Solvation of an Excess Electron in Pyrrolidinium Dicyanamide Based

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Solvation of an Excess Electron in Pyrrolidinium Dicyanamide Based Ionic Liquids Changhui Xu and Claudio J. Margulis* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *

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

1. INTRODUCTION Room-temperature ionic liquids (RTILs) have attracted considerable attention as possible environmentally benign media because of intrinsic properties that are particularly well suited for applications in energy storage and charge transport, with particular emphasis on batteries, capacitors, fuel cells, and spent nuclear fuel recycling. Key to the rational design of any and all such applications is an understanding of electronic charge transfer and transport. Are electrochemical cathodic limits always cationic and anodic limits always anionic? Recent electrochemical studies1−8 and computational studies from our group9,10 and others2,11 indicate that at least in the cathodic limit this is not necessarily the case. These studies found that when combined together in an RTIL, certain anions such as bis(trifluoromethylsulfonyl)amide ([Tf2N−]) are more prone to accept an excess electron than alkylammonium and pyrrolidinium based cations. Fragmentation of [Tf2N−] has also been ubiquitously observed in radiation chemistry experiments.12−19 Clearly, a better understanding of ionic liquids in the presence of excess electrons is needed since charge localization necessarily correlates with distinct cationic and anionic chemical reactivity. Whereas it is perfectly reasonable to think of an ionic liquid as composed of two different moieties (the cations and the anions), each moiety with its own HOMO and LUMO levels, we have shown in the past9 that the liquid landscape significantly influences the relative alignment of cationic and anionic levels. By this we mean that whereas cationic (anionic) © 2014 American Chemical Society

HOMO to LUMO energy gaps may be similar across liquids, the way energy levels align to determine the overall HOMO to LUMO gap for the whole liquid (not of each of the ionic subcomponents) depends on the collective liquid structure. This fact alone makes straightforward predictions about the localization of excess charge, ionization energies, electrochemical potentials, and chemical reactivity as well as possible mechanisms of excess charge solvation quite difficult. Gas phase or cluster calculations may not be adequate and more challenging condensed phase calculations may be needed. On the positive side, the slow molecular dynamics of many RTILs and the fact that they can reorganize to form strong Coulombic traps provide a fascinating platform to study the kinetics of excess charge relaxation, solvation dynamics, and reactivity. The question of how RTILs perform under ionizing conditions (radiolysis, photolysis, electrochemical extremes, etc.) is important for many real-world applications. One of the most desirable properties of ionic liquids is low viscosity. It is no surprise then that [Tf2N−] based ILs are very popular since they often possess low viscosities and can be prepared optically clean. However, recent computational work from our group10 and experimental studies2−8 appear to indicate that, at least in combination with alkylammonium and pyrrolidinium cations, one important reaction pathway is that Received: October 30, 2014 Revised: December 18, 2014 Published: December 18, 2014 532

DOI: 10.1021/jp5108922 J. Phys. Chem. B 2015, 119, 532−542

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The Journal of Physical Chemistry B in which the [Tf2N−] anions accept the excess electrons and this leads to fast anionic fragmentation. In a quest to better understand solvation dynamics and the kinetics of charge relaxation and reactivity, one must therefore seek other anions that on the time scale for solvation do not break and form liquids of low viscosity. Recently, Shkrob et al.16 studied the effect of anion substitution on radiation reactions and fragmentation of frozen ionic liquids at 77 K using EPR and found that [CF3SO3−] and [N(CN)2−] (DCA) were relatively stable among the smaller anions they studied. Furthermore, many ionic liquids based on the DCA anion are of low viscosity, have high conductivities, and are shown to be suitable for several different electrochemical applications.20−36 If we focus on electrons generated by photoexcitation or by radiation processes, two types of experiments exist that simulations should attempt to make contact with (i) studies of the reactions of secondary reactive species (radicals, radical anions) by the analysis of final products in the ionization processes and (ii) studies of the reactivity of primary species (excess electrons and excess holes) by examination of transient optical absorption bands of these species. The second type of studies suggested that the two major primary speciesthe excess electrons and excess holeshave very distinct optical properties, lifetimes, and reactivities with respect to scavengers. Several computational studies9,10,37−39 have discussed the pattern of charge localization of an excess electron or an excess hole in ionic liquids. In particular, a recent article from our group10 has probed the early ab initio dynamics of excess charge localization on the femtosecond to early picosecond time scales in RTILs consisting of aliphatic cations coupled with the [Tf2N−] anion. This article provided an intuitive physical description of the elusive experimentally postulated40−55 early “dry” and “trapped” excess electron states. In all cases we have so far studied, the dry electron state is characterized by delocalization of the excess electron over several ions and a very broad transient low-energy absorption band. The lifetime of the dry electron state is very short. In the case of alkylammonium based systems coupled with the [Tf2N−] anion it lasts about 50 fs. At least in this particular reaction channel, as the excess electron localizes the broad and transient low-energy band disappears, and the nascent trapped electron state forms. We find that in the case of [Tf2N−] the process of trapping also involves undesirable anionic dissociation. Our simulated bond dissociation pathway is consistent with experimental observations.2−8,12−14,19 Even though these findings explain different spectroscopical and electrochemical observations, they may not necessarily account for all observables in photoionization and radiation chemistry experiments. In our earlier study,10 a significant portion of the near-infrared (NIR) signal on a picoseconds time scale arises from interfragment electron transfer transitions that should become rare at long time as fragments drift apart. It is therefore unlikely that this reaction pathway will fully account for the long-time (hundreds of nanoseconds) NIR band characteristic of excess electrons. Other alternative electron localization and solvation pathways including the localization in cavities could be at play and should be explored. What is clear from prior studies56,57 is that the probability of finding a preformed cavity in ionic liquids of a size capable of supporting a solvated electron is vanishingly small. The probability for spontaneously forming anionic cavities (i.e., solvated mostly by positive charge) must be even smaller. An electron must either

find one of these cavities or form a cavity. It is therefore likely that only a fraction of all generated electrons will survive at long times to become cavity electrons giving rise to a typical broad NIR signal. In ionic liquids the ease of finding or forming a cavity may yet be complicated by viscosity. It is only natural that there will be electrons localizing on ions and others that will survive to become cavity electrons. Each of these will have characteristically different spectroscopic signatures. In this article we focus on the room-temperature ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide ([Pyrr1,4][DCA]) since both electrochemical20−27 and radiation chemistry experiments16,58 point to the conclusion that the DCA anion may be a very suitable candidate to study electronic processes in ionic liquids. We will show in subsequent sections that, in analogy with what we observed for the [Pyrr1,4][Tf2N] system, in the case of [Pyrr1,4][DCA] an important pathway also involves excess electron localization on the anions. However, as opposed to the fragmentation we observe in the case of the popular [Tf2N−] anion, the DCA anion does not fragment on the time scale of our studies. Furthermore, a pathway in which the excess electron localizes in a cavity solvated mostly by cations that share excess electronic density is also explored as a possible origin for the commonly observed long-time NIR excess electron band. One of the biggest hurdles when attempting to correlate experimental and computational studies is the mismatch of time scales accessible to ab initio molecular dynamics (AIMD) when compared to experiments. Whereas pioneering ultrafast55 optical experiments on excess electrons in ionic liquids exist, many more address the picosecond,45,48 nanosecond, and longer time scales.40,41,46−52 Our previous studies9,10 have only been able to describe the very early stages of excess charge distribution. In the current article, we would like to go beyond the study of dry and trapped electron states and derive some understanding of what the “solvated electron” state on a several nanoseconds time scale may be for the different reaction pathways at play. We attempt to access these longer time scales by different approaches described in section 2.

2. METHODS The bulk of the work presented here is based on condensed phase classical and ab initio molecular dynamics simulations as well as conjugate gradient (CG) ab initio optimizations. The simulation protocol has been described in detail previously.10 Briefly, two independent classical molecular dynamics simulations each with 10 [Pyrr1,4][DCA] ion pairs (350 atoms) were run to generate initial bulk liquid configurations to later be used in AIMD studies. We used the GROMACS59,60 package with a modified version of the Canongia Lopes and Pádua force field parameters61−63 in which for the alkyl chain part of [Pyrr1,4+] dihedral parameters were replaced for those in the improved OPLS-AA force field.64 Equilibration of the classical system involved several steps. First, we rescaled charges and pressure to finally arrive at 1 bar and full charges at 300 K, and then we ramped the temperature up to 500 K and back to 300 K. Once the two trajectories were deemed at equilibrium at 300 K, extra 6 ns in the constant number of particles, temperature, and pressure ensemble using the Nosé−Hoover65,66 thermostat and Parinello−Rahman67 barostat were run to arrive at final classical snapshots. Densities were consistent with experimental values20−23 at about 0.95 g cm−3, and radial distribution functions were comparable to 533


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The Journal of Physical Chemistry B

cationic-solvated cavity environment that could potentially support the bound states of an excess electron. This new system was reoptimized using the CG procedure in SIESTA to study the nature of SOMO, LUMOs, and the optical spectrum and excess electron density.

those obtained for simulations of larger systems with the same force field. For these two trajectories, final liquid structures were minimized quantum mechanically to a force less than 0.04 eV/Å per atom in order to adjust bonds lengths, angles, and dihedrals to the DFT potential. Minimized structures were used as initial configurations for 2.5 ps AIMD runs that ramped the temperature all the way back to 300 K. At this point systems were run for another picosecond at constant volume and temperature. The final liquid snapshots were used for spin restricted single point calculations (S = 1/2) in the presence of an excess electron. The purpose of this was to generate an anzatz for the density matrix to be used in our spin unrestricted AIMD simulations. Our production runs in the presence of an excess electron were in the constant number of particles, volume, and energy ensemble with a time step of 1 fs. Two production trajectories each of 5 ps in duration were generated in the presence of an excess electron. All quantum mechanical studies were carried out using the PBE68 flavor of the generalized gradient approximation as coded in SIESTA.69−71 A description of the generation of pseudopotentials used in this study was already given in ref 10. As in our recent study,10 we used the double-ζ plus polarization basis set with an energy shift of 25 meV and a mesh cutoff of 250 Ry. This mesh cutoff results in a real space grid resolution close to 0.1 Å. Because of the size of our periodically replicated system, Brillouin sampling was only conducted on the Γ-point. For most our trajectories it was sufficient to set the electronic temperature to 100 K to smear the Fermi−Dirac occupation function. However, in the case of [Pyrr1,4][DCA] with an excess electron, the electronic temperature was set to 1 K in order to eliminate the possibility of spin contamination. Optical spectra were computed following an identical procedure as described in ref 10. In order to provide qualitative understanding of what long time solvated excess electrons may look like, a single final AIMD configuration was used as initial condition for 6 ns long classical molecular dynamics in the constant number of particles, pressure, and temperature ensemble. As will be described in subsequent sections, excess electron localization imposes significant geometrical changes on a doubly charged DCA anion, and therefore, force field parameters for this species require adjustments. For this purpose, equilibrium angles, bond lengths, and charges were derived for a doubly charged DCA structure that was minimized using the Gaussian72 program at the spin unrestricted MP2/6-31G(d,p) level of theory. Charges were fitted using the ESP-CHELPG73 method. This procedure is reasonable since gas-phase geometry and spin density on the DCA anion with the extra charge are similar to those derived from our condensed phase DFT dynamics studies in the presence of the excess electron. Five snapshots from the last 2 ns of the classical trajectory were saved for DFT optimization using the CG algorithm implemented in SIESTA. This was done in order to adjust the classically derived snapshots to the quantum potential. Because cavity formation is a very rare event on the time scale and length scale accessible to AIMD, in order to also investigate the possible nature of a cavity bound solvated electron that could give rise to the long-time NIR signal, the DCA anion on which the excess electron localized was removed from the classically equilibrated and ab initio optimized IL. This resulted in a new system with 10 cations, 9 anions, and an excess electron. In this way, the procedure attempted to create a

3. RESULTS We start by analyzing the electronic nature of neat [Pyrr1,4][DCA]. Figure 1 shows the calculated optical absorption

Figure 1. Calculated optical absorption spectra for neat [Pyrr1,4][DCA]. No signal is observed at longer wavelength.

spectrum as well as the DFT eigenvalues for this liquid at 300 K. The gap between HOMO (blue line in the inset) and LUMO (red line in the inset) is computed to be about 3.95 eV, which appears to be consistent with electrochemical measurements that set this number to be about 0.5 eV larger.20−23,27 Since the neat liquid is transparent, it is expected that any IR or visible absorption band after the introduction of an excess electron or hole will be due to the extra charge. In our calculations the excess electron is in the spin-up channel. We therefore refer to the singly occupied state in which the excess electron is as SOMO. We use the same terminology to identify the state in which an unpaired electron is upon the formation of a hole. At time zero, before any possible chemistry or solvent reorganization, one could reasonably expect that the SOMO of [Pyrr1,4][DCA] with an excess hole would resemble the HOMO of the neat liquid. A similar argument could be made about the excess electron. At time zero, the SOMO of [Pyrr1,4][DCA] with an excess electron should resemble the LUMO of the neat liquid. Following this line of reasoning and without any further calculations involving an excess charge, one can rely on the neat liquid projected density of states (PDOS) to provide an estimation of the possible distribution of excess electron or hole spin density among cationic and anionic species. Figure 2 shows that in the case of neat [Pyrr1,4][DCA], consistent with mass spectroscopic studies,74 the HOMO level is almost exclusively anionic. Instead, the LUMO level has contributions both from cations and anions. We should therefore expect that at time zero the SOMO of [Pyrr1,4][DCA] with an excess electron should have both cationic and anionic components and the SOMO of [Pyrr1,4][DCA] with an excess hole should almost exclusively involve DCA. In electrochemical jargon, and based only on the neat liquid, one would predict the anodic limit to be anionic and the cathodic limit to be mixed. In subsequent subsections, we should contrast this crude prediction in the absence of excess charge with results in the presence of an excess electron paying particular attention to the time evolution of charge localization. 534


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In other words, electrochemistry and spectroscopy appear to be sampling different reactivity pathways. Much of this discussion carries to the case of [Pyrr1,4][DCA]. Therefore, in sections 3.1.1 and 3.1.2 we attempt to address these two possible mechanisms of excess electron localization. 3.1.1. Excess Electrons That Localize on Ions. We have found in previous studies10 that the simplest method to track the evolution of excess charge distribution in ILs is by Mulliken population analysis. This is certainly not the most accurate way to get at individual atomic charges but is a very useful tool to follow in time the reorganization and localization of charge during the transition from dry to trapped states. Figure 3 shows Mulliken charges for all ions in our condensed phase [Pyrr1,4][DCA] system in the presence of an excess electron as a function of time. Before about 70 fs, all anionic (cationic) charges fluctuate around similar values. The noninteger nature of the charges are in part due to the nonminimal and nonorthogonal form of the basis set and the inaccuracy of the Mulliken method. Before 70 fs no special ion holds significantly more charge than others of the same class. The electron is at this point delocalized. We see in the left panel of Figure 3 that between 70 and 150 fs one of the DCA anions becomes doubly charged. This is the beginning of the trapped electron state. Our ab initio trajectories indicate that trapping of the excess electron occurs on a 200 fs time scale. This is the same type of subpicosecond evolution that leads from dry to trapped electron states in the cases of [Pyrr1,4][Tf2N] and [N1113][Tf2N],10 and we speculate that a delocalized to localized transition in the early subpicosecond regime is likely generic behavior for many other ionic liquids. A different way to depict the transition from dry to trapped states is by visual inspection of the spin density (ρα − ρβ) shown in Figure 4. Figure 4 shows how a delocalized spin density at time zero evolves in time to localize mostly on a single anion with some small fraction of the density on two adjacent cations. Furthermore, anionic localization of the electron is not symmetrical with one-half of DCA accepting most of the excess density as can be seen at 200 fs in Figure 4. Yet another way to identify the transition from dry to trapped states is by means of the excess electron time dependent absorption spectrum. As can be gleaned from Figure 5, the signature of the dry to trapped transition is the rapid change on a subpicosecond time scale of the broad lowenergy band that characterizes the dry electron state and the appearance of visible transitions as the excess electron localizes mainly on one of the DCA anions. When the excess electron becomes trapped, transitions with high oscillator strength between 500 and 700 nm become possible which promote the

Figure 2. Cationic and anionic projected density of states for neat [Pyrr1,4][DCA]. All energies are shifted by the Fermi energy.

In radiolysis experiments, excess electrons and holes are generated via ionization processes induced by high-energy radiation. The ejected electrons can travel to new places far away from their parent ions. Instead, in photolysis experiments where the photon energy is just above the ionization threshold an electron can be left with little kinetic energy to travel. In the current study, excess negative charge is introduced by adding an electron to a pre-equilibrated ionic liquid configuration and therefore resembles the case in which the electron is far from the parent ions. As we have done in prior studies,10 such initial pre-equilibration as described in the Methods section, is first done with a classical force field and later via ab initio minimization to adjust bonds and angles to the quantum potential energy surface followed by AIMD simulations in the absence of extra charge. The result of this procedure is used as initial condition for AIMD simulations in the presence of an extra electron. 3.1. From Dry to Trapped and the Long-Time Excess Electron Solvated State. In water and alcohols, the radiolysis yield of solvated electrons that give rise to a characteristic broad optical absorption band can be estimated to be 1.5−2 times larger than in alkylammonium [Tf 2 N − ] based ionic liquids.40,41,50 Even if all excess electrons generated by radiolysis in water and alcohols were cavity electrons, this yield discrepancy would suggest that a non-negligible fraction of excess electrons in ILs do not survive to become cavity electrons. Furthermore, at least in the case of [Tf2N−] based anions combined with aliphatic cations, evidence for multiple mechanisms of excess electron localization and reactivity arise from spectroscopic and electrochemical results. Electrochemistry tells us that [Tf2N−] anions determine the cathodic limit and fragment, implying that excess electrons may localize on and react with them. Instead, spectroscopically we know that long-lived solvated electrons display the characteristic broad NIR band typical of cavity electrons likely solvated by cations.42

Figure 3. Mulliken charges for all ions in [Pyrr1,4][DCA] in the presence of an excess electron as a function of time. 535


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Figure 4. Spin density (ρα − ρβ) for [Pyrr1,4][DCA] in the presence of an excess electron. The isosurfaces were plotted using the same isovalue 0.002 |e|/bohr3. Highlighted are ions with most associated spin density.

[N1113][Tf2N] and [Pyrr1,4][Tf2N] is that the transition between dry and trapped electron states is not accompanied by bond dissociation. Whereas no bond dissociation is observed in any of our AIMD simulations in the case of the DCA ion in which an excess electron has localized, both fast subpicosecond intramolecular changes and longer time scale solvation start to take place. Figure 6 depicts the different intramolecular structural changes associated with excess electron localization that occur in this trajectory on a 150 fs time scale. We recall from Figure 4 that the excess electron localization on the doubly charged DCA anion is asymmetrical. Almost all excess electron density is on one-half of the anion and structural changes are much more pronounced on this side. We see from Figure 6 that both CN bonds slightly elongate, but the one labeled B4 on the side of DCA with most excess spin density elongates the most. Significant elongation of about 0.15 Å occurs in the case of the C−N bond labeled B2 in Figure 6 which as well as B4 is on the side of DCA where the excess electron localizes the most. The most dramatic intramolecular change occurs for the N−C−N angle on the side of DCA where the excess electron localizes. In this case an angle shift of about 50° can be observed. Ab initio dynamics beyond the several picoseconds regime becomes computationally prohibitive for significantly large condensed phase systems. In order to gain insight into the fate of the excess electron at longer time scales, we used a final AIMD snapshot at 5 ps for further classical molecular dynamics simulation. The partial charges for the special doubly charged anions were obtained using an ESP fit using the CHELPG method as coded in Gaussian72 and bond lengths and angles where adjusted in the force field to account for the significant changes that DCA undergoes upon charge localization. The [Pyrr1,4][DCA] liquid with a doubly charged anion was then simulated classically in the constant temperature pressure and number of particles ensemble for 6 ns. The final configuration

Figure 5. Optical absorption spectra of [Pyrr1,4][DCA] in the presence of an excess electron for different time snapshots.

excess electron to excited states that have significant overlap with the localized SOMO. At time zero after the introduction of an excess electron and before any type of structural relaxation, the dry electron SOMO is delocalized and close in energy to other adjacent delocalized LUMOs that are similar in character. Transitions between these are in the NIR or even lower in energy and, as we have previously noted,9,10 are translational in nature. By this we mean that by low-energy excitations an electron can be made to move from one part of the liquid to a location nearby. This broad low-energy absorption characteristic of the “dry” electron state disappears when the electron becomes localized (the “trapped” electron state). However, a new type of transition in the NIR becomes at times possible when there is good overlap between the trapped state localized SOMO and its corresponding LUMO↑. As an example of this, Figure 5 shows that there is almost no NIR intensity for this particular trajectory at 3 ps, but transitions in the NIR are possible again at 5 ps. Another trajectory shows similar results at different times. What is very different in the case of [Pyrr1,4][DCA] when compared to systems containing the [Tf2N−] anion such as 536


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Figure 6. Schemes on top left and right label bonds and angles and show the asymmetric spin density localization for the doubly charged DCA anion. From left to right and top to bottom, a comparison between the time history of CN bond lengths of a regular DCA anion and those in the doubly charged anion; a comparison between the time history of C−N bond lengths of a regular DCA anion and those in the doubly charged anion; a comparison between the time history of N−C−N angle of a regular DCA anion and those in the doubly charged anion; and a comparison between the time history of C−N−C angle of a regular DCA anion and that in the doubly charged anion.

as well as four other snapshots along the trajectory were subjected to quantum mechanical geometry optimization to bring back the system to the quantum potential energy surface. Electronic structure calculations were then conducted on the relaxed condensed phase liquid structures. This procedure has limitations; the most obvious one is that because of the nature of the classical force field, charge localization will be certain and solvent relaxation will only favor a deep localized trap. In other words, charge transfer as well as possible localized to delocalized transitions will be suppressed. Whereas we are aware of these limitations, we hope that the results provide insight into a very likely channel of solvent relaxation in the presence of an excess electron. Figure 7 shows the spin density in solvent equilibrated [Pyrr1,4][DCA] with an excess electron. The 6 ns classical equilibration of the system followed by ab initio minimization resulted in excess charge localization in exactly the same DCA anion emerging from our original several picosecond long AIMD study with some density spilling to surrounding cations. As was previously observed, localization of the charge remains mostly on one side of the doubly charged anion with participation of two cations. The involvement of [Pyrr1,4+] cations hints at the important role that these play in stabilizing the excess solvated electron. Right panels in Figures 3 and 8 show that the fully solvated doubly charged DCA anion preserves the same excess charge it gained in the early subpicosecond regime but that solvent reorganization lowered the energy and stabilized the fluctuations of SOMO. The

Figure 7. Spin density for [Pyrr1,4][DCA] in the presence of an excess electron at 6 ns. The isosurfaces was plotted at an isovalue of 0.002 |e|/ bohr3.

computed spectrum at 6 ns in Figure 5 shows very small NIR intensity and a peak in the 400−700 nm region. Interestingly, the experimental long time spectrum of excess electrons in RTILs of varied nature is often characterized by a broad band in the NIR. The mechanism explored in section 3.1.1 does not lead at long times to this band; it instead leads to 537


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shows absorption spectrum right before electron transfer, during electron transfer, and right after electron transfer. We see that whereas before and after electron transfer only small NIR intensity is observed, during electron transfer a very broad absorption band appears. This is because both SOMO and LUMO↑ have significant projections on the two DCA anions that are undergoing electron transfer, and this results in large oscillator strength for this transition. 3.1.2. Excess Electrons That Localize in Cavities. At time zero after an excess electron is produced in an IL, this is expected to be broadly delocalized. Whether later localization will be on ions or in cavities likely depends on the electron’s ability to find or create a suitable trap. Because of the small size of our simulation box (10 ion pairs) and because our simulations are at constant number of particles, volume, and energy (NVE ensemble) the likelihood of generating what would look like a preformed anionic vacancy where an excess cavity electron could localize is vanishingly small. In a real liquid even if such ionic configuration happened with very low probability, the delocalized nature of the dry electron may result in its ability to find these at least for a fraction of the excess electrons produced by radiation or photoexcitation. We hypothesize that electrons unable to find or create a cavity properly solvated by positively charged cationic heads will localize on DCA anions instead. Such electrons are likely spectroscopically silent in the NIR at long times. The subset of electrons that is able to localize in a cavity will give rise to a long time NIR band. In order to establish the possibility to model in our simulations a cavity bound electron as well as to probe the nature of its electronic transitions we removed from the liquid snapshot in Figure 7 the DCA anion on which the excess electron had localized. This resulted in a periodically replicated system with 10 cations, 9 anions, and an excess electron that was further optimized using conjugate gradient. After the CG procedure, the electron localized in the cavity and was fully solvated by cations that provide a Coulombic trap that supports bound electron states. Figure 10a shows the spin density of the system with an excess electron localized in the cavity. Most of the spin density is inside the cavity, but some of it spills to adjacent cations highlighted in green. Figure 10b shows the typical s-type “particle in a box ground state” wave function of a cavity electron. Adjacent cationic hydrogens appear to contribute to the cavity wave function with the same sign (depicted in red), but cationic carbon atoms appear to surround the cavity with p-orbital lobes of opposite sign

Figure 8. Energy diagram for [Pyrr1,4][DCA] in the presence of an excess electron. The band diagram at negative time represents the eigenenergies of the neat liquid without an excess electron. At positive times, the blue and red dashes represent SOMO and LUMO↑ eigenvalues, respectively. At negative times they represent HOMO and LUMO eigenvalues of the neat liquid.

a state that is dark in the near-IR. In section 3.1.2 we explore the case of excess electrons in cavities that likely give rise to the typical NIR band at long times. It is worth mentioning that several other possible origins for NIR transitions can be proposed beyond the cavity state which we believe is the dominant component at long times. The first one is electronic transitions from delocalized states. We see such transitions in the “dry” electron state and the possibility of electrons going back and forth between the localized and delocalized configuration has been observed by Bu and co-workers38 on methylpyridinium chloride. Another source for NIR transitions in the localized state is between SOMO and LUMO↑ when these states have good overlap. Yet another possible origin for this broad optical band is interionic electron transfer discussed in section 3.1.1.1. 3.1.1.1. Interionic Electron Transfer. Electron localization leading from the dry electron state to the trapped electron state is fast on the subpicosecond regime. However, this does not necessarily mean that once initial localization occurs the charge has to remain on the same DCA anion. One of our trajectories shows just the opposite. Localization first occurs on one DCA anion, but fluctuations in the system induce electron transfer onto a different DCA anion. The time-dependent spin density describing the electron transfer process is shown in Figure S1. Figure 9a shows the evolution of the charge localization as electron transfer occurs between 500 and 650 fs. Such electron transfer events have a very clear spectral signature. Figure 9b

Figure 9. (a) Mulliken charges as a function of time as intermolecular electron transfer occurs between 500 and 650 fs. (b) Absorption spectrum before, during, and after intermolecular electron transfer. During charge transfer a broad absorption band appears that is absent before and after. 538


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Figure 10. (a) Spin density of a cavity bound excess electron in [Pyrr1,4][DCA] at isovalue 0.001 |e|/bohr3, (b) s-type SOMO wave function of a cavity bound excess electron, and (c) p-type LUMO↑ wave function of a cavity bound excess electron.

(depicted in blue). Thus, this renders some cationic C−H bonds antibonding and possibly labile. The first three “excited states” (LUMO↑, LUMO↑+1, LUMO↑+2) of the cavity bound electron are, as expected, of p nature.75−80 Figure 10c shows that LUMO↑ has two cavity lobes of different sign corresponding to a state with a single node. As in the case of the SOMO state, the wave function also spills to adjacent solvating ions. Figure 11 shows that the transition between the s-type state and the p-type states of the cavity electron are in the NIR. Other transitions also contributing to the broad NIR band are associated with charge transfer to solvent, mostly to adjacent solvating cations. The cavity electron broad NIR band appears to be of higher intensity than that of the dry electron. We suspect that at long times this is the predominant channel giving rise to the broad NIR spectrum of the excess electron in [Pyrr1,4][DCA], with delocalization, electron transfer, and SOMO to LUMO↑ transitions of electrons localized on DCA also contributing to the signal at least at short times.

Figure 11. Spectrum of the cavity electron showing a broad NIR band of high intensity. The NIR band is only from transitions of the cavity electron (red); other electrons in the liquid in the opposite spin channel only appear to absorb below 400 nm (blue).

scopic outcomes. For example, in the case of [Tf2N−] based liquids the ionic localization channel should result in early anionic fragmentation. Significant evidence exists for such outcome from electrochemical and other measurements. On the other hand, such fragmentation does not provide a mechanism to account for the long-time NIR band observed spectroscopically. Therefore, a fraction of electrons is likely able to find or form anionic cavities that can support localization.

4. CONCLUSIONS An initially dry excess electron must localize. Whether an electron survives to localize on a cavity or it localizes on ions should result in significantly different chemical and spectro539


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In the case of [Pyrr1,4][DCA] in the absence of cavity localization, we find that the excess electron localizes asymmetrically on a single DCA anion with some spin density shared by adjacent solvating cations. This causes significant distortion but not early fragmentation of the anion. This is in contrast with and in clear advantage over other low-viscosity ionic liquids based on the [Tf2N−] anion where our studies and electrochemical measurements hint at significant anionic fragmentation. In the case of the DCA anion, since electron localization at short time appears to be asymmetric but not destructive, our studies beg for ultrafast Raman measurements that can provide further evidence of this mechanism. This mechanism of electron localization may be important; however, it may not necessarily account for the NIR band characteristic of the solvated electron at long times. We find that when localization of the excess electron is inside a cavity solvated by positive charge, the typical broad NIR band naturally manifests as a consequence of particle in a box s- to ptype electronic transitions as well as charge transfer to solvent transitions. The intensity of these is very high and the energy at which these appear is reasonably consistent with experimental findings. SOMO and LUMO↑ states are in the cavity but adjacent cations also contribute. It is possible that at longer times such cationic involvement could result in further chemical reactivity such as proton detachment.

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ASSOCIATED CONTENT

S Supporting Information *

Atomic partial charges, optimized bond lengths and angles of a doubly charged DCA anion, and Figure S1 showing the time evolution of the spin density during an intermolecular electron transfer event. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.J.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract DE-SC0008644 awarded to C.J.M. We thank Dr. James Wishart, Prof. David Blank, and Dr. Raluca Musat for instructive discussions. We also gratefully acknowledge the University of Iowa for a generous allocation of high performance computational resources.

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REFERENCES

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Solvation of an Excess Electron in Pyrrolidinium Dicyanamide Based

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