Physical Absorption Of CO2 in Protic and Aprotic Ionic Liquids: An

Aug 12, 2015 - The physical absorption of CO2 by protic and aprotic ionic liquids such as 1-ethyl-3-methyl-imidazolium tetrafluoroborate was examined ...
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Physical Absorption Of CO2 in Protic and Aprotic Ionic Liquids: An Interaction Perspective Ekaterina I. Izgorodina,*,† Jennifer L. Hodgson,† Derick C. Weis,† Steven J. Pas,‡ and Douglas R. MacFarlane† †

School of Chemistry, Monash University, Wellington Road, Clayton, VIC 3800, Australia Maritime Division, Defence Science and Technology Organisation, 506 Lorimer Street, Fishermans Bend, VIC 3207, Australia

‡

S Supporting Information *

ABSTRACT: The physical absorption of CO2 by protic and aprotic ionic liquids such as 1-ethyl-3-methyl-imidazolium tetrafluoroborate was examined at the molecular level using symmetry adapted perturbation theory (SAPT) and density functional techniques through comparison of interaction energies of noncovalently bound complexes between the CO2 molecule and a series of ionic liquid ions and ion pairs. These energies were contrasted with those for complexes with model amines such as methylamine, dimethylamine, and trimethylamine. Detailed analysis of the five fundamental forces that are responsible for stabilization of the complexes is discussed. It was confirmed that the nature of the anion had a greater effect upon the physical interaction energy in non functionalized ionic liquids, with dispersion forces playing an important role in CO2 solubility. Hydrogen bonding with protic cations was shown to impart additional stability to the noncovalently bound CO2···IL complex through inductive forces. Two solvation models, the conductor-like polarizable continuum model (CPCM) and the universal solvation model (SMD), were used to estimate the impact of solvent effects on the CO2 binding. Both solvent models reduced interaction energies for all types of ions. These interaction energies appeared to favor imidazolium cations and carboxylic and sulfonic groups as well as bulky groups (e.g., NTf2) in anions for the physical absorption of CO2. The structure−reactivity relationships determined in this study may help in the optimization of the physical absorption process by means of ionic liquids.

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INTRODUCTION The use of ionic liquids (ILs) for CO2 absorption has been of increased interest in recent years due to growing applications in postcombustion carbon capture. CO2 capture in liquids can occur via either physical absorption, in which the CO2 is dissolved in the liquid phase without changing its chemical structure, or by chemical absorption, whereby chemical bonding occurs between CO2 and the capture agent molecules. Chemical absorption has been investigated using various functionalized ILs. Functionalities utilized include acetate anions,1,2 amine functionality tethered to imidazolium-based cations,3 either with or without an amino acid anion,4,5 or some combination of functional groups. The chemical absorptions of these ionic liquids were shown to match those of common amines such as MEA and in some cases even exceeded them.6−10 However, ways to tackle the high viscosities of these materials once loaded with CO2 need to be found to avoid a significant disadvantage for practical applications.4,8,11 On the other hand a number of successful studies have confirmed that physical absorption of CO2 by non functionalized ionic liquids increases with pressure,12 and uptake as high as 0.8 mol fraction has been observed at 120 bar.13 The easy and low energy-cost reversibility of physical absorption © 2015 American Chemical Society

generates an interest in whether the CO2 capture by these non functionalized ionic liquids can be improved at ambient pressure. When absorbed through a physical absorption mechanism far less energy is required to remove the absorbed CO2 in the regeneration step; compared with the bondbreaking desorption step in standard amine solutions, less than a quarter of the energy is required remove CO2 physically absorbed into ionic liquids.14,15 It has previously been seen that the nature of the anion is a major factor determining the dissolution of CO2 in non functionalized ILs.14 While alkyl chains did not affect the solubility of CO2, their fluorination appeared to have a positive impact.13 In physical absorption, the tight packing of ions does not appear much disturbed by the insertion of CO 2 molecules.16 For example, for [C2mim][PF6], the PF6− anions are rearranged by a small displacement to form larger voids to hold CO2 molecules. The CO2 molecules lie above or below the imidazolium rings or close to the alkyl chains. Anions such as NTf2 and methide were shown to aid in CO2 solubility and Received: May 28, 2015 Revised: August 12, 2015 Published: August 12, 2015 11748

DOI: 10.1021/acs.jpcb.5b05115 J. Phys. Chem. B 2015, 119, 11748−11759

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The Journal of Physical Chemistry B this was attributed to fluorinated methyl groups.13 In addition, the CO2 physical absorption in ionic liquids was not accompanied by a significant increase in volume as in organic solvents, thus indicating that CO2 was unlikely to interfere with the bulk structure of the liquids. Computational quantum chemistry provides a method for the study of structures and energetics in this context. When theoretical chemistry calculations are performed alongside experiments, they can be used to aid in the interpretation of experimental results and in the prediction of properties that are difficult to measure. CO2 physical absorption by ionic liquids has not been studied using fully ab initio techniques. In previous studies, predictions of CO2 solubility in ILs were made using Henry’s law constants from COSMO-RS calculations of vapor pressures.17,18 The Maginn group have significantly contributed to the understanding on the mechanism of CO2 solvation in non functionalized ionic liquids.14,19−21 In hexafluorophosphate- and tris(pentafluoroethyl)trifluorophosphate-based ionic liquids the addition of CO2 did not affect the liquid structure of the ionic liquid even with 50% of CO2 introduced in the simulation.19 Recently, a first-principles molecular dynamics simulation by Firaha and Kirchner also confirmed that upon solvation CO2 did not disrupt the ionic liquid network in ethylammonium nitrate.22 Studying the effects of different cation and anion components in the ionic liquid may lead to development of better ionic liquids for CO2 absorption through physical absorption. More understanding of interaction between CO2 and ionic liquid ions at the molecular level is needed for a rational design of ionic liquids with increased capability for CO2 absorption.23 Previous experimental studies of the CO2 absorbing capabilities of ionic liquids have focused on aprotic ILs. It is expected that hydrogen bonding in protic ionic liquids will affect the CO2 absorbing capability of an ionic liquid. In this work, the interaction energies and mechanism of physical absorption with discrete cations, anions and single ion pairs are investigated with a view to understanding structure−reactivity relationships. A number of aprotic (based on quaternary ammonium, quaternary phosphonium and imidazolium) and protic (based on primary, secondary and tertiary ammonium) cations were selected as well as routinely used anions such as halides, carboxylates, sulfonates, BF4, PF6, and NTf2. Some cations such as C2mim+ and anions such as PF6− and NTf2− were chosen based on the findings of previous studies by Maginn and Brennecke49,57 reporting higher absorption of CO2 in these ionic liquids. Simple protic cations were chosen to study the effect of strong hydrogen bonding between cations and anions in protic ionic liquids on the CO2 physical absorption. It has to be noted that the current study of individual interactions between the CO2 molecule and ionic liquid ions and ion pairs provides the first step in selecting the best ionic liquid ions to improve the physical absorption of CO2. The findings of this work will need to be followed up by molecular dynamics simulations that can further provide insight into the effect of CO2 dissolution on the liquid structure of the selected combinations of cations and anions.

through the rotation of all bonds to ensure that global minima were located. Similarly, for complexes of ions with CO2 full conformational screening was performed with all possible interaction sites examined and conformations within the fragment explored after the introduction of CO2. Geometry optimizations were performed at the MP2/aug-cc-pVDZ level of theory in the presence of the polarizable continuum model (CPCM)25−27 as implemented in the GAUSSIAN09 software package. For larger systems, such as the large ions +N(CH2CH3)4, +P(CH2CH3)4 and NTf2− and for calculations on ion pairs, optimizations and frequencies calculated at the MP2 level become computationally expensive. For these systems the M06-2X functional was used. All geometry optimizations were carried out in the presence of an ethanol solvent field, while analogous calculations for the complexes of CO2 with ammonia (NH3) and amines such as methylamine (NH2CH3), dimethylamine (NH(CH3)2) and trimethylamine (N(CH3)3), a water solvent field was applied as aqueous solutions of amines are generally used as CO2 absorbents. Ethanol possesses a similar dielectric constant to the majority of ionic liquids.28 Some cations and anions in this study show large conformational spaces, i.e., a large number of conformations within a few kJ mol−1, as in the case of the +N(CH2CH3)4 and + P(CH2CH3)4 cations. In complexes involving interacting molecules, such as ion pairs or ions physically bound to CO2, conformational screening becomes even more complex. Therefore, prescreening of the conformational space at a lower level of theory becomes necessary for these larger systems. A prescreening method using molecular mechanics was recently applied in a study predicting the cellulose solvating capabilities of ionic liquids.29 The current study provides a framework for the assessment of a more rigorous prescreening approach for systems consisting of multiple fragments where additional degrees of freedom must be considered. In this study the simulated annealing method in AMPAC 1030 utilizing semiempirical calculations was used to search the conformational space and generate a diverse conformer library. AMPAC 10 combines semiempirical calculations with simulated annealing for automated conformational screening. In general, simulated annealing attempts to locate an acceptable approximation to global minimum using statistical mechanics. This method was first described in 1983 by Kirpatrick et al. where simulation was coupled with Boltzmann’s law at decreasing temperatures.31 Therefore, the procedure of simulated annealing followed by semiempirical optimization produces an annealing-assisted lowest energy conformer upon which further optimization at higher levels of theory can be performed. Geometry optimizations on annealing-assisted conformers in this work were performed at the PM6-D3H4 or RM1-D3H4 levels as indicated.32 These methods represent versions of the original methods PM633 and RM1,34 modified to include additional terms for dispersion and hydrogen bonding. In order to assess the success of the simulated annealing method at locating conformers close to the global minimum as found through traditional conformational screening the moderately sized systems of CO2 complexed with the ions + N(CH2CH3), CHOO− and NTf2−, and the ion pairs [NH3(CH3)][CH3COO] and [N(CH3)4][CH3SO3] were examined. The lowest energy conformations given by annealing were reoptimized at either the MP2/aug-cc-pVDZ35 or M062X/aug-cc-pVDZ level and compared with the global minimum structures in order to determine the energy difference between

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THEORETICAL PROCEDURES Ab inito molecular orbital and density functional calculations in this work were carried out using GAUSSIAN09.24 For all species, frequency calculations confirmed that convergence to a local minimum had been achieved. For individual ions in the study, traditional full conformational screening was carried out 11749

DOI: 10.1021/acs.jpcb.5b05115 J. Phys. Chem. B 2015, 119, 11748−11759

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The Journal of Physical Chemistry B

Table 1. Absolute Differences, ΔE, in Total Energy of Fragments and Fragment···CO2 Complexes and Interaction Energies, ΔEINT, of Fragment−CO2 Complexes between Geometries Determined through an Annealing-Assisted Screening Method Coupled with Semi-Empirical Optimization and Global Minimum Geometries Determined through a Traditional Full Conformation Screening (All Energies Given in kJ mol−1) fragment

ΔE(fragment)a

N(CH2CH3)4 NTf2− CH3COO− [NH3(CH3)][CH3COO] [N(CH3)4][CH3SO3]

c

+

ΔE(fragment···CO2 complex)a

ΔEINTb

c

0 4.2c − 1.9 0.3c

0.4c 0.6c 0.3 1.5 0c

2.7 14.5c 0.9 1.5 0c

a Species optimized at the MP2/aug-cc-pVDZ level in ethanol using the polarizable continuum model (CPCM). bCalculated at the MP2/aug-ccpvTZ//MP2/aug-cc-pVDZ level in ethanol using the polarizable continuum model (CPCM). cM06-2X/aug-cc-pVDZ optimization (CPCM in EthOH).

smaller basis set, the dispersion component of interaction energy is usually underestimated by approximately 2 kJ mol−1, whereas the charge transfer component is overestimated by 1 kJ mol−1. A comparison between the two basis sets for some ions is included in electronic Supporting Information. For the complexes between CO2 and single ion pairs, the latter were taken as individual fragments in SAPT2+3 calculations. Free energies of solvation, △Gsolv, were calculated using the CPCM42 and SMD43 solvation models at the MP2/aug-ccpVTZ level of theory using the following expression:

the lowest annealing-assisted conformation and the conformation corresponding to the global minimum. The absolute energy differences between the conformations at the MP2/augcc-pVDZ or M06-2X/aug-cc-pVDZ level were determined as well as the difference produced in the interaction energies by using the lowest annealing-assisted conformations in its calculation. In practical terms, the annealing method is not necessarily expected to produce one lowest energy conformation, but a range of lower-energy conformations to be further screened. Therefore, the semiempirical energies of the lowest annealing-assisted conformation and the annealing-assisted conformation corresponding to the MP2/aug-cc-pVDZ or M06-2X/aug-cc-pVDZ global minimum structure were also compared in order to determine a cutoff range of annealingassisted conformers which should be included in reoptimization. The interaction energies (EINT) in Table 1 of complexes between CO2 and various common ionic liquid cations, anions and ion pairs were calculated at the MP2/aug-cc-pVTZ level of theory and defined as the energy difference between an interacting complex of CO2 with various individual ions or ion pairs (IPs), and the energies of the individual species within the complex. Single-point energy calculations of the latter were performed within the basis set of the whole complex according to the Boys and Bernadi approach36 to account for the basis set superposition errors. For complexes between the CO2 molecule and single ion pairs, the latter were taken as individual fragments to eliminate the calculation of the cation−anion interaction in interaction energy. These MP2 interactions energies were also corrected for solvent effects using the CPCM model with ethanol as solvent. To analyze the interplay of five fundamental forces (electrostatic, exchange, induction, dispersion and charge transfer) on interaction of CO2 with ions and ion pairs a state-of-the-art approach, symmetry adapted perturbation theory (SAPT),37 was adopted in this study. Specifically, the highest level of truncation available so far, SAPT2+3,38−40 in combination with the aug-cc-pVTZ basis set35 was used as implemented in the PSI4 software.41 Because of high computational expense of SAPT2+3 small- to medium-sized cations and anions were chosen in the study. Only a few larger systems were selected in this study such as those containing + N(CH2CH3)4, +P(CH2CH3), C1mim+, C2mim+, −NTf2, and the ion pairs. For these the use of aug-cc-pVTZ was not feasible and therefore, the aug-cc-pVDZ basis set was used instead. There is usually a good agreement within a couple of kJ mol−1 between SAPT2+3 interaction energies using both the aug-ccpVDZ and aug-cc-pVTZ basis sets. Notably, when using the

complex complex ΔGsolv = [Esolv − Egas ]

−

∑ fragments

fragment fragment [Esolv − Egas ]

(2)

where solv stands for either CPCM or SMD model. Theoretical detail on these models is given below in the text. Interaction energies accounting for solvent effects with either CPCM or SMD are referred further in the text as solvation interaction energies that were calculated by adding CPCM or SMD ΔGsolv to gas phase SAPT2+3 interaction energies. The geodesic scheme44 was adopted to study charge distribution and net charge transfer between the CO2 molecule and fragments. The particular scheme based on restrained electrostatic potential was established in our previous work to be reliable for analyzing charge transfer in ionic liquids cluster.45 These calculations were performed using a quantum chemical package, GAMESS-US,46,47 in combination with the TZVPP basis set.48

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RESULTS AND DISCUSSION Semiempirical Optimization. Table 1 shows the absolute energy differences, ΔE, of traditional fully screened global minimum structures with structures determined through conformational prescreening using a simulated annealing method coupled with semiempirical optimization as implemented in AMPAC 10. Table 1 also shows the difference produced in the interaction energies of the fragments with CO2 calculated using the lowest annealing-assisted conformations rather than the global minimum structures. Energy differences between the conformations at the MP2/aug-cc-pVDZ or M062X/aug-cc-pVDZ level were determined through reoptimization of the annealing-assisted conformers and these reoptimized geometries used in the calculation of the interaction energies at the MP2/aug-cc-pvTZ level. The first two columns in Table 1 show energy differences for individual fragments, i.e., individual ions and ion pairs, and for the corresponding complexes with CO2. For systems where this value is listed as 0 the lowest 11750

DOI: 10.1021/acs.jpcb.5b05115 J. Phys. Chem. B 2015, 119, 11748−11759

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The Journal of Physical Chemistry B

Table 2. Structures of Physically Bound Complexes and Interaction Energies of CO2 with Fragment Cations in This Studya

a

Atom colors: orange = carbon, blue = nitrogen, red = oxygen, and white = hydrogen). (a) Calculated with the aug-cc-pVDZ basis set.

significant in the cases of more complex systems. Therefore, as expected, the annealing method will be more effective if used not to produce only one lowest energy conformation, but a range of lower-energy conformations for further screening using either a DFT- or wave function-based method. For all test cases, at the semiempirical level there is less than a 10 kJmol−1 difference in the conformational energies between the annealing-assisted lowest energy conformation and the annealing-determined conformation corresponding to the global minimum structure; this being the conformation found during the annealing method screening which, when reoptimized at either the MP2/aug-cc-pVDZ or M06-2X/aug-ccpVDZ level, results in the global minimum structure. Therefore, in order to ensure the global minimum structure is selected through the prescreening annealing, all conformers within 10 kJ mol−1 should be further reoptimized with ab inito methods. SAPT2+3 Interaction Energies in Gas Phase. Tables 2−4 show the complexes of various ionic liquid cations, anions,

energy annealing-assisted conformation matched within computational tolerance the global minimum structure found from full conformational screening. Differences in conformation between the annealing-determined structures and the global minimum structures cause energy differences that are generally within chemical accuracy (1 kcal mol−1), however a larger difference of 14.5 kJ mol−1 is seen for the −NTf2···CO2 complex. The final column of Table 1 shows the effect of using the lowest annealing-assisted conformations to calculate interaction energies of the fragments with CO2. Differences in conformation between the annealing-determined structures and the global minimum structures cause absolute energy differences of 0.0 to 1.5 kJ mol−1 in the interaction energy with CO2 for the systems studied. While the differences for the five example systems studied are small and within chemical accuracy (1 kcal mol−1), the small magnitude of the interaction energies (see discussion below) means that errors could possibly become 11751

DOI: 10.1021/acs.jpcb.5b05115 J. Phys. Chem. B 2015, 119, 11748−11759

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The Journal of Physical Chemistry B

Table 3. Structures of Physically Bound Complexes and Interaction Energies of CO2 with Fragment Anions in This Studya

a

Atom colors: orange = carbon, blue = nitrogen, red = oxygen, and white = hydrogen. (a) Calculated with the aug-cc-pVDZ basis set.

configurational screening only searches for the minimum interaction energy between the two species. Analysis of the CO2 orientation reveals that the CO2 molecule tends to position itself to maximize the resulting interaction with the fragment. Therefore, when complexed with a cation, one of the slightly negatively charged oxygen atoms is oriented toward the positively charged cation center. In the case of the smaller ammonium cations such as ammonium, methylammonium and dimethylammonium, a lack of steric hindrance around the nitrogen center allows for formation of a N−H···O hydrogen bond of between 2.01 and 2.03 Å. Ordinarily for neutral complexes, this short hydrogen bonding distance would be indicative of a strong interaction. In the case of protic cations the interaction varies between −29 and −35 kJ mol−1. As was observed previously, interionic distances might not necessarily indicate the relative strength of interaction due to interplay of all five fundamental forces.50 In the case of the complexes with tetramethylammonium and tetramethylphosphonium CO2 is located much further away from the cation center at 3.6−3.7 Å; however, the interaction energies only decrease by 10 kJ mol−1 on average. Aprotic cations show insensitivity with respect to binding with CO2, with interaction energies falling between −15.3 and −20.5 kJ mol−1, thus indicating that there is a different interplay of fundamental forces in protic vs aprotic cations. For the imidazolium ring structures the CO2 molecule lies above the ring with one oxygen interacting with the π delocalized system. Surprisingly, this stabilization only produces interaction energies similar to those of the tetraethylammonium

ion pairs and neutral molecules with CO2 along with the noncovalent interaction energies calculated using the SAPT2+3/aug-cc-pVTZ level of theory in the gas phase using CPCM-optimized geometries. The SAPT2+3 interaction energies are also shown graphically in Figure 1, in which they are ranked in order of energy and grouped by charge. The calculated interaction energies of the fragments with CO2 are all negative, indicating that CO2 is thermodynamically stabilized by these fragments. The strength of the CO2 intermolecular complexes ranges from −3.1 kJmol−1 for methylated ammonia, N(CH3)3, to −43.9 kJ mol−1 for the formate anion. As a reference point the interaction energy of the addition of CO2 to H2O is −13.0 kJmol−1. In general, the ions and ion pairs show to varying degree, higher thermodynamic stability by forming a complex with CO2 than the complex of CO2 and water. This is expected from previous studies indicating that CO2 is generally more soluble in ILs than in molecular solvents.49 Trends within the data also show that in general more stability is imparted by the noncovalent binding of CO2 to anions than the similar binding to cations, although this is not always the case. Ion pairs show interaction energies similar to those of the cations. The neutral amines show stronger interactions, increasing with methylation. The structures of the fragment···CO2 complexes are shown in Tables 2-4. In all optimized geometries the OCO bond angle is close to 180°, indicating that CO2 forms a noncovalent interaction with the fragment. It has to be pointed out that only physical binding is examined in these examples, as the 11752

DOI: 10.1021/acs.jpcb.5b05115 J. Phys. Chem. B 2015, 119, 11748−11759

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The Journal of Physical Chemistry B

Table 4. Structures of Physically Bound Complexes and Interaction Energies of CO2 with Ion Pairs, Amines, and Water in This Studya

Atom colors: orange = carbon, blue = nitrogen, red = oxygen, and white = hydrogen. (a) ΔGsolv (SMD) was estimated at the HF level of theory. (b) Calculations performed with the aug-cc-pVDZ basis set. (c) Calculations performed at the MP2/aug-cc-pVTZ//MP2/aug-cc-pVDZ level using water as solvent.

a

and tetraethylphosphonium cations in the range of −15 kJ mol−1. The physical binding of CO2 to an anion occurs with the partial-positively charged carbon atom oriented close to the negative center on the anion. Apart from binding to halogen anions, in general more negative interaction energies are seen for CO2 adding to an anion rather than a cation. This is in agreement with previous studies showing that the nature of the anion plays a greater role in determining the dissolution of CO2

in ionic liquids, with the cation playing a secondary role.14,51 Interaction energies for CO2 with anions ranges between −23.9 kJmol−1 for PF6 to −43.9 kJmol−1 for HCOO−. In order of decreasing stability of the fragment···CO2 complex, the interaction energies are HCOO− > CH3SO3− > CF3SO3− ≈ CH3COO− ≈ NTf2 > −BF4 > CF3COO− ≈ Cl− > Br− > −PF6. Previously published experimental solubilities of CO2 in imidazolium-based ionic liquids were found to decrease in the series of −NTf2 > CF3SO3− > −PF6 ≈ −BF4,13,18 which forms a 11753

DOI: 10.1021/acs.jpcb.5b05115 J. Phys. Chem. B 2015, 119, 11748−11759

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The Journal of Physical Chemistry B

The physical binding of CO2 with various amines was also examined. In some of the amines examined it is known that a chemical reaction with CO2 occurs leading to the formation of carbamate. This chemical absorption will be the focus of a future study, however in cases that can lead to well-known aqueous amine reactions with CO2, the physical absorption of CO2 is an earlier step on this reaction pathway. For the purposes of this study only noncovalently bound complexes between CO2 and ionic liquid ions were considered. Two favorable interaction sites for CO2 with amines were also examined: (1) global minimum (GM) complexes, in which the CO2 molecule directly interacts with the nitrogen atom and (2) hydrogen-bonded (HB) complexes, in which CO2 forms a hydrogen bond with either N−H bond or hydrogens of the methyl group. Surprisingly, the interaction energies of the global minimum complexes are comparable with those of the charged species in this study falling between −17.2 and −25.3 kJ mol−1, while in the hydrogen bonding systems, as expected, less stability is imparted by the physical addition of CO2. In the latter interaction energies drop below −12.9 kJ mol−1. In general, slightly stronger interactions are seen between CO2 and protic ammonium cations than in the equivalent neutral species, with interaction energy becoming less negative with increasing methylation. Hydrogen bonding seems to have a significant impact on the strength of CO2 binding. Introduction of methyl groups into ammonium cations decreases hydrogen bonding, increasing steric hindrance and resulting in a weaker interaction with CO2. This trend is reversed for neutral amines. The strongest interaction is observed for the fully methylated amine, N(CH3)3, further indicating a difference in interplay of fundamental forces between primary, secondary and tertiary amines. The binding of some ion pairs with CO2 was also examined in this study. Of particular interest is the physical capture of CO2 by ion pairs representative of protic ionic liquids. Ion pairs (IPs) considered include primary, secondary, and tertiary ammonium cations and formate, acetate and sulfonate anions. It should be noted that when CO2 is complexed with the [+NH4][CHOO−] ion pair the lowest energy conformation showed proton transfer to the anion, however no transfer was seen in the global minimum structure of the ion pair itself. For the purposes of this study the lowest energy nontransferred ion pair was chosen to determine the physical binding interaction energy. The interaction energies for the acetate and formate ion pairs considered are similar, occurring between −27.8 and −31.2 kJ mol−1. From Figure 1, it can be seen that the interaction energies of ion pairs fall at around the same values as the corresponding protic cations. The ion pair example chosen to represent an aprotic IL, [N(CH3)4][CH3SO3], shows an interaction energy of −38.8 kJmol−1, which is the highest compared to those of the protic ion pairs. This is attributed to the strong binding of CO2 with the mesylate anion. It has to be pointed out that interaction energy of CO2 with an ion pair is not expected to be the sum of interaction energies of individual cation and anion. It is likely that this type of interaction energy would be dominated by the ion giving a stronger interaction. For example, the CO2 molecule interacts with the tetramethylammonium cation in the amount of −20 kJ mol−1, whereas the mesylate anion, CH3SO3−, results in a twice as much stronger interaction of −41 kJ mol−1. The [N(CH3)4][CH3SO3] ion pair combining these two ions has interaction energy of −38.8 kJ mol−1 that is similar to that of the CO2···anion complex. A similar trend is observed for the other four ion pairs studied,

Figure 1. Interaction energies (in kJ mol−1) of the fragment···CO2 complexes: anions are shown in blue, cations in red, ion pairs in green and amines and water in orange. Solid colors represent SAPT2+3 gas phase interaction energies, whereas patterned colorsSAPT2+3 interaction energies augmented with SMD solvation energies. Solid and dashed black lines show the range of numbers for gas-phase and solvent phase interaction energies, respectively.

moderate agreement with the presented results. SAPT2+3 interaction energies for BF4 and PF6 differ by CF3SO3 > BF4 > PF6, which becomes in good agreement with the experimentally observed one.

Figure 2. SAPT2+3 interaction energies (shown in green) together with their induction (shown in blue) and the dispersion (shown in red) components. Other components such as positive exchangerepulsion and negative electrostatics are included in Table 5. All energies are given in kJ mol−1. For the neutral amine complexes GM represents the global minimum, whereas HB stands for a hydrogenbonded complex. 11754

DOI: 10.1021/acs.jpcb.5b05115 J. Phys. Chem. B 2015, 119, 11748−11759

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The Journal of Physical Chemistry B

Table 5. Energetic Components of Interaction Energy (in kJ mol−1) in Fragment···CO2 Complexes Calculated Using the SAPT2+3 Approach and Geodesic Net Charge Transfer (in e) from the Fragment to CO2 fragment

EELST

EEXCH

EINDa

EDISP

ECT

NCT

NH4 + NH3(CH3) + NH2(CH3)2 + NH(CH3)3 + N(CH3)4 + N(CH2CH3)4 + P(CH3)4 + P(CH2CH3)4 C1mim+ C2mim+ − NTf2 HCOO− CH3COO− CF3COO− CH3SO3− CF3SO3− Cl− Br− − BF4 − PF6 [NH4][HCOO] [NH3(CH3)][HCOO] [NH3(CH3)][CH3COO] [NH2(CH3)2][HCOO] [N(CH3)4][CH3SO3] NH3 (GM)b NH2(CH3) (GM)b NH(CH3)2 (GM)b N(CH3)3 (GM)b NH3 (HB)c NH2(CH3) (HB)c NH(CH3)2 (HB)c N(CH3)3 (HB)c H2O

−21.2 −19.9 −18.9 −13.2 −14.1 −12.4 −14.8 −12.7 −11.2 −11.0 −35.7 −39.8 −29.9 −27.9 −39.0 −28.8 −26.4 −26.9 −24.9 −20.0 −29.1 −32.7 −32.5 −32.5 −54.4 −15.8 −20.2 −27.1 −40.0 −3.9 −4.2 −5.1 −2.0 −14.9

15.4 18.2 19.1 11.3 15.4 17.1 17.7 18.6 18.8 20.3 44.6 32.4 31.8 31.3 37.1 28.2 19.3 23.0 20.4 19.3 28.8 33.8 35.4 35.0 65.7 15.8 23.4 34.1 52.0 6.8 8.5 11.5 8.2 13.7

−21.9 −20.5 −18.6 −8.2 −8.7 −6.4 −8.7 −6.4 −6.0 −5.6 −11.0 −16.8 −12.4 −9.2 −14.2 −10.3 −10.6 −9.5 −10.0 −8.0 −7.2 −12.9 −7.6 −8.5 −14.0 −2.9 −4.3 −6.2 −9.8 −1.2 −1.1 −1.7 −0.9 −2.7

−7.5 −9.3 −10.8 −9.3 −12.6 −13.7 −14.7 −14.8 −16.9 −19.3 −28.3 −19.7 −21.1 −22.7 −24.9 −21.2 −10.9 −12.4 −14.5 −15.2 −20.2 −19.4 −24.9 −24.5 −36.2 −10.0 −16.1 −21.8 −27.4 −5.1 −8.0 −12.1 −8.4 −9.1

−0.9 −1.1 −1.2 −0.4 −0.6 −0.8 −0.6 −0.9 −0.4 −0.5 −3.0 −4.4 −3.0 −2.2 −2.9 −2.2 −3.3 −4.0 −1.6 −1.4 −2.0 −3.1 −2.3 −2.3 −4.4 −1.2 −1.9 −2.4 −4.6 −0.4 −0.4 −0.5 −0.4 −0.9

0.042 0.071 0.061 0.014 0.020 0.022 0.021 0.011 0.021 0.024 −0.071 −0.055 −0.055 −0.048 −0.069 −0.059 −0.055 −0.052 −0.054 −0.052 −0.013 −0.001 −0.041 −0.024 −0.054 −0.021 −0.033 −0.040 −0.087 0.027 0.030 0.005 −0.001 −0.006

+

a

The induction component includes charge transfer. bCorresponds to the global minimum (GM) structure. cCorresponds to the hydrogen-bonded (HB) structure.

delocalized systems imidazolium cations show the strongest dispersion contribution to the overall stabilization of CO2 complexes. Although the carboxylate and sulfonate anions of lower basicity (such as formate and mesylate) are dominated by the dispersion component, induction also plays an important role. It is not surprising that binding between CO2 and more basic anions with a rather diffuse negative charge such as NTf2, CF3COO, and CF3SO3 is mainly driven by dispersion forces. A similar trend is observed for the ion pairs (be it protic or aprotic) and global minimum structures of neutral amines, with the dispersion component reaching −25.3 kJ mol−1 for the complex with N(CH3)3 and −36.2 kJ mol−1 for [N(CH3)4][CH3SO3]. It can be concluded that dispersion component plays by far the more significant role in determining the strength of CO2 binding to aprotic ions with a rather diffuse charge. Less basic anions seem to benefit from additional induction, thus allowing them to compete with basic ones for stronger affinity to CO2. On the basis of these findings, protic cations coupled with formate, acetate and mesylate can be potential candidates for strong physical absorption of CO2. The question here is to what extent do solvation properties of ionic

with interaction energy being weaker or similar to that of the constituent anion, thus further confirming that interaction between the CO2 molecule and a single ion pair does not exceed interaction energy of CO2 with each individual ion. To this end, protic ionic liquids show rather high levels of physical capture (comparable to aprotic ILs) due to the presence of hydrogen bonding to the CO2 molecule.6 In addition, aminefunctionalized protic ionic liquids may contribute to significant chemical capture of CO2 and this process will be examined in a future work. Interaction Energy Decomposition. In order to further understand the interplay of fundamental forces on the strength of binding between the CO2 molecule and ionic liquid ions, the interaction energy was decomposed using SAPT2+3, with the main components, electrostatic, exchange-repulsion, induction and dispersion, being shown in Table 5 and Figure 2. Analysis of Figure 2 clearly demonstrates that the stabilization of protic cations is dominated by the induction component due to the presence of hydrogen bonding, whereas for quaternary ammonium and phosphonium cations dispersion plays a more important role. Because of the presence of the π 11755

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Figure 3. Comparison of Geodesic net charges in CO2 complexes with various fragments. (Atom colors: orange = carbon, blue = nitrogen, red = oxygen, and white = hydrogen.)

Sq = −V

liquids affect the CO2 binding in these intermolecular complexes. Charge Transfer. Table 5 shows net charge transfer (NCT) from the fragment to the CO2 molecule in the studied complexes. The Geodesic NCT numbers are similar for the different cations, anions and ion pairs when no hydrogen bonding is present, however greater charge transfer occurs between CO2 and the hydrogen bonded protic cations, especially +NH3CH3, and anions such as −NTf2 and mesylate. It is not surprising that in the case of cations, CO2 becomes slightly positively charged, whereas anions donate some of their electron density to CO2, making it slightly negatively charged. Figure 3 also shows the overall and individual charges in the [N(CH3)4][CH3SO3] and [NH3(CH3)][CH3COO] ion pairs when bound to CO2. The protic ionic liquid does not show significantly greater charge transfer between the ion pair and CO2 than the aprotic ionic liquid. When bound to an ion pair, the CO2 acquires an overall negative charge. Overall, the NCT numbers are not sufficiently large to indicate that charge transfer plays a big role in stabilizing the CO2 complexes in ionic liquids. Comparison of CPCM and SMD. Solvent effects were accounted using two solvation models, CPCM and SMD. CPCM, the conductor-like polarisable continuum model, belongs to a commonly used implicit solvent model that accounts for solvation effects through a polarizable dielectric medium.42 SMD is a universal solvation model that can be applied to both charged and uncharged solutes.43 Both models separate free energy of solvation into two main components: (1) bulk electrostatic contribution which represents the solution of the Poisson equation for electrostatics and (2) solute−solvent interactions (cavitation and dispersion) arising from the direct contact between the solute and solvent molecules. The biggest differences between the two models lie in the treatment of the apparent charge density of the solute and structural detail of the solute. In the CPCM model the charge density is divided into smaller areas (called tessarae), each of these being assigned a f inite point charge. These solvation charges are calculated through the solution of the following equation:

(1)

where S is individual components to electrostatic potential arising from solvation charges, q, and V is the total electrostatic potential. Solvation charges are initially generated by means of the fast multipole method.52−54 In addition, the finite dielectric constant associated with the medium is changed to infinity in the CPCM model to account for a conductor medium instead of a dielectric medium. The response between the induced polarization in the solvent is assumed to be linear with respect to the electrostatic field of the solute, thus generalizing the structure of the solute. This model works best for polar solvents. The generated ideal unscreened charge density is scaled to reflect the finite dielectric constant of a polar solvent. In the SMD model the charge density of the solute is described as a continuous electronic density calculated quantum mechanically.55 In this formulation the cavity-dispersion interactions are explicitly represented via atomic surface tensions. The latter depend on the solvent parameters such as their acidity and basicity.56 This allows us to account for structural detail of the actual solute molecule. The advantage of the SMD model also lies in parametrization for a wide range of solutes including ionic species, thus making it attractive for studying solvation energies in ionic liquids. Free energies of solvation calculated using both CPCM and SMD models are given in Tables 2−4. The direct comparison of solvation energies is given in Figure 4. Overall there is a good correspondence between the CPCM and SMD free energies of solvation for the cations. These energies are calculated to be quite positive, with protic cations having higher positive values. For both protic and aprotic cations the difference between the two models is within a couple of kJ mol−1. The biggest difference comes for the imidazolium cations that inevitably form additional dispersion interactions with CO2 due to the presence of the π delocalized system as was supported by the SAPT analysis (see above). Contrary to CPCM, the SMD model predicts even slightly negative solvation energies, indicating that SMD can indeed take the structural detail of the solute into account. It is worth noting that for the anions the CPCM consistently produces higher solvation energies by 5 kJ mol−1 on average. A bigger difference in free energies of solvation comes for neutral species. The CPCM model is 11756

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mol−1 (the estimates include error bars), which is in a slightly better agreement with the SMD solvation model. Relatively low numbers of interaction energies between CO2 and ionic liquid ions presented in this study point out that solvation of CO2 might not be able to disrupt the strongly bound ionic network (identified as heterogeneity) usually present in ionic liquids, thus supporting the findings of previous MD simulations that CO2 did not affect the liquid structure of ionic liquids.19 Recently, Firaha and Kirchner22 have shown through ab initio MD simulations of CO2 in neat ethylammonium nitrate that CO2 molecules indeed could not break the hydrogen bonded network and had to adopt the already existing voids in the structure, which further supports the finding of this study. In summary, no significant difference between particular cations and anions was found when the SMD/CPCM solvation model was applied, highlighting a rather narrow room for improvement when it comes to the physical absorption of CO2 by ionic liquids due to the weakly polar nature of the CO2 molecule itself. On the basis of the more reliable SMD solvation interaction energies imidazolium cations combined with sulfonate- and carboxylate-based anions as well as bulky anions such as NTf2 seem to be the best candidates for the CO2 capture. On the basis of the presented result it can be concluded that ions with a strong dispersion component (be it through bulky substitutes such as NTf2 or delocalized systems such as imidazolium) should be preferred for the physical absorption of CO2. Comments on Transferability of Presented Results to the Liquid Structure. Geometry optimizations of CO2 complexes studied in this work were optimized using the CPCM solvation model and therefore, it is expected that intermolecular distances observed will be in good correlation with those from MD simulations, thus warranting a direct transferability of interaction energies of studied systems to condensed systems. For example, CO2 is located at around 3.5 Å from the C1mim+ and C2mim+ cations measured as the distance between the C2 atom in the imidazolium ring and the carbon atom in the CO2 molecule. In contrast, for the PF6 anion the CO2 molecule sits further away from the phosphorus center at 3.8 Å (measured from the carbon center on CO2). In the MD simulations reported by Brennecke and Maginn14 similar distances were observed between CO2 and ions in the [C4mim][PF6] and [C4mmim][PF6] ionic liquids. This is also evidenced by the calculated solvation interaction energies of individual anions that were also in good agreement with measured enthalpies of CO2 absorption in aprotic ionic liquids.49,57

Figure 4. Comparison of CPCM and SMD total interaction energies.

incapable in distinguishing between various degrees of methylation in the amines, whereas SMD produces a consistent increase in solvation energy with increasing methylation for the GM structures. The latter observation is more consistent with a polar solvent not favoring solvation of neutral methyl groups. The analysis of Tables 2−4 further reveals that the SMD model favors solvation of hydrogen-bonded complexes over global minimum ones, with solvation energies being slightly negative. Figure 4 shows the relationship between solvation interaction energies using both models. As one can see, based on the described trends above it is not surprising that CPCM tends to produce smaller solvation interaction energies on the absolute scale compared to those from SMD. Overall the solvation effects reduce interaction between CO2 and ions by 13 kJ mol−1 in the SMD model and 16 kJ mol−1 in the CPCM model. It becomes obvious that the effect of including a solvent field decreases the calculated stabilization imparted by the formation of the fragment···CO2 complex, as ions are greatly stabilized in the presence of a strong solvent field. In the complexes between ions/ion pairs and CO2 some of the solvation sites become occupied by CO2. Since the CO2 molecule is only weakly polar, it cannot impart similar stabilization as ionic liquids ions themselves, thus resulting in positive free energies of solvation and moderate solvation interaction energies. The noticeable change in inclusion of free energies of solvation in gas phase interaction energies lies in the fact that the gap between highest and lowest interaction energies shrinks to just 10 kJ mol−1 for all three of the studied groups: cations, anions and neutral amines. Solvation interaction energies of cations fall between −6.1 and −17.2 kJ mol−1 in the SMD model and −0.7 and −11.8 kJ mol−1 in the CPCM model. For the anions these ranges change slightly to −11.8 to −21.8 kJ mol−1 for SMD and −8.2 to −18.5 kJ mol−1 for CPCM. Protic cations have turned out to produce solvation interaction energies on the low side due to the fact that the favorable for solvation hydrogen bonding is now “blocked” with the CO2 molecule. All studied carboxylate- and sulfonate-based anions as well as bulky NTf2 have solvation interaction energies on the high side around −20 kJ mol−1. It seems that the strong dispersion component in the fragment···CO2 complexes is by far the most important in ensuring that solvation interaction energies remain favorable for the physical absorption of CO2. Previous estimates of the enthalpy of physical absorption of CO2 by imidazolium-based ionic liquids with the PF6 and NTf2 anions49,57 have been measured to be around −10 to −20 kJ

■

CONCLUSIONS SAPT2+3 gas phase interaction energies of the noncovalently bound complexes between CO2 and ions/ion pairs fall into a wide range between −12.4 to −38.8 kJ mol−1, with anions and protic cations showing the strongest interactions. Because of its weakly polar nature, CO2 was shown not to disrupt the ionic network, thus resulting in positive free energies of solvation, with a few exceptions for imidazolium cations and amines. For a diverse series of anions of varying basicity, cations including protic ammoniums and a few ion pairs, the solvation interaction energies were found to fall into a rather narrow range of about 10 kJ mol−1 within each group, indicating that scope for finetuning the physical absorption of CO2 is rather limited. Despite protic cations showing favorable gas phase interaction energies, hydrogen bonding of CO2 to these cations results in highly 11757

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(10) Gurkan, B. E.; Gohndrone, T. R.; McCready, M. J.; Brennecke, J. F. Reaction kinetics of CO2 absorption in to phosphonium based anion-functionalized ionic liquids. Phys. Chem. Chem. Phys. 2013, 15, 7796−7811. (11) Gutowski, K. E.; Maginn, E. J. Amine-functionalized task-specific ionic liquids: a mechanistic explanation for the dramatic increase in viscosity upon complexation with CO2 from molecular simulation. J. Am. Chem. Soc. 2008, 130, 14690−14704. (12) Pinto, A. M.; Rodríguez, H.; Colón, Y. J.; Arce, A.; Arce, A.; Soto, A. Absorption of carbon dioxide in two binary mixtures of ionic liquids. Ind. Eng. Chem. Res. 2013, 52, 5975−5984. (13) Aki, S. N. V. K.; Mellein, B. R.; Saurer, E. M.; Brennecke, J. F. High-pressure phase behavior of carbon dioxide with imidazoliumbased ionic liquids. J. Phys. Chem. B 2004, 108, 20355−20365. (14) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why is CO2 so soluble in imidazoliumbased ionic liquids? J. Am. Chem. Soc. 2004, 126, 5300−5308. (15) Ramdin, M.; de Loos, T. W.; Vlugt, T. J. H. State-of-the-art of CO2 capture with ionic liquids. Ind. Eng. Chem. Res. 2012, 51, 8149− 8177. (16) Huang, X.; Margulis, C. J.; Li, Y.; Berne, B. J. Why is the partial molar volume of CO2 so small when dissolved in a room temperature ionic liquid? Structure and dynamics of CO2 dissolved in [Bmim+] [PF6-]. J. Am. Chem. Soc. 2005, 127, 17842−17851. (17) Zhang, X.; Liu, Z.; Wang, W. Screening of ionic liquids to capture CO2 by COSMO-RS and experiments. AIChE J. 2008, 54, 2717−2728. (18) Palomar, J.; Gonzalez-Miquel, M.; Polo, A.; Rodriguez, F. Understanding the physical absorption of CO2 in ionic liquids using the COSMO-RS method. Ind. Eng. Chem. Res. 2011, 50, 3452−3463. (19) Zhang, X.; Huo, F.; Liu, Z.; Wang, W.; Shi, W.; Maginn, E. J. Absorption of CO2 in the ionic liquid 1-n-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([hmim][FEP]): a molecular view by computer simulations. J. Phys. Chem. B 2009, 113, 7591−7598. (20) Wu, H.; Maginn, E. J. Water solubility and dynamics of CO2 capture ionic liquids having aprotic heterocyclic anions. Fluid Phase Equilib. 2014, 368, 72−79. (21) Perez-Blanco, M. E.; Maginn, E. J. Molecular dynamics simulations of CO2 at an ionic liquid interface: adsorption, ordering, and interfacial crossing. J. Phys. Chem. B 2010, 114, 11827−11837. (22) Firaha, D. S.; Kirchner, B. CO2 absorption in the protic ionic liquid ethylammonium nitrate. J. Chem. Eng. Data 2014, 59, 3098− 3104. (23) Zhang, Y.; Yu, P.; Luo, Y. Absorption of CO2 by amino acidfunctionalized and traditional dicationic ionic liquids: Properties, Henry’s law constants and mechanisms. Chem. Eng. J. 2013, 214, 355− 363. (24) Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Scalmani, G., Barone, V., Mennucci, B., Petersson, G. A.et al. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (25) Cancès, E.; Mennucci, B.; Tomasi, J. A new integral equation formalism for the polarizable continuum model: Theoretical background and applications to isotropic and anisotropic dielectrics. J. Chem. Phys. 1997, 107, 3032−3041. (26) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Ab initio study of ionic solutions by a polarizable continuum dielectric model. Chem. Phys. Lett. 1998, 286, 253−260. (27) Mennucci, B.; Tomasi, J. Continuum solvation models: A new approach to the problem of solute’s charge distribution and cavity boundaries. J. Chem. Phys. 1997, 106, 5151−5158. (28) Izgorodina, E. I.; Forsyth, M.; MacFarlane, D. R. On the components of the dielectric constants of ionic liquids: ionic polarization? Phys. Chem. Chem. Phys. 2009, 11, 2452−2458. (29) Parviainen, A.; King, A. W. T.; Mutikainen, I.; Hummel, M.; Selg, C.; Hauru, L. K. J.; Sixta, H.; Kilpeläinen, I. Predicting cellulose solvating capabilities of acid−base conjugate ionic liquids. ChemSusChem 2013, 6, 2161−2169. (30) AMPAC 10; Semichem, Inc.: Shawnee, KS, 1992−2013.

positive solvation energies, thus showing only moderate overall interaction energies comparable to that of water. The SMD model was found to quantify solvent effects better than CPCM by capturing the structural detail of each ion, such as the delocalization of the imidazolium ring and the methylation effect in amines. Dispersion forces were established to play a key role in providing ionic liquids with improved physical absorption of CO2. Bulky functional groups (such as NTf2), carboxylic and sulfonic groups as well as groups with the π delocalization are the best candidates for physical absorption.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b05115. Table S1, containing differences between interaction energies two basis sets, aug-cc-pVDZ, and aug-cc-pVTZ, at the SAPT2+3 level of theory (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(E.I.I.) E-mail: [email protected]. Telephone: +61 3 9905 8639. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank NeCTAR, the Monash eResearch Centre, and the National Computational Infrastructure for generous allocations of computer time. E.I.I. would like to thank the Australian Research Council for a DP grant (DP1095058) and a Future Fellowship (FT110100612). D.R.M. is also grateful to the Australian Research Council for support under the Australian Laureate Fellowship program.

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REFERENCES

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