicinity of Li+, but mainly outside the solute, so that the anion may be called a contact electron – solute pair.

1. (LiClO4)-

Molecule

(LiClO4)22.

structure

Figure 11 (d) shows that the ClO distance has a large fluctuation between 1.4 and 1.6 Å. This fluctuation is essential, because we could verify by inspection of the MC configurations that the length of the ClO bonds determines the energy of the excited state. More specifically the low energy wing of the band corresponds to configurations with elongated ClO distances, and conversely the high energy wing of the band corresponds to configurations with shorter ClO distances.

lumo+2

lumo+1

Hybrid Simulations of the dimer. The optimizations of the (LiClO4)2- dimer at 0 K shows that it has at least three stable isomers. The structures and MPW1K and B3LYP / aug-cc-pvtz, free energies at 300 K, in the vacuum and in THF, are shown on Figure 10. We have checked that all the vibration frequencies are real. These results call for the following comments:

lumo

homo Figure 13. homo and lumo orbitals of the monomer and dimer anions, from two geometries taken in the MPW1K simulations.

Table 2. Vacuum DFT values (in Å) of the ClO bond distances in free ClO4- and in the anions (LiClO4)n-. functional

O–Li

Free

n=1

n=2

n=3

-

bond

ClO4

MPW1K

no

1.484

1.464

1.458

1.451

B3LYP

no

1.523

1.504

1.496

-

MPW1K

yes

-

1.506

1.503

1.487 and 1.507

B3LYP

the lumo orbital is of a pσ type, with a nodal surface on the Li atom and spreading over the perchlorate.

yes

-

1.560

1.550

-

The scaling of the ClO4- anions can be exerted in the simplest way with the +2.6% parameter of the free anion. We call this method the “one parameter” scaling. It can also be deduced from the MPW1K and B3LYP optimum geometries of the monomer anion at 0 K in the vacuum. The values of the ClO distances are given in Table 2. It can be seen that two types of ClO bonds may be considered, according to whether the O atom is bound to the Li+ cation, or not. For O atoms bound to Li+ the B3LYB values are 3.6% larger than the MPW1K ones, for the others O atoms they are only 2.7% larger. Using these two percentages for the ClO4- scaling will be called “two parameter” scaling hereafter. The spectra obtained with the “one parameter” and “two parameter” scalings of the geometries are shown in Figure 12 (bottom, left). It can be seen that the procedures yield close results. The

1.

In THF (where the spectra are recorded) the “linear” ClO4- Li2+ ClO4- dimer is the most stable isomer for both functionals.

2.

In the vacuum (where the simulations are done) the “inserted” isomer, of the type Li ClO4- Li+ ClO4-, is the most stable for both functionals.

This raises the question that the simulations of the dimer should converge toward the inserted configuration. We have found, nevertheless, that if the simulation is started with a configuration of the linear ClO4- Li2+ ClO4- type, then the whole simulation continues on this type of dimer. We think that this feature is due to our sampling mode in the MC simulation. The sparing of the linear conformation of the dimer is proved by the LiO rdfs of Figure 11 (b), which are identical to that of the monomer. We also have found out that the B3LYP simulation of the dimer is actually feasible in the vacuum, i.e. without self-reduction of the dimer. We thus have in hand two long simulations of 30000 steps with the two functionals. The spectra of the dimer are given in Figure 12 (right) and the structure and orbitals of a particular configuration are given in Figure 13 (right). It can be seen on Figure 12 that the full spectrum is quite analogous to that of the monomer, with two low energy bands. The low energy band is due to h → l, l+1 transitions, the intense band at 600 nm is due to h → l+2, l+3 transitions. The l+2 and l+3 orbitals are π orbitals of the Li2+ fragment, with a diffuse character. Since the band at 600 nm is absent from the measured spectra, we consider that the corresponding transitions are artifacts of the PCM method. The fact that B3LYP and MPW1K simulations are both possible enables a further appraisal of our hybrid method, more demanding than the simple case of the ClO3- and ClO4- anions. The absorption spectra obtained with B3LYP and with the MPW1K / B3LYP method with one and two parameter configuration

9


The Journal of Physical Chemistry scaling are shown on Figure 12 (bottom right). The two parameters scaling uses two percentages analogous to those used for the monomer, with the values 3.1% and 2.6%, as can be deduced from Table 2. It can be seen that: 1.

The two parameterizations of the hybrid method yield two spectra, very close to each other. This suggests that the one parameter scaling will be sufficient for larger oligomers.

2.

The hybrid method yields excellent values of λmax, this confirms the ability of this method, already suggested by the study of ClO3- and ClO4-.

3.

The hybrid method underestimates the infrared wing of the band. The pure B3LYP spectrum is actually in better agreement with the experiment, as will be seen in the discussion,

The homo and lumo orbitals of the dimer can be seen on Figure 13 (right). It can be seen that: 1.

The homo is essentially the σg orbital of Li2+. It is clearly more localized than the homo of the monomer. We have no contact pair in this case, rather an anion.

2.

The lumos spread over one perchlorate or the other, just like for the monomer. We have found configurations, by chance almost symmetrical, for which the lumos are σg and σu combinations of the lumos of the two perchlorates.

actually in much better agreement with the recorded spectrum than the MPW1K spectrum (see further discussion). This suggests that the B3LYP simulation and its striking rdf of LiLi distances are realistic. Note that free energy curves with two minima are not rare in molecular simulations. This is the case for examples for the dimer of Cl- in water.53,54 In the present case the double minima cannot be attributed to the energy: we have calculated the potential curve of the dimer, with constrained geometry optimization, and observed only one minimum (not shown). The other possibility is that the outer free energy minimum is due to increasing entropy of the perchlorates, as the LiLi distance increases. This should be investigated by molecular simulations, but this work is out of the scope of the present article. We have separated two contributions to the B3LYP spectrum, relative to the two peaks of the LiLi rdf. To this aim we have calculated two spectra, due to configurations with the LiLi distance smaller, or larger than 3.60 Å. This results in the two spectra of Figure 14. It can be seen that small LiLi distances mainly contribute the high energy part of the spectrum, and that conversely larger LiLi distances mainly contribute the low energy part. This can be understood with the same argument as for the ClO distances in the monomer: for larger LiLi distances the somo orbital is destabilized and the excited state gets lower. A large overlap can be seen, nevertheless.

Molecule

(LiClO4)3-

(LiClO4)4-

In Figure 11 (a) we show the LiLi rdfs in the B3LYP and MPW1K simulations. These rdfs are very different: 1.

The MPW1K rdf spreads between 2.9 and 3.8 Å and is slightly non symmetrical

2.

The B3LYP rdf is very striking, with two peaks. It spreads between 2.8 and 4. Å, with a depression at 3.6 Å.

homo

structure dLiLi < 3.60 Å

8000

Figure 15. homo orbitals of the trimer and tetramer anions, from geometries taken in the MC simulations

dLiLi > 3.60 Å total absorbance

ε (L mol-1 cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 14

6000

4000

2000

0 600

800

1000

1200

1400

1600

1800

λ (nm)

Figure 14. B3LYP absorption spectrum of the dimer (LiClO4)2-, with extraction of two components corresponding to small ( < 3.60 Å) and large ( > 3.60 Å) LiLi distances. The B3LYP rdf suggests that the free energy LiLi curve displays two minima, separated by a low barrier. It may be argued that the rdf is not converged, but we emphasize that with the same number of MC steps (30 000) the MPW1K rdf seems converged. The B3LYP spectrum, with a very large manifold of two bands, is

Hybrid Simulations of the trimer and tetramer. The MPW1K optimization of the trimer shows that in the case of LiO bond, two ClO distances are present (Table 2). We could not achieve the B3LYP optimization of the trimer, because of ClO bond breaking whatever the geometry guess. Therefore, the one parameter scaling is the only one possible. We have undertaken MPW1K simulations of the trimer and the tetramer in the vacuum, with 20 000 MC steps, and calculated the B3LYP absorption spectra. The shape of the somos can be seen on Figure 15. It can be seen that in the most stable tetramer the electron is confined in a Li44+ tetrahedron. The full spectra of the trimer and tetramer are given in Figure 16. It seems that no spurious artefactual transitions interferes in these cases. It can be seen that the spectrum spreads over the whole 400 – 1800 nm range, with a larger extinction coefficient in the visible. The corresponding LiLi and ClO rdfs are given in Figure 11.

10


Page 11 of 14

Discussion

(LiClO4)-

1153 nm

To understand the results of pump-probe experiments on LiClO4 solutions in THF, we have to consider two points:

(LiClO4)2-

15000

ε (L mol-1 cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(LiClO4)3(LiClO4)4-

854 nm

10000

(1) the studied solutions contain a concentration-dependent distribution of oligomers. Conductivity measurements reported in the literature showed mainly the presence of neutral species in the LiClO4/THF solutions. Moreover, our NMR measurements clearly evidenced the presence of neutral oligomers. Their distribution depends on the concentration of the salt.

1159 nm

5000

0 400

600

800

1000

1200

1400

1600

1800

λ (nm) Figure 16. Simulated absorption spectra of the oligomers (LiClO4)n- (n = 1 - 4). Comparing the spectra of the oligomer anions. We have gathered the absorption spectra of the four oligomers with n = 1 4 in Figure 16. In this Figure we give our “best” spectra, namely: for the monomer the h → l hybrid MPW1K / B3LYP spectrum, for the dimer the h → l, l+1 B3LYP spectrum and for the trimer and tetramer the full hybrid MPW1K / B3LYP spectra. These spectra raise the following comments: -

-

The absorption band is well defined for the monomer, located at 1150 nm; with increasing n of the oligomer, it becomes more and more spread with a high energy edge shifting more and more to the blue. The extinction coefficient is large for the monomer, smaller and smaller for the other oligomers. We think that this is due to the variable extension of both the somo and the virtual orbitals. On one hand the somo gets more localized when n increases due to the larger cationic charge of the metallic core: Li+, Li22+, Li33+ and Li44+. On the other hand the virtual orbitals are more and more delocalized when n increases because they spread over more and more ClO4- anions. These two circumstances combine and provoke a drop of the transition moments. The band maxima are shifted toward shorter wave lengths as the oligomers are larger. We think that this is due to the larger stabilization of the additional electron, as the number of Li+ cations increases. Table 3 shows the average values of the somo energy along the MC simulations. It can be seen that in the clusters with increasing n, from 1 to 4, the electron is more and more bound. Note that in the trimer the electron is only slightly more bound than in the dimer. Table 3. Mean value of the somo energy (eV) for different anions along the Monte-Carlo simulation Anions (LiClO4)(LiClO4)2- (LiClO4)3- (LiClO4)4energy -1.90 -2.97 -3.04 -3.60

(2) the species formed after irradiation can have broad absorption spectra with possibly several maxima. The simulated spectra enable to label the spectra resulting from data analysis by MCRALS. Table 4 reports the maximum of the NIR absorption band of the monomer (LiClO4)-, dimer (LiClO4)2-, and trimer (LiClO4)3- anions resulting from these methods. The agreement between them is very good: the difference on the maximum of the absorption bands is lower than 0.1 eV. Table 4. Maximum of the absorption band (eV) of the anions in NIR domain obtained by molecular simulation and by deconvolution of the experimental data with the MCR-ALS method. E(eV)

(LiClO4)-

(LiClO4)2-

(LiClO4)3-

Experimental

1.13

1.33

0.97

Simulation

1.08

1.45

0.95

The anion of the LiClO4 monomer, absorbing with a maximum at 1100 nm, can only be effectively observed at low concentrations (0.05 mol L-1 and 0.1 mol L-1). This suggests that this anion reacts with a neutral molecule to form the dimer which presents a higher stability: −

Li(ClO4 ) − + Li(ClO4 ) → [ Li(ClO4 )]

2

(2)

In concentrated solution, the dimer can also be formed directly by the following reaction: −

es− + [ Li (ClO4 )]2 → [ Li (ClO4 ) ]2

(3) The maximum of the absorption band of the dimer is located at 930 nm with a shoulder at 1200 nm. The kinetics obtained by the MCR-ALS analysis (Figure 7) show that the anionic dimer is the dominant species formed in solution. The molecular simulations also suggest that this dimer is much more stable than the anionic monomer, trimer and tetramer. This is clear when B3LYP simulations are considered: these simulations are impossible for the monomer, trimer and tetramer because the ClO bond breaking, resulting from a selfreduction, occurs very rapidly. We have verified that these simulations are also impossible for the secondary isomers of the dimer, the inserted and the bridging ones, for the same reason. Considering first the heavier oligomers (n = 2, 3, 4), we have found that the bond breaking occurs most rapidly when one (or more) ClO4- is in bridging position between two

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Li atoms. This is the case for the trimer and tetramer, as can be seen on Figure 15, and also for the secondary isomers of the dimer, as can be seen on Figure 10. In these cases, the dissociated O atom get inserted inside the Li2+, Li32+ or Li43+ cluster, forming a lithium oxide cluster Li2O+, Li3O2+ or Li4O3+. Further self-reduction is probably possible. Only the most stable isomer of the dimer, the linear ClO4-Li2+ClO4- dimer, resists to this bond breaking, probably because none of the ClO4- ligands is bridging. Consequently, a B3LYP simulation of 30 000 MC steps has been possible for this linear dimer, without ClO bond breaking. Comparing now the monomer and the linear dimer, which both have no bridging ClO4-, we attribute the larger stability of the dimer to the fact that the Li2+ cluster is much more stable than the Li atom, as can be seen from the somo energies of Table 3. The remarkable stability of the linear dimer with respect to self-reduction is therefore due to its unique structure, with a stable Li2+ core and two nonbridging perchlorates. The kinetics showed that the anions appear simultaneously just after the pulse and not consecutively, confirming that they are produced mainly by the direct reaction between the solvated electron and the oligomers.

Page 12 of 14

absorbing in the NIR. In the case of magnesium, the absorbing species was attributed to the reduced anion [Mg(ClO4)2]- in which ClO4- is also self-reducing.51 The method elaborated during this work can be applied to the domain of highly concentrated electrolytes used in Li-batteries, in which the salt (usually LiPF6) is at molar concentration. In fact, during the cycle of charge and discharge of these batteries, the electrolytes are decomposed and one of the paths to explain this decomposition is the electron attachment to the solute.

ACKNOWLEDGMENT The authors thank Jean-Marie Teuler who develops and maintains the GIBBS simulation code and Daniel Ortiz and Jean-Pierre Baltaze for the NMR measurements.

AUTHOR INFORMATION Corresponding Author * [email protected] and [email protected]

Author Contributions Concluding remarks As LiClO4 is not dissociated in THF solutions, the solvated electron is scavenged by the neutral oligomers to form anions according to the following reaction:

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. All authors contributed equally.

REFERENCES

−

es− + [ Li(ClO4 )]n → [ Li(ClO4 )]n

(4)

The concentration of free Li+, even in molar LiClO4 solution, is very low and even if a rate constant of 1012 mol-1 L s-1 is considered between es- and Li+, the pair formation would take more than 10 ns, in contradiction with our observations in the ps range. Therefore, Li+ does not form a pair with the solvated electron in THF. It is worth noting that in similar conditions an absorption band at 1180 nm was previously assigned to an (Li+, es-) pair. In fact, we can now affirm that the maximum at 1180 nm was due to the overlap of several absorption bands. It is important to note that the (LiClO4)n- anions are not stable with respect to self-reduction and they decay by forming subproducts of perchlorate anions. This point is very important because, in polar solvents or in low polarity solvent but at very low concentration, when ClO4- is not associated with Li+, there is no reaction between es- and ClO4-. But when LiClO4 is not dissociated (in low polarity solvents) the anion is formed. It is not stable and can disappear by forming Li+ and ClO3-. In that case, Li+ plays the role of a catalyst for the dissociation of ClO4-. The dissociation of organic anions by electron attachment has been studied by pulse radiolysis and it was shown that the reaction is stepwise.55 First the anion is formed and then the anion is dissociated. Here, a similar mechanism is observed for an inorganic molecule, LiClO4. The barrier for the dissociation of the anion depends on the size of the oligomer. Picosecond pulse radiolysis measurements of THF solutions containing different concentrations of Mg(ClO4)2, Ca(ClO4)2 and Sr(ClO4)2 showed also that the reaction of the solvated electron with these molecules leads to the formation of a new species

1 Gurudutt, K. N.; Ravindranath, B. Reaction of Oxiranes with Lithium: Deoxygenation Leading to Olefins. Tetrahedron Lett. 1980, 21, 1173−1174. 2 Forman, M. A.; Dailey, W. P. The Lithium Perchloratediethyl Ether Rate Acceleration of The Diels-Alder Reaction: Lewis Acid Catalysis by Lithium Ion. J. Am. Chem. Soc., 1991, 113, 2761–2762. 3 Pocker, Y.; Spyridis, G. T. Electrostatic Modulation by Ionic Aggregates: Charge Transfer Transitions in Solutions of Lithium Perchlorate−Diethyl Ether. J. Am. Chem. Soc. 2002, 124, 7390– 7394. 4 Yamada, Y.; Furukawa, K.; Sodeyama, K.; Kikuchi, K.; Yaegashi, M.; Tateyama, Y.; Yamada, A. Unusual Stability of Acetonitrile-Based Superconcentrated Electrolytes for FastCharging Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 5039–5046. 5 Sodeyama, K.; Yamada, Y.; Aikawa, K.; Yamada, A. Tateyama, Y. Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte. J. Phys. Chem C. 2014, 118, 14091–14097. 6 Hart, E. J.; Boag, J. Absorption Spectrum of the Hydrated Electron in Water and in Aqueous Solutions. J. Am. Chem. Soc. 1962, 84, 4090−4095. 7 Gould, R. F. Ed. Solvated Electron Advances in Chemistry Series. 1965 8 Baxendale, J. H.; Wardaman, P. Direct Observation of Solvation of the Electron in Liquid Alcohols by Pulse Radiolysis. Nature. 1971, 230, 449.

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Identification of Transient Radical Anions (LiClO4)nâ (n = 1â3) in THF

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