Vapors from Ionic Liquids: Reconciling ... - American Chemical Society

Oct 31, 2012 - Ionic liquids (IL) were long thought to be completely nonvolatile with .... techniques, the cloud of evaporating microdroplets provides...
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Vapors from Ionic Liquids: Reconciling Simulations with Mass Spectrometric Data Brenno A. D. Neto,† Eduardo C. Meurer,‡ Renan Galaverna,‡ Benjamin J. Bythell,§ Jairton Dupont,∥ R. Graham Cooks,⊥ and Marcos N. Eberlin*,‡ †

Laboratory of Medicinal and Technological Chemistry, University of Brasília, Institute of Chemistry, Brasília, DF 70904-970, Brazil ThoMSon Mass Spectrometry Laboratory, Institute of Chemistry, University of Campinas, Campinas, SP 13085-970 Brazil § National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States ∥ Institute of Chemistry, Federal University of Rio Grande do Sul, Porto Alegre, RS 91501-970, Brazil ⊥ Aston Laboratory for Mass Spectrometry, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States ‡

S Supporting Information *

ABSTRACT: The species involved in the distillation of aprotic ionic liquids are discussed in light of recent simulations and mass spectrometric data obtained by various techniques. New mass spectrometric data collected via laser-induced acoustic desorption and the thermal desorption of ionic liquids are also presented as well as additional DFT calculations. The available evidence of theoretical simulations and mass spectrometric data suggests that the distillation of ionic liquids occurs mainly via neutral ion pairs of composition CnAn [C+ = cation and A− = anion], followed by gas-phase dissociation to lower order ion pairs and then dissociation of hot CA to C+ and A−, followed by ion/molecule association events to give [CnAn−1]+ or [Cn−1An]− ions to a degree that depends on the amount of internal energy deposited into the neutral CnAn clusters upon evaporation.

SECTION: Molecular Structure, Quantum Chemistry, and General Theory

I

The recent observation that both protic and most particularly aprotic IL can be distilled without degradation5 was a major paradigm change in IL chemistry and one of the most intriguing findings about these exotic but very useful chemical species. Note that protic ionic liquids are volatile by their nature because proton-transfer equilibrium between both the cation [C−H]+ and the anion A− allows for the formation of neutral molecular species C and A−H that readily evaporate and recombine restoring the protic ionic liquid upon condensation. Soon after that report, different proposals were made regarding the nature of the species present in the vapor of aprotic IL and whether isolated ions or neutral supramolecular aggregates were present.6 Although it seemed that a consensus was reached about the prevalence of the CA neutral ion pair in the vapor of IL,7 the matter is still under debate. Recently, Chaban and Prezhdo8 reported detailed atomistic simulations of the vapor phase of IL in equilibrium with the liquid that seem to provide crucial insights into the debate. Interestingly, the simulations predicted that the saturated vapor of IL, at relatively high temperatures in the range of 1000−1075 K,

onic liquids (IL) were long thought to be completely nonvolatile with negligible vapor pressures, as are also displayed by classical molten salts.1 But room-temperature IL, especially those based on imidazolium cations, such as 1 (see below), are unique chemical entities due to a 3D cooperative network with H bonds between cations and anions.2 This 3D preorganized H-bonded structures provide structural directionality via an entropic effect3 whereas classical salts form aggregates only through simple ionic bonds. As a consequence, comparatively weak supramolecular interactions control the structures of IL, whereas the charges rigidly control the structure within classical salts. IL are better described, therefore, as H-bonded polymeric “zwitterionic supramolecules” of the type [(C)n(A)n−y)]y+[(C)m−y(A)m)]y−, where C+ = cation and A− = anion. This structural description applies to the condensed phase and is apparently maintained to a large extent in the gas phase;4 hence, the supramolecular interactions of IL should govern their main and unique properties, including their volatility and the species generated in the process of volatilization.

Received: October 8, 2012 Accepted: October 31, 2012 Published: October 31, 2012 © 2012 American Chemical Society

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Figure 1. Schematic of the cooling effect provided by APCI for hot IL aggregates that may eventually codistill with the solvent from the minute APCI droplets.

could be explained by the relatively low thermal evaporation temperature of ca. 522 K used for MS, whereas their simulations were performed at temperatures of 1000−1075 K. The neutral ionic pair [C+A−] would therefore lack the vibrational energy necessary to break its ionic bond. The authors also pointed to the fact that their simulations demonstrated the existence of free anions in the vapor of IL whereas, presumably based on the EI(−)-MS data, the detection of anions in the vapor of IL still presents a challenge to MS. Here we wish to contribute to this discussion and show that there are no inconsistencies between the current body of physical MS data and the numerical simulations. In fact, we believe that the simulations presented by Chaban and Prezhdo8 as well as by others10 perfectly fit the available MS data, as well as the new MS data presented herein, on IL vapor and distillates. The failure of EI(+)-MS to detect the higher order aggregates predicted by the simulations is hardly a surprise. EI is a quite “hard” ionization technique that would certainly transfer significant amounts of internal energy to the nascent ions; hence, any supramolecular aggregates of [CnAn−1]+ or [Cn−1An]− composition would likely be promptly disrupted under EI conditions. In fact, even molecules with strong covalent bonds are often unable to accommodate the amount of vibrational internal energy deposited in the nascent ions produced upon gas-phase EI; hence, extensive dissociation occurs, sometimes leading to spectra that do not contain intact molecular ions.11 The fact that anions from the vapor of IL were not detected by EI(−)-MS is also not surprising because electron capture yields in EI are known to be typically orders of magnitude lower than those for electron detachment,11 and dissociative ion pair formation by EI is almost unknown. The exclusive detection via EI(+)-MS of C+ ions from the vapor of IL is therefore consistent with the limited capability of EI(±) to investigate via MS the actual composition of IL vapor. Detection of anions or more loosely bonded CnAn aggregates is indeed a severe limitation for EI(−)-MS, but other MS techniques perform well for such species. Several ionization techniques also perform well in both ionization modes, so anion detection and characterization presents no particular challenge to MS.

consists of a complex set of neutral and charged species including CnAn up to n = 3 and [CnAn−1]+ and [Cn−1An]− charged species, that is, CA, C2A2 and C3A3, C+, A−, [C2A]+, and [CA2]−. The simulations also indicate that the neutral ionic pair CA is the most abundant species and that the distribution of species depends on the IL. The distribution was also shown to be sensitive to the vapor’s temperature and pressure. The increase in temperature was found to disfavor CA slightly and increase the abundance of the charged species. Mass spectrometry (MS) has been by far the technique most employed to investigate the structures present in IL vapors.9 For instance, IL have been applied as efficient, low-noise matrices for MALDI-MS.9 They have also been studied in their own right by ESI-MS,9a where they show both C+ and A− ions and under gentler conditions the cluster ions CnAn−1+ and Cn−1An−. Another noteworthy study uses chiral IL as solvents for enantioselective photoisomerization, that is, as chiral induction solvents.9 To compare the predictions of simulation with experimental data, Chaban and Prezhdo elected to focus on MS data, more specifically on recent data acquired using electron ionization mass spectrometry (EI-MS).8 They argue that the detection of only isolated cations via EI(+)-MS of IL provides evidence that the vapor consists exclusively of the [C+A−] ion pair and that the observed C+ ions are formed by a mechanism involving electron detachment from the neutral ion pair [C+A−], followed by the rupture of the loosely bonded [C+---A] aggregate. No C+ was detected when the ionizing electron beam was turned off. The inability to detect anions using EI(−)-MS was also pointed out, and no information about the negatively charged species present in the vapor could be obtained by EI(−)-MS. The authors also noted that similar conclusions, the predominance of CA, had been reached by APCI-MS9a and TOF-MS9b measurements. The authors8 also pointed out that no [CnAn−1]+ species were detected in the EI(+)-MS experiments; hence, it remained unclear whether the higher order ion pairs C2A2 or C3A3 predicted by the simulations would have been accessible via EI(±)-MS. Chaban and Prezhdo ended their discussion by noting the differences between the physical and numerical experiments, presumably that the EI(+)-MS data point to the predominance of CA species and their simulations point to a wide range or neutral CnAn and charged [CnAn−1]+ and [Cn−1An]− aggregates, and suggesting that these differences 3436

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Figure 2. APCI(−)-MS of a acetonitrile solution of ionic liquid 1. Note the detection of A− of m/z 279.9, CA2− of m/z 699.1, and C2A3− of m/z 1118.3 at nearly the same proportion as predicted by the simulations of Chaban and Prezhdo.8

APCI-MS and without employing any auxiliary ionization step (such as the electron beam used in APCI) or evaporative cooling. These data therefore also fit to the simulation results. They show that C+ and A− as well as ionic aggregates coexist with the neutral aggregates in the vapor of IL particularly when more energy is used for evaporation. Therefore, it seems that the MS measurements done at low and high temperatures using an auxiliary beam of electrons to detect the neutral aggregates as well as those in which no such means of ionization is used match the simulations presented by Chaban and Prezhdo8 and others10,12 and that the atomistic organization of IL vaporization can now be adequately described by both experiment and theory (Figure 3 and Scheme 1). To further test this rationale, we have performed additional MS measurements. In experiment A (Figure 4), laser-induced

In fact, we were delighted to note that the simulations presented by Chaban and Prezhdo8 as well as other pieces of evidence summarized in a recent review article by Rebelo et al.12 were all very close to our MS measurements using APCI.9a In fact, even the predicted relative numerical composition of the CnAn supramolecular neutral aggregates (CA ≫ C2A2 > C3A3) as well as the experimentally observed upper limit for n = 3 match very closely our APCI-MS data.9a During the electrohydrodynamic spray process of APCI, IL vaporize together with the solvent from the evaporating droplets.13 At these low temperatures (temperatures considerably lower than the boiling point or decomposition temperature of IL), CnAn ion pairs of the same composition up to n = 3 were therefore indirectly detected. In the atmospheric pressure spray techniques, the cloud of evaporating microdroplets provides a cooling environment (Figure 1) that may have prevented gasphase dissociation. In contrast with the harsh EI conditions used in conventional EI in vacuum, ionization using an atmospheric pressure electron beam that is known to provide a quite gentle ionization environment with the formation of low energy ions13 was used in APCI for ionization and detection of both negatively charged and positively charged species. Ions in the relative abundance order CA > C2A2 > C3A3 were observed in both polarities, namely, APCI(+) and APCI(−)-MS. Figure 2, as an example, shows the detection of A−, [CA2]−, and [C2A3]− in the APCI(−)-MS of ionic liquid 1. We have also observed that no charged species such as C+ or − A could be detected when the voltage was turned off during APCI(+) or APCI(−). This experimental observation also agrees with the conclusion of Chaban and Prezhdo8 and others10 that evaporation at low temperatures such as lowtemperature distillation (or the cooling environment experienced by the gaseous species generated in APCI) should contribute to the formation of stable and cool supramolecular CnAn aggregates with insufficient internal energy to fragment to form charged species. Using ionizing conditions that sharply contrast with those of APCI, however, one of us has also reported MS data via atmospheric-pressure thermal desorption ion mass spectrometry (APTDI-MS).14 Under these harsh conditions, C+ and A− were detected together with the aggregate cation [C2A]+. This MS observation was made in experiments using much higher thermal evaporation temperatures than those employed during

Figure 3. Schematic representation at the atomistic level of the distillation process for IL. If less energy is provided, then neutral CnAn up to the n = 3 ion pairs escapes from the liquid surface to the gas phase and remains as such, in the relative proportion CA > C2A2 > C3A3. If more energy is provided, then the hot gaseous CnAn ionic pairs dissociate (also see Scheme 1) to lower order ones and eventually to C+ and A−, which may react further with CnAn to form the set of CnAn, [CnAn−1]+, [Cn−1An]−, C+, and A− species predicted by the simulations. The possibility that vibrationally excited higher order ion pairs such as C2A2 may also dissociate directly to C2A+ or CA2− is also considered. 3437

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Scheme 1. Route for the Formation of the Set of CA, C2A2, and C3A3, C+, A−, [C2A]+, and [CA2]− species (Figure 3B) Predicted by the Simulations of Chaban and Prezhdo8 As a Function of Temperature from a Relatively Low-Temperature Vapor of Aprotic IL Composed Mainly of Neutral Ion Pair CnAn Species

is, the actual transition structure energy necessary to produce these products will be greater than or equal to these values. The relative Gibbs free energies (ΔG298) of the species are presented to provide an illustration of the likely kinetic component of the various structures. For the formation of ionic species from vibrationally excited neutral CnAn species, the calculations support the general idea summarized in Scheme 1 that ionic species are formed more favorably from the disruption of the larger aggregates to CA and then to C+ and A−. Further reactions of these ions with CnAn would form stabilized “solvated” cations and anions. The energetics summarized in Table S1 in the Supporting Information and illustrated in Figure 6 show that the distillation of the ionic pair CA is comfortably the most favorable reaction. Fragmentation reactions generating smaller, charged species are substantially more energetically demanding (Figure 6, for example). The calculations indicate that increased temperatures like those modeled by Chaban and Prezhdo8 logically favor greater numbers of smaller clusters and ions as well. For example

Figure 4. (A) LIAD of a thin layer of IL deposited on the surface of an aluminum foil with the MS detection of no ions and (B) fast thermal desorption of IL from an incandescent filament with detection of both naked cations and anions as well as ionic aggregates formed in the gas phase by dissociation of neutral CnAn ion pairs.

C3A3 → C2A 2 + CA ΔH(ΔG)298,rxn = +85(41) kJ mol−1

C2A 2 → 2CA

acoustic desorption (LIAD) was performed. LIAD15 is known to provide one of the gentlest approaches to desorb molecules from surfaces via acoustic waves and to yield rather cool gaseous species. An aluminum foil surface was covered with a thin layer of IL, and near complete LIAD desorption was observed after a few minutes. However, no significant ion current could be detected from the IL vapor, even when using a sensitive ion trap mass analyzer (Bruker HCT, Bremen, Germany). This result seems to support IL evaporation via neutral hence MS undetectable ion pairs CnAn. In experiment B, however, very fast desorption was performed by depositing IL on the surface of a wire filament, which was heated nearly instantaneously to incandescence. In this case, [CnAn−1]+ or [Cn−1An]− species could be detected in either the positive or negative ion modes (Figure 5). This result seems to support the idea that isolated C+ and A− ions as well as [CnAn−1]+/ [Cn−1An]− ionic aggregates detected by MS in experiment B did not originate by evaporation directly from the liquid surface but were formed via gas-phase dissociation of vibrationally excited hot gaseous CnAn according to Scheme 1. To estimate the energetics of IL distillation, we also performed B3LYP/6-31+G(d,p) calculations on a hypothetical minuscule cluster of IL [N,N-dimethyl imidazolium·BF4]n (n = 1−3). Figure 6 illustrates this for the C2A2 supramolecule. We then compared the relative energy (ΔH298) of the clusters to the energy of the product clusters, ions, or both. This provides an estimate of the minimum energy necessary to generate the smaller clusters (neutral or charged) from the larger ones; that

ΔH(ΔG)298,rxn = + 81(35) kJ mol−1

C2A 2 → C2A+ + A− ΔH(ΔG)298,rxn = +301(247) kJ mol−1 C2A 2 → CA 2− + C+ ΔH(ΔG)298,rxn = +322(266)kJ mol−1

CA → C+ + A−

ΔH(ΔG)298,rxn = + 344(302) kJ mol−1

From a combination of the available MS data and theory, it seems therefore that IL distill (i.e., escape from the liquid surface and move into the gas phase) as discrete neutral ion clusters of general formula CnAn, experimentally, up to n = 3 as detectable species, in the relative abundance order CA > C2A2 > C3A3.16 Whereas neutral ion pairs are likely to distill, the amount of vibrational energy deposited into these neutral aggregates during the distillation process or the final temperature of the vapor phase is crucial to determine the equilibrium between neutral and charged species in the IL vapor. Because the interactions of the lighter gaseous supramolecules CnAn are weaker than those observed in the condensed phase, where much larger supramolecular assemblies dominate,17 they can be thermally disrupted relatively easily, particularly for the higher order aggregates. The “hot” ionic pairs break down to lower order aggregates and then to the end neutral product of this cascade, CA. If enough internal energy remains, CA may 3438

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Figure 5. (A) APTDI(+)-MS and (B) APTDI(−)-MS of IL vapors produced by thermal desorption experiment as described in Figure 4B.

Figure 6. B3LYP/6-31+G(d,p) calculations (see the Supporting Information) for the energetics of hypothetical “distillation” of CA (where in the case C+ = N,N-dimethylimidazolium and A− = BF4−), C+, or A− from the surface of a minuscule C2A2 IL cluster.

dissociate to the naked C+ and A− ions (Scheme 1). When free gaseous ions C+ and A− are formed, for instance at 1000−1075 K temperatures used in the simulations, they may stay isolated or aggregate further with CnAn species via gas-phase ion/ molecule reactions to form [CnAn−1]+ or [Cn−1An]− ionic aggregates.

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DFT Calculations. To provide an estimate of the energetics of IL structures to inform our discussion of IL distillation, we performed density functional calculations on hypothetical, tiny clusters of [N,N-dimethyl imidazolium.BF4]n (denoted C2A2 in Figure 6). Other multiple families of structures were also investigated. Structures were optimized with the B3LYP/631+G(d,p) functional.18 Local energy minima were confirmed with frequency calculations. The Gaussian set of programs was used.19 Standard statistical mechanical formulas provide the enthalpy and Gibbs free-energy adjustment from 0 to 298 K. The relative energies of the various separated components were also calculated (Table S1, Supporting Information) with the production of charged or neutral products. The calculations also imply that the formation of ionic species from substantially larger, vibrationally excited neutral CnAn species will be

METHODOLOGY

Mass Spectrometry. Both the LIAD and APTDI-MS experiments were performed in a HCT ultra mass spectrometer (Bruker Daltonics). Mass spectra were acquired over the 50−2000 m/z range in the positive ion mode. General operating conditions were as follows: Cone voltage of 30 V, extractor cone voltage 3 V, and source temperature of 250 °C. 3439

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comparatively energetically favorable due to the substantial degree of charge stabilization by solvation available in these systems (Table S1, Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

Details of the DFT calculations such as energies and optimized structures. 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]. Author Contributions

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

The authors acknowledge financial support from the following agencies: CNPq, FAPESP, CAPES, FAPDF, LNLS, and NSF CHE 0848650 (RGC). Notes

The authors declare no competing financial interest.

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REFERENCES

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The Journal of Physical Chemistry Letters

Letter

R. J.; Corfield, J. A.; Taylor, A. W.; Gooden, P. N.; Villar-Garcia, I. J.; Licence, P.; Jones, R. G.; Krasovskiy, V. G.; Chernikova, E. A.; Kustov, L. M. Measuring and Predicting Delta H-vap(298) Values of Ionic Liquids. Phys. Chem. Chem. Phys. 2009, 11, 8544−8555. (17) Dupont, J. From Molten Salts to Ionic Liquids: A “Nano” Journey. Acc. Chem. Res. 2011, 44, 1223−1231. (18) (a) Becke, A. D. a New Mixing of Hartree-Fock and Local Density-Functional Theories. J. Chem. Phys. 1993, 98, 1372−1377. (b) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (c) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.

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dx.doi.org/10.1021/jz301608c | J. Phys. Chem. Lett. 2012, 3, 3435−3441