Competitive pi interactions and hydrogen bonding within imidazolium ionic liquids.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 3238

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Competitive pi interactions and hydrogen bonding within imidazolium ionic liquids† Richard P. Matthews,* Tom Welton and Patricia A. Hunt* In this paper we have explored the structural and energetic landscape of potential p+–p+ stacked motifs, hydrogen-bonding arrangements and anion–p+ interactions for gas-phase ion pair (IP) conformers and IP-dimers of 1,3-dimethylimidazolium chloride, [C1C1im]Cl. We classify cation–cation ring stacking as an electron deficient p+–p+ interaction, and a competitive anion on-top IP motif as an anion–donor p+–acceptor interaction. 21 stable IP-dimers have been obtained within an energy range of 0–126 kJ mol 1. The structures have been found to exhibit a complex interplay of structural features. We have found that

Received 5th November 2013, Accepted 23rd December 2013

low energy IP-dimers are not necessarily formed from the lowest energy IP conformers. The sampled

DOI: 10.1039/c3cp54672a

alone, moreover the IP-dimers are recovering additional key features of the local liquid structure. Including

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dispersion is shown to impact both the relative energy ordering and the geometry of the IPs and IP-dimers, however the impact is found to be subtle and dependent on the underlying functional.

range of IP-dimers exhibits new structural forms that cannot be recovered by examining the ion-pairs

1. Introduction Ionic liquids (ILs) are a class of solvents composed entirely of ions. These ions are generally bulky, asymmetric heteroaromatics (e.g. 1,3-dialkylimidazolium or 1-alkylpyridinium) or ammonium or phosphonium cations combined with a wide range of inorganic anions (e.g. Cl , Br , [NO3] , [BF4] , triflouromethanesulfonate ([CF3SO3] ) and bis(trifluoromethylsulfonyl)imide ([NTf2] )). The multitude of unique cation–anion combinations has lead to ILs being described as designer solvents.1 Coupled with favourable physicochemical properties such as negligible vapour pressure, thermal stability, wide liquid range, hydrophobicity, solubility and acidity, ILs have attracted widespread academic and industrial interest.1–4 This has lead to varied and far reaching applications; including synthesis and catalysis,4,5 extraction and separation processes,6,7 CO2 capture/absorption materials,8 the pre-treatment and dissolution of lignocellulosic biomass9–12 and solvation of simple carbohydrates.13 Recent developments have Department of Chemistry, Imperial College London, London, SW7 2AZ, UK. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Details of results for selected MP2 calculations with inclusion of the counterpoise correction, details for constructing potential IP-dimer structures, figure of additional IP-dimer motifs, figure and discussion on the use of 1 and 2 in the naming of IP-dimer structures, figure indicating energy bands for [C1C1im]Cl IP-dimer structures, figure of centre-of-ring (COR)-COR RDF for [C2C1im]Cl, figure showing p+–p+ stacked structures from MD trajectories, figure of potential energy surface joining M_FS_SF_R and D_F1T_TF_A structures. Binding energies, ZPE and BSSE corrections, DEDisp, DG, DH and TDS for [C1C1im]Cl IPs and IP-dimer conformers at several levels of theory and estimates of the dispersion energies for IPs and IP-dimers. See DOI: 10.1039/c3cp54672a

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prompted the application of ILs within confined environments, such as narrow pores and thin films. Within these environments ILs function as solvents for fuel storage and energy devices14–16 and as lubricants.17,18 ILs can further facilitate the dispersion and solvation of single wall carbon nanotubes in conductive gels.19 This last application is based on the aromatic character of the IL imidazolium cation, which facilitates p-type interactions between the surface of the aromatic carbon nanotube and imidazolium cations. The local nano-structural organisation and physicochemical properties of ILs arise as a direct consequence of the characteristics of the constituent ions and the intermolecular interactions present between the ions in individual systems. As with conventional molten salts, such as NaCl and LiCl, ILs are dominated by Coulombic interactions. Combining asymmetric ions weakens these forces. Other interactions include dipole–dipole, dipole–induced dipole, dispersion and hydrogen bonding (H-bonding). The combination of the Coulombic and weak intermolecular interactions gives rise to the unique physical and chemical environment present in each IL. Imidazolium ILs have a heteroaromatic ring and provide a sub-class of ILs in which p–p interactions are possible, moreover these ILs undertake H-bonding which in-part determines structuring in the IL. In this article we investigate dispersion and p–p interactions and how these compete with and compliment H-bonding in imidazolium based ILs. H-bonding within ILs is a key interaction and an underlying driver for local structuring in both solid and liquid phases. The study of H-bonding in ILs has been multifaceted with a range of experimental and computational methods employed.20–26

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The nature and effect of H-bonds on the properties of ILs is a key area of debate, and no universally accepted definition of a H-bond within ILs has been established. Traditional H-bonding involves complex attractive and repulsive intermolecular interactions represented by X–H Y, where a H-atom intercedes between two electronegative species X and Y, such as O or N. Standard H-bonds are primarily electrostatic interactions with covalent and dispersion contributions.27 The concept of C–H Y H-bonds can be extended to ionic liquids, where the H-bond acceptor and H-bond donor both belong to charged species, CH+ Y . Aprotic imidazolium based ILs typically exhibit weak H-bonds. In general, weak hydrogen bonds have energies less than 17 kJ mol 1.27 However, an individual IL ion will typically undertake multiple H-bonding interactions within the liquid environment, increasing the overall energy contribution from H-bonds. In concert with H-bonding, p-type interactions are observed in protein folding28 and ion selectivity,29,30 and have been found to play essential roles in molecular recognition,31,32 self-organised electronic materials33 and in organic nanodevices.34,35 p-type interactions are also observed within some ILs. p–p stacking of aromatic rings occurs in the crystal structures of [C2C1im][NO3],36 [C2C1im]2[SO4]–H2O,36 [C1C1im][OTf]37 and [C1C1im][NTf2].38 However, no stacking has been observed in the crystal structures of the analogous [C2C1im][OTf]36 and [C2C1im][NTf2]36 or in the [C1C1im][NTf2] liquid phase.38 For cations with alkyl chains of an intermediate length alkyl–alkyl (dispersion) interactions play a more dominant role.39 For cations with long alkyl chains (now liquid crystals), inter-digitation may facilitate the alignment of the cation headgroups such that p–p stacking again becomes relevant. Such a feature has been suggested in a study of the phase behaviour of [CnC1im]Br n = 12,14 and 16, with p-xylene and water.40 p–p interactions facilitate the formation of IL–benzene liquid clathrates in [C1C1im][PF6] and [C2C1im][NTf2].41 IL mixtures with benzene have also been shown to play a key role in structuring at graphene– and carbon nanotube–IL interfaces.42,43 p–p stacked structures have been proposed based on NMR data for [C2C1im]Cl in dichloromethane.44 The local structure of cation–cation interactions within neat [C4C1im][BF4] and the methylated [C4C1C1im][BF4] ILs have been studied using NOE.45 Imidazolium ILs have been analysed for angular correlations of the C2–H vectors identifying parallel and antiparallel correlations.46 Ab initio MD simulations have also revealed the formation of p–p interactions in [C2C1im]X, X = Cl and SCN .47 Thus, p-type interactions have been recognised as a key component in the local structuring of some imidazolium based ILs. In order to better understand p–p cation stacking and anion–p interactions within ILs, it is useful to briefly review the characteristics of neutral p–p stacking and of (singly) charged ion–p systems. The prototypical p–p system is the benzene dimer. For this system three preferential structural p–p arrangements exist, Fig. 1. These conformers are ‘face-toface’ denoted stacked, ‘edge-face’ denoted T-shape, and ‘offset’ denoted parallel displaced. The stability of these structures


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Fig. 1 Geometries, quadrupole moments and ESPs (blue is positive and red is negative) of typical p–p aromatic interactions.

can be rationalised by the attractive/stabilising arrangements of the quadrupole moments associated with the aromatic ring, Fig. 1.48 In these arrangements there is a partial negative p electron density above and below the plane of the ring and a partial positive charge on the periphery of the rings. This charge distribution leads to favourable electrostatic interactions in the T-shape and parallel displaced conformers, with the stacked or traditional p–p structure the least favoured.48 However, if the ring polarity of one of the rings is reversed, e.g. by fluorinating the benzene ring, the stacked p–p structure is favoured. In addition to electrostatic interactions, dispersion interactions have been shown to have an effect in determining preferential p–p arrangements.49,50 Several other types of interactions involving aromatic p-systems exist. These include cation–p, anion–p and XH–p (where X = C, N, O), Fig. 2. Cation–p and anion–p interactions have been extensively studied.50–53 Both systems can incorporate atomic (e.g. Na+/Cl –p) and molecular (e.g. [NH4]+/[ClO4] –p) cation/ anion–p interactions. Computational studies have revealed that a cation preferentially sits above the p-ring whereas an anion sits in the plane of the ring coordinating via H-bonds. The preference of the anion position can be altered to above the ring if the p-system is electron deficient (e.g. C6F6).

Fig. 2

Possible cation–anion interactions with aromatic p-rings.

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Cation–p and anion–p interactions are comparable in energetic terms; both are dominated by electrostatic and induction contributions.50 The induction term recovers the interaction of the permanent multipole moments of one monomer with the induced multipole moments on another monomer. The anion–p interaction includes a small dispersion component which increases with increasing anion size, whereas the cation–p interaction has a minimal dispersion component.49,50 An extension of the p–p and cation–p interactions is the p+–p interaction. In p+–p systems the cation is a charged p-ring (p+), such as an imidazolium or pyridinium cation. This type of interaction has been exploited in the design and assembly of host–guest complexes.54 p+–p interactions have both the characteristics of conventional p–p and of cation–p systems, but cannot be represented by a simple sum of p–p and cation–p interactions.49 Structurally, a T-shaped arrangement with one X–H of the p+ ring directed towards the centre of the neutral p-ring is more stable than a displaced-stacked structure. However, this changes in the presence of solvent molecules and/or counter-ions. Displaced structures have large dispersion and electrostatic energy contributions together with a moderate induction energy contribution, whereas T-shaped structures have large electrostatic energy contributions together with substantial dispersion and induction energy contributions.49 Thus, in the following, the terminology of ‘‘p–p stacking’’ is clarified to include a distinction between neutral and charged interactions. We will refer to imidazolium rings as p+-rings not neutral p-rings, and interactions will be referred to as p+–p+ interactions. Moreover, the traditional conceptualisation of p–p stacking is not the most stable arrangement for aromatic rings; the T-shape and displaced stacked structures are more stable than the ‘‘sandwich’’ stacked structures. Only very recently, in the context of host–guest chemistry and fundamental ab initio studies of p–p interactions, have p+–p and p+–p+ interactions in general been recognised as a distinctive contributing factor in structuring.49,55–57 Here we consider imidazolium based ILs as having the potential to undertake p+–p+ interactions leading to structuring within an IL. Possible p+–p+ motifs include: (a) anti-parallel stacked, (b) parallel stacked, (c) rotated stacked and (d) T-shaped, Fig. 3.45 Moreover, a parallel displaced (Fig. 3e) motif is also possible. Like p–p and p+–p, p+–p+ interactions involve significant dispersion and electron correlation.58 A key difference with respect to p–p and p+–p interactions is the large repulsive Coulombic forces exerted by the positively charged p+-rings. Screening via the inclusion of counterions or a suitable solvent environment helps reduce the large repulsive component and has been found necessary for the formation stacked structures.58 Thus, the T-shape and displaced stacked are lower in energy than the stacked structure (like the more conventional p–p interactions). Moreover, (in the presence of counterions) the T-shape and displaced stacked p+–p+ and p+–p structures, have increased binding energies in comparison to equivalent p–p interactions in the order; p+–p+ > p+–p > p–p.49 The importance of dispersion and exchange contributions in p–p, p+–p and p+–p+ interactions necessitates the use of


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Fig. 3 Schematic of [C1C1im]+ ring IP-dimer cation structural arrangements. These include (a) anti-parallel, (b) parallel, (c) rotated stacked arrangements, (d) T-shaped and (e) parallel displaced.

computational methods that can accurately recover these contributions. Studies of dispersion interactions have traditionally been carried out using MP2 and CCSD(T) methods. However, the computational expense of these methods is considerable. The dispersion interaction is prominent over medium(E2–5 Å) and long-ranges (>5 Å) and attempts to correct this have been made via the inclusion of a large set of parameters, as in the M0X functionals59 or the inclusion of long-range corrections as used in oB97X.60 More recently DFT methods with an external empirical dispersion correction such as Grimme’s-D2 and -D3 corrections61,62 have provided an alternative means of studying p–p interactions at much reduced computational cost.63 DFT-D methods have been shown to provide energetic accuracies comparable to the CCSD(T)/CBS and SAPT levels for intermolecular interactions.64,65 Moreover, overbinding observed in p–p interactions at the MP2 and DFT-D2 levels is corrected in DFT-D3 by an improved description of medium range (2–5 Å) interactions.62,65 Dispersion corrected methods have been applied to IL systems. Grimme’s-D2 correction and the DCACP correction have been shown to produce results within the error of the MP2 method.66 Moreover, the D3 dispersion correction combined with standard hybrid and GGA functionals has been shown to reproduce potential energy curves for the dissociation of cation–anion ion pairs to within the errors of the CCSD(T)/ CBS method.67 Functionals incorporating a large proportion of HF exchange have been shown to perform well suggesting medium range interactions are important for ILs.68 B3LYP geometries followed by energy evaluations using GGA functionals with the inclusion of a dispersion correction or metaGGA functionals have been applied to IL ion pairs.69 These studies indicate that functionals with a large HF exchange contribution and including dispersion corrections are suitable for calculating IL ion-pairs. Thus, we have selected a range of density functionals, which include HF exchange and dispersion corrections to study the role of dispersion in p+–p+ and anion–p+ interactions.


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Generally ab initio quantum chemistry studies are carried out on cation–anion ion pairs (IP). Ion pair structures provide a qualitative understanding of electronic and structural characteristics, however it has become increasingly evident that more than one IP is required to better model the liquid state. Ideally, to fully represent a liquid state, a large number of cations and anions should be studied, however this is currently not possible using high level quantum chemical methods. Typically quantum chemical reports have included a minimal number of selected small clusters.70–73 One recent study has sampled an extended range of potential structures.74 We have found that combining the lowest energy IP structures into larger clusters does not characterise all of the lowest energy ion-pair dimers, and thus it is important to explore a larger configuration space of cluster geometries. As cluster size increases it is not possible to establish that the lowest energy conformer has conclusively been obtained. Moreover, an increasingly large number of low energy conformers occur. A good sampling of these low energy structures is important because they will all be present in the liquid environment. A less complete sampling of higher energy structures is still desirable to determine the extent of structural variation and to identify the boundaries of the conformer space wherein clusters are still stable. Thus, we have systematically generated IP-dimer structures of [C1C1im]Cl to elucidate a full range of cluster forms. In this paper, we explore the structural and energetic landscape of potential p+–p+ stacked motifs and the competition with H-bonding for gas-phase [C1C1im]Cl IP-dimers using ab initio gas phase methodologies. The role of dispersion forces on the formation of these structures is explored with Grimme’s empirical dispersion corrections and a range of DFT functionals is examined and compared with MP2. Furthermore, we explore the impact of zero point energy (ZPE) and basis set superposition error (BSSE) corrections.

2. Computational methods DFT calculations have been carried out with the Gaussian 09 suite of programs.75 A range of functionals based on B3LYP76,77 and oB97X78 together with MP2 and single point CCSD(T), have been used to explore the role of dispersion. The oB97X functional employs 100% Hartree–Fock (HF) exchange for longrange electron–electron interactions and the B97 exchange functional for short-range interactions. Partitioning of the inter-electronic distance r is controlled by the parameter o. Grimme’s-D261 and -D362 dispersion corrections which incorporate C 6, and C 6 together with C 8 dispersion terms respectively have been added to the B3LYP functional (here on referred to as B3LYP-D2 and B3LYP-D3). The oB97X-D functional implements the Grimme-D2 empirical correction formulism modified by a different damping function because of the long-range dispersion already recovered.60 The damping function used in oB97X-D was subsequently employed in Grimme’s-D3 correction.62


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All IP and IP-dimer calculations at the DFT and DFT-D levels have been carried out with the aug-cc-pVTZ basis set. MP2 optimisation and single point CCSD(T) calculations of [C1C1im]Cl ion-pairs have also been carried out using Dunning’s correlation consistent aug-cc-pVTZ basis set. Due to computational constraints, aug-cc-pVDZ basis sets have been employed for the MP2 level calculations of [C1C1im]Cl IP-dimers. Every IP-conformer was matched with all other IP-conformers generating E150 potential [C1C1im]Cl IP-dimers. Those with clear steric clashes were eliminated and the remaining 80 IP-conformers have been initially optimised at the B3LYP-D2/ aug-cc-pVDZ level resulting in 21 unique stable IP-dimers. Each of these was then characterised employing the range of methods and basis sets outlined above. All structures have been fully optimised under no symmetry constraints and have been confirmed as minima by vibrational analysis. Each structure has been individually optimised for each method. This process was followed because (a) different optimised geometries may occur with the inclusion of dispersioncorrections and (b) very small energy differences are to be evaluated. This eliminates the risk of creating artificial differences in energy because the underlying fixed structure is closer or further away from the real (optimised) structure of the method employed. Optimisation convergence criteria have been tightened from the Gaussian 09 defaults to 10 9 on the density matrix and 10 7 on the energy matrix. The numerical integration grid was improved from the default to a pruned (optimised) grid of 99 radial shells and 590 angular points per shell. These enhanced criteria have been maintained for the vibrational and counterpoise calculations. Vibrational frequencies and zero-point vibrational energy corrections (ZPE) have been obtained within the harmonic approximation for each structure. Basis set superposition error (BSSE) has been determined using the counterpoise method.79 Previously, MP2 level intermolecular geometries including p–p interactions have been shown to be indifferent to basis set size when the counterpoise correction has been included at each step of the optimisation.80,81 To test the validity of our MP2 geometries, optimisations have been carried out on two representative IP-dimer structures at the MP2 level with and without the counterpoise correction included at every step of the optimisation. The resulting geometries (ESI,† Fig. S1) for both procedures showed minimal differences. Thus, (expensive) counterpoise corrected optimisations have not been carried out for the remaining IP-dimer conformers; counterpoise corrections have been determined subsequent to optimisation. In order to explore the Cl position relative to a central [C2C1im]+ ring we have generated spatial distribution functions (SDFs) using the TRAVIS software package.82 These have been generated from the last 5 ns (total simulation length was 20 ns) of classical MD simulations previously reported by us for the [C2C1im]Cl IL.22 Simulations were carried out with 128 IPs at 450 K employing the canonical (NVT) ensemble within the DL_POLY simulation package.83 Equations of motion were integrated using the leapfrog Verlet algorithm with a 1 fs time step.84 A Nose–Hoover thermostat with a temperature relaxation


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time of 0.2 ps was used.85 The modified SHAKE86,87 algorithm was used to constrain bonds terminating in H atoms.

Paper Table 1 [C1C1im]Cl front-conformer binding energies (D EB) in kJ mol 1, ZPE and BSSE corrected, at various levels of theory using the aug-cc-pVTZ basis set and an estimate of the dispersion contribution, DEDisp

DEB (kJ mol 1)

3. Results and discussion


3.1

[C1C1im]Cl ion-pairs

Stable IP-conformers (1 cation and 1 anion) have been determined with the aim of generating suitable structures to combine to form ion-pair dimers (IP-dimers: 2 cations and 2 anions). For [C1C1im]Cl IPs five anion positions have been identified as stable minima; front, top/bottom, side, back and meth, Fig. 4. The anion in the front, side and back positions sits approximately in the plane of the [C1C1im]+ ring and includes primary (11) [C–H]+ X H-bonds between the halide and the ring protons. Secondary (21) H-bonds between the halide and the methyl protons are also present in the front and side structures, Fig. 4. When the halide is in the top/bottom position it resides above/below the plane of the [C1C1im]+ ring. When the imidazolium is [CnC1im]+, n denotes the number of carbon atoms in an alkyl chain, as in the case of 1-butyl-3-methylimidazolium [C4C1im]+, the top position is on the same side of the ring as the alkyl group. However, for [C1C1im]+ the top and bottom structures are equivalent. Binding energies (DEB) using different methods, ZPE and BSSE corrected, and the relative dispersion contributions (DEDisp = DEmethod DEB3LYP) for the [C1C1im]Cl front conformer are presented in Table 1. Dispersion energy contributions are E7–15 kJ mol 1 for the other IP-conformers; top, side and back (further details are provided in the ESI,† Table S1). Conformer energies (DE), ZPE and BSSE corrected, relative to the front-conformer calculated at several levels of theory are presented in Table 2. DZPE is generally below 2 kJ mol 1 for the full range of structures examined with no effect on the relative order of the binding energies (ESI,† Table S2). Nevertheless, the absolute ZPE correction is large; ZPE ranges from E364–373 kJ mol 1 for the different levels of theory (ESI,† Table S3). There is a very small variation in DBSSE (0.1 kJ mol 1) for different DFT functionals (ESI,† Table S4). Moreover, the absolute value of BSSE for the front IP-conformers is also small, in the order of 0.5 kJ mol 1 (ESI,† Table S3) for all of the DFT and DFT-D methods employed.

Fig. 4 Possible anion positions around the [C1C1im]+ (a) in-plane with primary (11) and secondary (21) H-bonding and (b) the top/bottom and meth interactions.


B3LYP oB97X B3LYP-D2 oB97X-D B3LYP-D3 MP2

DEDisp (kJ mol 1)

386.84 396.47 394.33 393.07 395.01 392.61

0.00 9.63 7.49 6.23 8.17 5.77

Table 2 [C1C1im]Cl IP conformer energies (DE) in kJ mol 1, ZPE and BSSE corrected, reported relative to the front structure. CCSD(T)* = CCSD(T)/ aug-cc-pVDZ//B3LYP-D3/aug-cc-pVTZ

B3LYP oB97X B3LYP-D2 oB97X-D B3LYP-D3 MP2 CCSD(T)* Front 0.00 Top 4.40 Side 34.01 Back 61.30 Meth —

0.00 1.40 35.56 61.92 —

0.00 1.63 33.98 62.21 94.16

0.00 3.04 35.91 63.48 —

0.00 1.75 34.34 61.91 94.15

0.00 2.71 30.61 53.23 —

0.00 4.63 34.55 61.36 —

BSSE and DBSSE are significantly larger at the MP2/aug-ccpVTZ level (E16 kJ mol 1 for the front structure). Nevertheless, DBSSE corrections have no overall effect on the relative energy ordering for all the methods employed. Thus, a general conclusion is that ZPE and BSSE corrections do not alter the energy ordering of the structures for [C1C1im]Cl IPs. The front and top IP-conformers are the lowest in energy, they are also found to be approximately degenerate and the energy ordering varies with the method employed. The largest difference in energy between these structures is at the CCSD(T)/ aug-cc-pVDZ//B3LYP-D3/aug-cc-pVTZ level with the top structure being favoured by 4.63 kJ mol 1. The energy ordering for the remaining conformers; side (E34 kJ mol 1), back (E62 kJ mol 1) and meth (E94 kJ mol 1, where stable) is consistent for all the methods employed here. The energy ordering of the front and top structures is altered by the inclusion of dispersion. However, the effects are found to be subtle and method dependent. The B3LYP functional favours the front structure, but on addition of dispersion (D2 and D3) the top structure is favoured. Including dispersion appears to stabilise the top structure by E6 kJ mol 1 (Table 1). The oB97X functional, like B3LYP, favours the front structure. However, unlike B3LYP, on addition of dispersion oB97X-D favours the front structure even more. To further explore this method dependency, front and top structures have been computed using the B97-D3 and PBE-D3 functionals with the aug-cc-pVTZ basis set. For the PBE-D3 functional the top structure was found to be 0.2 kJ mol 1 more stable than the front structure, whereas no stable front structure was found for B97-D3. Thus, the addition of dispersion corrections for B3LYP and oB97X shifts the stability of the top conformer in opposite directions. The front structure is found to be the most stable using the B3LYP, oB97X and oB97X-D functionals while the top structure is favoured at the B97-D3, PBE-D3, B3LYP-D2, B3LYP-D3 and MP2 levels of theory.


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Free energy (DG), enthalpy (DH) and entropy (DTDS) data are provided in the ESI,† Tables S5–S8. Examination of DG indicates that the front and top structures are within 6 kJ mol 1 of each other. Moreover, the front structure is favoured for all of the DFT and DFT-D methods examined here, while the top structure is (just) favoured at the MP2 level. The DH ordering follows that of DE, indicating the change in relative ordering for DG has a temperature/entropic contribution (TDS). Structures that could be expected to be more sensitive to the inclusion of dispersion are the top and the meth conformers. Hence, each of these conformers has been considered in more detail. Previous results for the top IP of [C4C1im]Cl have shown that as more dispersion is recovered the H2–C2 Cl angle tends towards 901.22 This indicates that as more dispersion is recovered the anion moves along the C2–H2 bond vector and approaches a position above the C2 atom and not above the centre of the ring. A gradual increase in the H2–C2 Cl angle is observed when dispersion is included for the B3LYP functional; B3LYP (74.61) o B3LYP-D3 (77.91). In contrast, upon inclusion of dispersion the reverse trend is observed for oB97X; with oB97X-D (75.41) o oB97X (76.11). The H2–C2 Cl angle is 81.51 at the MP2 level. Including more dispersion does not significantly alter the H2–Cl interionic distance, all calculated values are within 0.03 Å of the MP2/aug-cc-pVTZ value (1.95 Å). The nature of the top (and bottom) ion-pair interaction is not clear. This could be an interaction between the Cl and the p+-cloud of the [C1C1im]+ ring, or a strong Coulombic interaction with the C2 atom. There is also H-bonding to the adjacent methyl C–H, Fig. 5b. Features that support the top-conformer as a C2-based Coulomb interaction include the Cl positioning directly above the C2 atom, which is highly positively charged, and the preference for anion–p systems to interact in-plane rather than with the p density. Features that support an anion–p+ interaction include the propensity for anions to position above the ring in electron deficient ring systems, such as C6F6. Moreover, anion structuring of this type is a well known motif in anion recognition complexes.51,52,88 The [C1C1im]+ ring has an electron deficient p+-cloud. We have also found dispersion effects, which indicate that the Cl is interacting with a p-cloud. Thus, evidence suggests a subtle interplay between charge and aromatic effects for this structural motif.

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The top and front structures obtained for [C1C1im]Cl are similar to those observed for the interaction of halides with neutral electron deficient arenes, such as s-triazine and 1,2,4,5tetracyanobenzene.89 Moreover, imidazolium and s-triazine have a common N–C–N motif, where a halide can interact either with the C sitting above the C2–H (C–H) bond vector or via a H-bond to the C2–H, Fig. 5. Studies of s-triazine with Cl have located both the top and front conformers as stable minima and the top structure is described as a Cl donor ring p–acceptor interaction.89 Thus, the top IP-conformer of [C1C1im]Cl may represent a (doubly) charged form of an anion–donor p–acceptor interaction. This is in analogy to the (doubly) ionic form of H-bonding obtained for the other IP-conformers, where the charge on the ions enhances the H-bonding. For the top IP-conformer the charge on the cation enhances the donor–acceptor properties, to the extent that the top IP-conformer is of comparable energy to the H-bonding front IP-conformer. In the meth IP-conformer the Cl interacts with a methyl group of the imidazolium cation along the N–CH3 bond vector. An alkyl–anion interaction of this type could be expected to be sensitive to the amount of dispersion recovered.22 The meth structure has previously been observed from MD simulations and using B3LYP and MP2 methods has been determined to lie approximately 100 kJ mol 1 above the most stable IP structure.22 However, characterising the nature of this critical point is problematic. For all the levels of theory examined here, other than B3LYP, B3LYP-D2 and B3LYP-D3, the meth structure was determined as a transition state leading to a front structure. Moreover, the lowest energy vibrations at the B3LYP, B3LYP-D2 and B3LYP-D3 levels were essentially 0 cm 1. Thus, the meth interaction in the isolated ion-pair is highly method and basis set dependent and is not merely a product of dispersion stabilisation. The meth interaction is not a stable IP structure, and appears to be a true product of the liquid environment, as it has been observed in MD simulations and in neutron diffraction experiments on [C1C1im]Cl.90 The results presented here indicate that including dispersion results in a change in the energy ordering of the front and top IP-conformers. Including dispersion also influences the H2–C2 Cl angle for the top IP-conformer and the stability of the meth IP-conformer. However, the parent functional also plays a distinctive role, as seen in the contrast between B3LYP and oB97X results. The addition of a dispersion-correction to the B3LYP functional is purely empirical, and there is no adjustment of the parameters used in formulating the functional. In contrast, for the oB97X functional there is a subtle interaction between the o and dispersion corrections.60 It has become clear that careful consideration must be given to the specific type of dispersion correction employed. 3.2

Fig. 5 (a) H-bonding and (b) anion–donor p–acceptor interactions observed for [C1C1im]Cl and [s-triazine]Cl.


Construction and naming of the ion pair dimers

IP-dimers have been constructed by combining the IP-conformers in various arrangements allowing for in- and out-of-plane cation– anion interactions and p+–p+ stacking. Details on cluster set-up


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Fig. 6 General structural motifs, anion positions and relative cation rotational orientations present in the [C1C1im]Cl IP-dimers.

and selection are provided in the Computational Methods section and a schematic overview of the construction of several IP-dimer conformers is provided in the ESI,† Fig. S2. From a large number of starting IP-dimers a reduced set of stable structures has been obtained. Five general motifs have been identified; middle, diagonal, outer, alternate and linear, Fig. 6. The middle (M) motif is identified by two parallel stacked cations, with the two anions lying in the middle, between the [C1C1im]+ rings. The stacked arrangement allows each anion to interact with both cations and a potential repulsive p+–p+ stacking interaction is possible. The diagonal (D) motif is identified by the positioning of the two anions at opposite diagonals of a rectangle with the [C1C1im]+ rings occupying the remaining corners. This arrangement allows for cation– anion interactions but little or no p+–p+ interaction. In the linear (L) and alternate (A) motifs the cations are arranged ‘‘end-on’’ or stacked respectively. Additional linear and alternate type motifs (ESI,† Fig. S3) are conceivable, but no stable structures have been obtained on optimisation. In both the linear and alternate motifs one anion resides between and interacts with both cations. The second anion can only interact with one cation and no p+–p+ stacking interactions are present. In the outer (O) motif two cations are placed in a stacked arrangement and the anions are positioned above and below the cations in standard top and bottom positions. This motif contains p+–p+ stacking interactions and each anion interacts with one cation only. Although this conformer may appear


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highly repulsive, stable structures were obtained. The stability of these structures may result from electrostatic screening,58 which will be more prevalent in the liquid phase.91 Thus, even though the anions are on the ‘‘outside’’ their presence reduces the net cation–cation repulsion. Starting from the basic general ‘‘cartoon’’ motifs presented in Fig. 6. An additional level of complexity is introduced by considering the location of the anions around the rings: front (F), side (S), meth (M), back (B), top (T) and bottom (Bt). Moreover, the relative rotational orientation of the [C1C1im]+ rings (parallel (P), rotated (R), anti-parallel (A) or T-shape (T)) is important. The naming of the IP-dimer conformers follows a four-step process. First, the parent part of the name is taken from the general motif, i.e. middle (M), diagonal (D), linear (L), alternate (A) or outer (O). Second the position of the anions relative to cation 1 is identified and third the position of the anions relative to cation 2 is identified, i.e. front (F), side (S), back (B), meth (M), top (T) or bottom (Bt). Because there are two anions, these denotations appear as a pair, e.g. FB indicates that the first anion is in a front (F), and the second anion is in a back (B), position relative to a given cation. The second and third sections of the name can also include numbers 1 or 2, which differentiate structures with a slight rotation of the upper imidazolium cation due to slightly different H-bonding motifs with methyl groups (described in more detail in the ESI,† Fig. S4). The final part of the name is based on the relative [C1C1im]+ ring orientations, i.e. parallel (P), rotated (R), antiparallel (A) or T-shape (T). The linear, alternate and outer structures have a reduced number of cation–anion interactions and hence have simplified names. A number of illustrative examples are provided in Fig. 7. Fig. 7a depicts the IP-dimer D_F1T_TF_A. This is a diagonal (D) structure with anion 1 in a front (F1) position and anion 2 in top (T) position relative to cation 1. Anion 1 is in a top (T)

Fig. 7 Example IP-dimers illustrating the naming conventions. The IP-dimers shown are (a) D_F1T_TF_A, (b) O_T_Bt_P and (c) L_FB_F_P.


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position and anion 2 in front (F) position relative to cation 2. The [C1C1im]+ rings are in an anti-parallel (A) arrangement. Fig. 7b shows the O_T_Bt_P IP-dimer constructed from an outer (O) motif. T indicates anion 1 is in a top position relative to cation 1 and Bt indicates anion 2 is in a bottom position relative to cation 2. P denotes that the [C1C1im]+ rings are stacked in a parallel orientation. The final example is the linear L_FB_F_P IP-dimer (Fig. 7c). FB indicates that anion 1 is in a front position and anion 2 in a back position relative to cation 1 and F indicates that anion 2 is in a front position relative to cation 2. The P indicates a parallel arrangement of the [C1C1im]+ rings. 3.3

Ion-pair dimer structures

21 stable [C1C1im]Cl IP-dimers of the middle, diagonal, alternate, linear and outer motifs have been obtained at the B3LYP-D3/augcc-pVTZ level. These structures are presented in Fig. 8–10. A number of structural features become evident on examining the middle conformers (with the Cl between the p+–p+ interacting rings), Fig. 8. The lower energy structures all have the Cl in front and side positions. IP-dimers comprised of all side structures (M_SS_SS_A) or front and back structures (M_FB_BF_A) do occur, but at a higher energy. The cation rings form all possible p+–p+ interactions; parallel, rotated or anti-parallel. The M_SS_SS_A conformer stands out as the only middle structure with minimal p+–p+ interaction, as the rings have become displaced laterally relative to each other. All of the diagonal conformers (Fig. 9) have a top IP structure as a dominant feature. Seven distinct IP-dimer conformers are obtained when the top structure is combined with the front, bottom and meth structures. The dominance of the top

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structure reduces possible p+–p+ stacked interactions, the cation rings are either displaced or adopt a T-shaped orientation. The D_TBt_BB_P IP-dimer conformer is distinct in that the rings form a V-shape blocked by the Cl from p+–p+ stacking. Linear, alternate and outer IP-dimer conformers (Fig. 10) are composed of at least one front or top structure. Linear conformers, which have no p+–p+ interactions, are found with combinations of front and back structures only. The optimisation of linear conformers starting from a front and side combination converged to middle conformers. Outer conformers have one Cl in a top position, while the second Cl in both outer IP-dimer conformers is in a bottom position. Both of these conformers, which may initially appear electrostatically unfavourable, are found as minima on the B3LYP-D3 potential energy surface. No parallel ring arrangement has been found, but the anti-parallel and rotated structures are stable. The alternate conformers have one Cl in front and side or front and bottom structures, and the second Cl in a top position. All of the outer IP-structures have a p+–p+ arrangement. 3.4

Ion-pair dimer energies

The binding energy DEB-ions = Ecluster 2 (Ecation + Eanion) of M_FS_SF_A (the lowest energy structure) is 909.89 kJ mol 1 at the B3LYP-D3/aug-cc-pVTZ level. Binding energies can also be calculated using IPs rather than the constituent ions (DEB-ion-pair = Ecluster 2 (IP)). However, the latter approach requires reference to a particular IP-conformer, the best choice of which is not obvious, as the lowest energy IP-conformer may not be the only contributor to the IP-dimer. Binding energies for the M_FS_SF_A conformer using both schemes are presented in Table 3, DEB-ion-pair was calculated using the energy of the front IP-conformer.

Fig. 8 Middle IP-dimer structures. Relative conformer energies (DE), set relative to the M_FS_SF_A structure, reported in kJ mol B3LYP-D3 level.


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Fig. 9 Diagonal IP-dimer structures. Relative conformer energies (DE), set relative to the M_FS_SF_A structure, reported in kJ mol B3LYP-D3 level.

Binding energies of all the IP-dimer structures at the B3LYP and B3LYP-D3 levels are provided and discussed in the ESI† (Table S9). The dispersion-corrected functionals have very similar DEB-ions, however DEB-ions for the B3LYP functional is E50 kJ mol 1 less. DEB at the MP2/aug-cc-pVDZ level lies between the B3LYP and dispersion-corrected methods. Relative conformer energies, Table 4, range from 0–126 kJ mol 1 and are represented on an energy level diagram in ESI,† Fig. S5. Two distinct breaks in DE are found between 10–25 kJ mol 1 and at 50–70 kJ mol 1, thus the IP-dimer conformers appear to group into distinct energy bands. We define IP-dimer energies as low DE o 10 kJ mol 1, medium 10 o DE o 50 kJ mol 1 and high DE > 50 kJ mol 1. The 13 low and medium energy structures are a mix of middle and diagonal conformers. The low-energy band has 4 middle and 4 diagonal conformers and the medium-energy band has 2 middle and 3 diagonal conformers. Thus, there is no obvious preference for middle or diagonal conformers in either energy band. The 8 high-energy conformers are dominated by alternate, linear and outer motifs. These high-energy conformers also exhibit collection into further distinct energy bands (ESI,† Fig. S5). We have focused on the low- and medium-energy conformers and DE at various levels of theory are presented in Table 5. The energy ordering of the low-energy structures is altered by the choice of functional. In general, diagonal conformers are favoured over the middle conformers for the B3LYP and oB97X functionals. Including dispersion alters this trend with the


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middle conformers now favoured over the diagonal conformers. The difference in energy between the dispersion and nondispersion-corrected structures is E2–10 kJ mol 1. In the low-energy band the difference in energy between IP-dimer conformers is o10 kJ mol 1 (B3LYP-D3) and a similar range holds for the other methods tested. Estimates of the error in the IP binding energies are 4.9 and 2.4 kJ mol 1 at the B3LYP-D3 and MP2 levels respectively.69 Thus, assuming the error in the binding energy of IP-dimers is approximately twice that of the IPs, differences in the binding energy in the region of 5–10 kJ mol 1 are of the order of the accuracy of the methods employed, and thus no firm conclusion can be made as to the minimum energy structure. The energy ordering of the conformers in the medium-energy band is indifferent to the chosen functional. The difference in energy between the dispersion and non-dispersion-corrected structures is E6–15 kJ mol 1 within this energy band. Examination of DG for the low-energy band IP-dimers (ESI,† Tables S10 and S11) indicates that the diagonal conformers are lower in free energy than the middle conformers for all DFT and DFT-D functionals, however the reverse is observed at the MP2 level. At the MP2 level the lowest free energy structure is M_FS_SF_R, which is a rotated version of M_FS_SF_A (lowest energy B3LYP-D3 IP-dimer structure). The DH ordering follows that of DE, indicating the change in relative ordering for DG has a temperature/entropic contribution. TDS contributions at 298.15 K are of the order of 0–10 kJ mol 1. Relevant data are provided in the ESI,† Tables S10–S13. ZPE and BSSE corrections are provided in the ESI,† Tables S14–S16. DZPE ranges from 2–5 kJ mol 1 depending


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Fig. 10 Alternate, linear and outer IP-dimer structures. Relative conformer energies (D E), set relative to the M_FS_SF_A structure, reported in kJ mol the B3LYP-D3 level.

Table 3 M_FS_SF_A [C1C1im]Cl IP-dimer binding energies (DEB) in kJ mol 1, ZPE and BSSE corrected, calculated at various levels of theory

DEB-ions (kJ mol 1) B3LYP MP2 oB97X-D oB97X B3LYP-D2 B3LYP-D3

857.29 894.25 904.19 908.25 909.42 909.89

DEB-ion-pair (kJ mol 1) 83.60 121.84 118.06 115.31 120.76 119.87

on the level of theory and thus impacts on the relative ordering of the lowest-energy conformers. BSSE for the DFT and DFT-D methods is o2 kJ mol 1 with only a small variation (0.5 kJ mol 1) between conformers. Thus, BSSE has no impact on the relative energy ordering of the IP-dimers for the DFT based methods. BSSE is significantly larger at the MP2/aug-cc-pVDZ level with values ranging from 50–72 kJ mol 1. BSSE for the IP-dimers can be computed with respect to IPs (2 components) or monomeric


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Table 4 [C1C1im]Cl IP-dimer conformer energies (DE) in kJ mol 1, ZPE and BSSE corrected, at the B3LYP-D3/aug-cc-pVTZ level relative to the M_FS_SF_A conformer

IP-dimer M_FS_SF_A M_FS_SF_R M_FS_FS_R D_F1T_TF_A D_F2T_TF_A D_F1T_TF_T D_F2T_TF_T M_FS_SF_P M_SS_SS_A D_TM_BF_T D_TB_BT_A

DE (kJ mol 1)

IP-dimer

DE (kJ mol 1)

0.00 0.42 1.66 2.32 3.29 3.68 4.88 7.49 26.60 32.91 35.58

M_FB_BF_A D_TBt_BB_P A_FS_T_A L_FB_F_P A_FBt_T_P A_TBt_T_T O_T_Bt_A O_T_Bt_R A_SF_T_A L_BB_F_A

37.42 48.83 72.02 75.02 78.38 81.16 100.87 103.84 106.57 125.99

ions (4 components). Within the framework of the counterpoise method, BSSE is known to increase with the number of components.81 We have computed BSSE for a selection of IP-dimers.


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Table 5 [C1C1im]Cl IP-dimer conformer energies (DE) in kJ mol 1, ZPE and BSSE corrected, for the low- and medium-energy bands. NS = structure is not stable at this level

Conformer

B3LYP oB97X

MP2

M_FS_SF_A M_FS_SF_R M_FS_FS_R D_F1T_TF_A D_F2T_TF_A D_F1T_TF_T D_F2T_TF_T M_FS_SF_P M_SS_SS_A D_TM_BF_T D_TB_BT_A M_FB_BF_A D_TBt_BB_P

0.00 NS 0.68 2.52 0.34 1.27 1.21 5.51 15.92 22.31 NS NS NS

0.00 0.50 1.02 0.24 1.25 2.09 3.17 7.22 18.92 31.63 28.95 33.44 38.01

0.00 1.20 1.32 5.21 3.48 3.50 1.90 NS 22.67 27.62 34.58 NS 46.13

B3LYP-D2 oB97X-D B3LYP-D3 0.00 1.85 2.85 6.38 7.71 7.90 8.71 9.12 28.55 37.96 39.80 38.07 52.92

0.00 0.17 1.97 3.65 4.28 5.41 6.98 7.26 29.50 36.13 40.05 38.73 NS

0.00 0.42 1.66 2.32 3.29 3.68 4.88 7.49 26.60 32.91 35.58 37.42 48.83

The BSSE for D_FT_TF_A (no p+–p+ stacking) is 32.4 kJ mol 1 and 55.1 kJ mol 1 employing 2- and 4-components respectively. The former BSSE is comparable with results obtained at the MP2/6-31+G(d) level for a representative [C1C1im][BF4] IP-dimer using the 2-component method.92 Similarly BSSE for M_FS_SF_R (which includes p+–p+ stacking) increases from 50.1 kJ mol 1 to 71.7 kJ mol 1 when changing from 2 to 4 components respectively. These results indicate that there is an increase of E20 kJ mol 1 in the BSSE on increasing the number of components. The presence of p+–p+ stacking interactions has resulted in an increase in the BSSE of E18 kJ mol 1. The p+–p+ stacked conformers at the B3LYP level (4.9 Å) have an increased ring– ring (centroid) distance compared to the MP2 level (3.4 Å), indicating that better overlap of the p+ charge clouds may be increasing BSSE at the MP2 level. Moreover, the MP2 method is also known to over bind p–p stacked structures.80 Nevertheless, the dispersion corrected DFT methods employed (using a augcc-pVTZ basis set) have very similar geometries to those at the MP2 level (for example the B3LYP-D3 ring-ring distance is 3.5 Å) and do not exhibit such a substantial BSSE. 3.5

Ion-pair dimer structure–energy relationships

A number of distinctive structural features become evident on examining the low and medium energy IP-dimers. It could be expected that the lowest energy IP-dimers would be a combination of the lowest energy front and top IP structures. However, the four middle conformers with low energy (including the lowest energy structure at the B3LYP-D3 level) are a combination of front and side IP structures. This is unexpected because the side IP-conformer is E35 kJ mol 1 higher in energy than both the front and top IP-conformers. Nevertheless, the diagonal IP-dimers in the low-energy band are composed of the expected front and top structures. In the middle-energy band IP-dimers are a combination of low-energy (top or front) and high-energy (back or meth) structures, or in the case of M_SS_SS_A a combination of all side positions for the Cl . A comparison of structures shows that relative ring orientation (parallel, antiparallel, or rotated) does not significantly


Table 6

Ring orientation for the middle front-side structural motifs

Structure M_FS_SF_A M_FS_FS_R M_FS_SF_R M_FS_SF_P

Anions

Rings

DE (kJ mol 1)

Front-side Front-side Front-side Front-side

Stacked displaced-anti-parallel Stacked displaced-rotated Stacked-rotated Stacked-parallel

0.00 1.66 0.42 7.49

impact on energies, Table 6. This indicates that Coulomb effects are dominant over the more subtle steric contributions arising from ring rotation. This could be expected to change for cations with larger alkyl groups. Where rings are stacked, the Cl prefer to remain on opposite sides of the periphery of the cluster, Fig. 11. Moreover, the Cl take up positions where they can interact with the largest number of C–H groups. Some high-energy IP structures (meth and back) are stabilised in the IP-dimers, for example, the back and meth structures in the D_TBt_BB_P and D_TM_BF_T IP-dimers, Fig. 12a. The dual side–side combination of M_SS_SS_A is also stabilised, Fig. 12b. The D_TM_BF_T IP-dimer exhibits a meth structure (Fig. 12c), and lies only 32.9 kJ mol 1 above the lowest energy IP-dimer (at the B3LYP-D3 level). The presence of the meth interaction in the IP-dimers indicates further that liquid phase local structuring is being recovered. This dimer is also unique in that the Cl occupy essentially one of each type of available position around the cations (top, meth, back, front), the single exclusion is the side position. In addition, new structural forms not possible for single IPs, are emerging; the out-of-plane or vertical position of the Cl is varying within the IP-dimers, Fig. 13. The p+–p+ ring distance (z-direction), however, stays roughly constant at around E3.3 Å. Middle IP-dimer conformers exhibiting p+–p+ interactions have been observed in MD simulations, and a pre-peak attributed to ring stacking is found in the center-of-ring radial distribution function (RDF) of [C2C1im]Cl, albeit at a larger distances of 4–5 Å (ESI,† Fig. S6). Thus, the Cl can remain essentially in-plane with the rings (similar to the IP-conformers), Fig. 13a, and the Cl can also move vertically to position between the cations. A number of structures are found with the Cl E 1.1 Å apart, Fig. 13b. Alternatively the Cl can be coplanar lying centrally between the cations (vertical displacement E0.1 Å), Fig. 13c.

Fig. 11 Cl anions remain on opposite sides and on the periphery of the [C1C1im]+ rings, maximising the H-bonding interactions.


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Fig. 12

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Examples of high-energy IP structures stabilised in the IP-dimers.

M_FS_SF_A structures. The Hn–Cn Cl angles for the remaining middle and alternate IP-dimer conformers are of a similar magnitude. H-bonding angles of between 341 to 451, with maxima between 281 and 341 have recently been reported in an MD study of [C4C1im]Cl and [C2C1im]Cl ILs.22 This type of H-bonding has not been observed for the isolated IPs and represents a new defining feature of the IP-dimer structures. This also suggests that the stability of these conformers is enhanced by the formation of the p+–p+ stacked arrangements and that the Cl are providing Coulomb stabilisation for the repulsive cation–cation interaction. We have generated spatial distribution functions (SDFs) of the Cl relative to a central [C2C1im]+ ring from the MD simulations reported in ref. 22, Fig. 14. The SDFs of the Cl relative to the [C2C1im]+ indicate that the (roughly) in-plane positions are the most favoured (50% above the bulk density). However, the likelihood of a Cl being in an out-of-plane position is still above the background density (30% and 20% above the bulk). The prominent presence of the side structure in the low energy IP-dimers is consistent with the high probability that this site is occupied in the MD simulations (side and front have similar weights in the SDF). The area around the methyl substituent at the lowest enhancement level is consistent with finding this motif in one of the medium-energy IP-dimers. The agreement of the results obtained from the IP-dimers and MD suggests that IP-dimers are beginning to recover key features of the local solution structure. Significantly, out-of-plane conformers are not found for the IPs but are found in the liquid phase. Rather than considering the anions in the liquid as centered in-plane and oscillating up and down relative to the ring, it is possible that the upper and lower portions of the SDF’s represent structures with the Cl sitting in stable positions between two rings. These structures are less common than those with Cl occupying in-plane positions, or it may be that stacked structures are less common (as stacking facilitates out-of-plane Cl positioning). Visual inspection of the trajectories shows cation stacking with surrounding Cl positioned in a mix of in-plane and middle or out-of-plane motifs (ESI,† Fig. S7). In these structures the Cl are positioned on the periphery of the molecule (Fig. 11),

Fig. 13 Horizontal and vertical displacements for the cations (blue) and anions (red) for the D_F1T_TF_A, M_FS_SF_R and M_FS_SF_A IP-dimer structures.

However, in this last case the rings are no longer precisely stacked, but are now slightly displaced. The rings can be displaced in two different directions, along the N–N vector ( y-direction), or along the C2–H vector (x-direction). Each displacement direction gives rise to slightly different types of steric interaction. For the above discussed middle (p+–p+ stacked) conformers and for several of the alternate conformers, the Cl are outof-plane and thus the H-bonds formed are not linear. For example, the H2–C2 Cl angles are 241 and 321, while the H4/5–C4/5 Cl angles are 461 and 341 for the M_FS_SF_R and


Fig. 14 Top and side views of the SDFs of Cl relative to the [C2C1im]+ at decreasing probability (50%, 30% and 20%) above the bulk density.


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and thus these out-of-plane conformers are distinct from the top structures obtained for the IPs and diagonal IP-dimers. Low energy IP-dimers can be middle (rings stacked) or diagonal (rings perpendicular) conformers. The propensity for a structure to be present in the liquid phase depends not only on the relative stability, but also on the barrier to interconversion. Hence, we have investigated the energy barrier for an IP-dimer to move from a p+–p+ stacked structure (M_FS_SF_R) to a diagonal conformer with no p+–p+stacking (D_F1T_TF_A), details of the PES scan are provided in the ESI,† Fig. S8. The barrier is E6.1 kJ mol 1 at the B3LYP-D3 level, whereas no barrier exists at the B3LYP level. Thus, the middle to diagonal interconversion should be facile in the liquid phase even at room temperature. This is consistent with the seamless SDF found for the liquid phase (Fig. 14), which merges the top and front motifs. However, the existence of barrier at the B3LYP-D3 level tends to support some p+–p+ structuring, consistent with recent results from AIMD simulations which included a dispersion correction.93 3.6

The Impact of dispersion effects on structure

The failure of conventional density functionals, including B3LYP, to accurately describe weak intermolecular interactions is well documented.94–96 The largest potential impact of dispersion on the IP-dimer structures studied here is in relation to p+–p+ stacking of the rings and the top position of the Cl . We have previously established that including dispersion moves the position of the Cl relative to the C2–H bond vector. In the top IP-conformer the H2–C2 Cl angle is E741 and E781 at the B3LYP and B3LYP-D3 levels respectively. The same effect can be observed in the IP-dimer structures that contain a Cl in the top position, this is principally evident in the diagonal conformers. The Cl shifts even further along the bond vector to a position directly above the C2 atom. For example, in D_F1T_TF_A and D_F1T_TF_T the H2–C2 Cl angle is E851 and E911 at the B3LYP and B3LYP-D3 levels respectively. Somewhat surprisingly both B3LYP and B3LYP-D3 show an increase in the C2–Cl distance, from E2.65 Å (IP) to E3.2 Å (IP-dimer). In addition, for the B3LYP structures there is a concomitant twisting of the [C1C1im]+ rings, Fig. 15a. IP-dimers calculated at DFT, DFT-D (D2 and D3) and MP2 levels have been overlaid in Fig. 15 to illustrate the impact of dispersion. The oB97X-D and B3LYP-D2 structures are not shown because they overlap with the oB97X and B3LYP-D3 structures respectively. Dispersion effects are evident in the M_FS_SF_R conformer (Fig. 15b) the tilt angle between the two planes of the [C1C1im]+ rings decreases from E1201 (B3LYP) to E961 at the MP2 level. Moreover, the vertical displacement between the centres of geometry of the rings is E1.5 Å larger when employing B3LYP (4.9 Å) compared to MP2 (3.4 Å). The dispersion corrected density functionals and the long-range corrected oB97X functional perform well, generating structures very close to those obtained at the MP2 level. B3LYP poorly describes the M_FS_SF_A conformer, Fig. 15c. Vertical and horizontal displacements between the [C1C1im]+ rings are in the order of E0.2–0.3 Å larger than those at the


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Fig. 15 (a) Relative twisting of the [C1C1im]+ rings in D_F1T_TF_A at the B3LYP (orange) and B3LYP-D3 (purple) levels (b) tilt angle between the planes of the stacked rings in M_FS_SF_R. (c) Horizontal and vertical displacements between the rings in M_FS_SF_A. MP2 (blue), B3LYP (orange), B3LYP-D3 (purple) and oB97X (green). For clarity the chloride anions are not shown.

DFT-D and MP2 levels. The larger displacement of the rings at the B3LYP level may limit potential orbital overlap of the p-orbitals in the [C1C1im]+ rings, thus under-representing p+–p+ stacking. A similarly poor match has been observed previously for [C1C1im]DCA–cyclopentadiene and [C1C1im]DCA–methyl acrylate clusters.66 These examples illustrate the poor ability of the B3LYP functional to describe intermolecular p+–p+ stacking interactions in the [C1C1im]Cl IP-dimers. The inclusion of long-range or dispersioncorrections provides DFT structures that are comparable (if not better) than at the MP2 level. Thus the use of DFT-D is extremely important for conformers that include p+–p+ interactions.

4. Conclusions In this paper, we have explored the structural and energetic landscape of potential p+–p+ stacked motifs, H-bonding arrangements and anion–p+ interactions for gas-phase IP-conformers and IP-dimers of [C1C1im]Cl. The distinctive nature of the charge deficient (cationic) aromatic interactions as opposed to ordinary p–p interactions has been highlighted, and particularly the requirement for Coulomb stabilisation by the anions. The competition between dispersive p+–p+ vs. anion–p+ (top structure) interactions has been elucidated. For the isolated IPs, the front and top conformers are the lowest in energy and almost degenerate, the front structure is favoured by the B3LYP, oB97X and oB97X-D functionals while the top structure is favoured by the B97-D3, PBE-D3, B3LYP-D2, B3LYP-D3 and MP2 levels of theory. However, only the MP2 method favours the top structure when DG is computed, indicating the role of TDS in determining the lowest Gibb’s


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free energy structure. Thus, the effects of including dispersion were found to be subtle and dependent on the underlying functional. The nature of the top (and bottom) ion-pair interactions has been interrogated; is the top IP-conformer an anion–p+ interaction, a Coulombic Cl C2 interaction or a conformer stabilised by H-bonding to the adjacent methyl C–H? Our conclusion has been that the top IP conformer may represent a (doubly) charged form of a (anionic) donor p–acceptor interaction, supported by the methyl H-bonding and Cl C2 Coulombic interaction. 21 stable IP-dimers have been obtained within an energy range of 0–126 kJ mol 1. The structures have been found to exhibit a complex interplay of structural features: middle vs. diagonal conformers, p+–p+ interactions that can be parallel, rotated or antiparallel, and anion locations which are a mix of top, front, side, and meth motifs. Structures have been grouped according to energy. The low energy structures (o10 kJ mol 1) are sensitive to the functional employed and dispersion effects, in part because the energy differences between the structures are of the order of the dispersion corrections. BSSE is found to be substantial at the MP2 level, but not for the DFT based methods. It has been found that low-energy middle (p+–p+ stacked) conformers tend to exhibit a front and side motif and in the low-energy diagonal conformers front and top motifs dominate. A low barrier (o6 kJ mol 1) was found for the conversion of one middle to one diagonal conformer, indicating that the general conversion from p+–p+ stacked to diagonal (top and front structures) may be a facile process in the liquid phase. The three lowest energy middle IP-dimers of [C1C1im]Cl (oB97X-D, B3LYP-D3 and MP2 level) are built up from front (E0 kJ mol 1) and side (E35 kJ mol 1) structures, and not the expected lowest energy front and top (both E0 kJ mol 1) structures. The presence of the side structure in the low energy IP-dimers is consistent with the similar probability of finding the Cl in the front or side positions (both in-plane and out-ofplane) in the SDF obtained from MD simulations. These middle IP-dimers also exhibit p+–p+ stacking interactions, such arrangements have been observed in MD simulations and in cation–cation RDFs. The relative energy was found to be essentially insensitive to ring orientation (parallel, antiparallel, or rotated). Coulomb effects appear to dominate the Cl positioning for the middle structures; favouring anions placed vertically between and on opposite sides of the cationic rings, while maximising H-bonding opportunities (through very bent bifurcated H-bonds). Moreover, the meth (E100 kJ mol 1) and back motifs, which are very high energy in the isolated IPs, have been found to be substantially stabilised in the IP-dimers (E33 kJ mol 1). The array of IP-dimer structures presents a number of new structural forms that cannot be recovered by examining the isolated ion-pairs. The large out-of-plane positioning, equal density of front and side structures and stabilisation of the meth structure, are all consistent with MD simulations of the


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liquid and indicate that the IP-dimer clusters are recovering key features of the local liquid structure. The role of dispersion in the formation of the IP and IP-dimer structures has been investigated employing a range of functionals. B3LYP-D2 and B3LYP-D3 include empirical dispersion corrections to the base B3LYP functional, while oB97X includes long range exchange corrections to recover dispersion, and oB97X-D includes an additional empirical correction. Comparisons have been made with respect to MP2 level calculations. Dispersion effects are important, however we found that the parent functional can play a distinctive role. Dispersion corrections created opposing shifts for B3LYP and oB97X in terms of the front/top IP conformer stability, top structural features and with respect to the stability of the meth IP-conformer. The impact of dispersion on the IP-dimer structures was found to be significant for p+–p+ stacking of the rings and the Cl top structure. At the B3LYP level the [C1C1im]+ rings are pushed further apart and twist open, while at the MP2 level and for the DFT-D methods they stack. In conformers with a top structure, dispersion shifts the Cl along the C2–H bond vector to a position approximately above the C2 atom, concomitant with a gradual increase in the H2–C2 Cl angle. Moreover, the C2–Cl distance increases by E0.55 Å on going from the IPs to the IP-dimers. We have explored the impact of zero point energy (ZPE) and basis set superposition error (BSSE) corrections on both IP-conformers and the IP-dimers. While the relative ZPE and BSSE are small for DFT based methods this increases substantially at the MP2 level. ZPE and BSSE corrections have no effect on the IP energy ordering of the conformers, but they do alter the energy ordering of the low-energy IP-dimers. The energy differences between p+–p+, anion–p+ and cation– anion H-bonding are all very small, o10 kJ mol 1, and their competition creates a very delicate balance of forces within the liquid environment. Summarising the key results. We have classified ring stacking as an electron deficient p+–p+ interaction, and the competitive top structure as an anion–donor p+–acceptor interaction. Including dispersion affects both the energy and geometry of the IPs and IP-dimers, however the impact is subtle and dependent on the underlying functional. BSSE corrections are important. The sampled range of IP-dimers has presented new structural forms that cannot be recovered by examining the isolated ion-pairs. The IP-dimers are not necessarily constructed from the lowest energy IP-conformers. Importantly, the IP-dimers appear to be recovering key features of the local liquid structure.

Acknowledgements R.P.M. acknowledges post-doctoral financial support provided through an ERC Advanced Investigator Grant held by Prof. Welton and Dr Hunt. The authors would like to thank the reviewers for their thoughtful comments, which have added to the clarity of the paper.


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Notes and references 1 P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3772–3789. 2 T. Welton, Chem. Rev., 1999, 99, 2071–2084. 3 N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123–150. 4 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576. 5 T. Welton, Coord. Chem. Rev., 2004, 248, 2459–2477. 6 J. G. Huddleston and R. D. Rogers, Chem. Commun., 1998, 1765–1766. 7 X. Han and D. W. Armstrong, Acc. Chem. Res., 2007, 40, 1079–1086. 8 X. Zhang, X. Zhang, H. Dong, Z. Zhao, S. Zhang and Y. Huang, Energy Environ. Sci., 2012, 5, 6668–6681. 9 R. P. Swatloski, S. K. Spear, J. D. Holbrey and R. D. Rogers, J. Am. Chem. Soc., 2002, 124, 4974–4975. 10 J. S. Moulthrop, R. P. Swatloski, G. Moyna and R. D. Rogers, Chem. Commun., 2005, 1557–1559. 11 D. A. Fort, R. C. Remsing, R. P. Swatloski, P. Moyna, G. Moyna and R. D. Rogers, Green Chem., 2007, 9, 63–69. 12 A. Brandt, M. J. Ray, T. Q. To, D. J. Leak, R. J. Murphy and T. Welton, Green Chem., 2011, 13, 2489–2499. 13 L. J. A. Conceiçao, E. Bogel-Łukasik and R. Bogel-Łukasik, RSC Adv., 2012, 2, 1846–1855. 14 P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854. 15 Y. Bai, Y. Cao, J. Zhang, M. Wang, R. Li, P. Wang, S. M. ¨tzel, Nat. Mater., 2008, 7, 626–630. Zakeeruddin and M. Gra 16 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati, Nat. Mater., 2009, 8, 621–629. 17 C. Ye, W. Liu, Y. Chen and L. Yu, Chem. Commun., 2001, 2244–2245. 18 F. Zhou, Y. Liang and W. Liu, Chem. Soc. Rev., 2009, 38, 2590–2599. 19 Y.-K. Yang, C.-E. He, R.-G. Peng, A. Baji, X.-S. Du, Y.-L. Huang, X.-L. Xie and Y.-W. Mai, J. Mater. Chem., 2012, 22, 5666–5675. 20 C. G. Hanke and R. M. Lynden-Bell, J. Phys. Chem. B, 2003, 107, 10873–10878. 21 P. A. Hunt, B. Kirchner and T. Welton, Chem.–Eur. J., 2006, 12, 6762–6775. 22 I. Skarmoutsos, D. Dellis, R. P. Matthews, T. Welton and P. A. Hunt, J. Phys. Chem. B, 2012, 116, 4921–4933. 23 V. Kempter and B. Kirchner, J. Mol. Struct., 2010, 972, 22–34. 24 K. Fumino, A. Wulf and R. Ludwig, Angew. Chem., Int. Ed., 2009, 48, 3184–3186. ´ska, D. H. Zaitsau, 25 K. Fumino, T. Peppel, M. Geppert-Rybczyn ¨ckerling and R. Ludwig, J. K. Lehmann, S. P. Verevkin, M. Ko Phys. Chem. Chem. Phys., 2011, 13, 14064–14075. 26 P. A. Hunt, J. Phys. Chem. B, 2007, 111, 4844–4853. 27 G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford University Press, Oxford, 1997. 28 R. Bhattacharyya, U. Samanta and P. Chakrabarti, Protein Eng., Des. Sel., 2002, 15, 91–100. 29 R. Kumpf and D. Dougherty, Science, 1993, 261, 1708–1710.


Paper

30 P. C. Kearney, L. S. Mizoue, R. A. Kumpf, J. E. Forman, A. McCurdy and D. A. Dougherty, J. Am. Chem. Soc., 1993, 115, 9907–9919. 31 H. Ihm, S. Yun, H. G. Kim, J. K. Kim and K. S. Kim, Org. Lett., 2002, 4, 2897–2900. 32 A. S. Mahadevi and G. N. Sastry, Chem. Rev., 2013, 113, 2100–2138. 33 V. Percec, M. Glodde, T. K. Bera, Y. Miura, I. Shiyanovskaya, K. D. Singer, V. S. K. Balagurusamy, P. A. Heiney, I. Schnell, A. Rapp, H.-W. Spiess, S. D. Hudson and H. Duan, Nature, 2002, 419, 384–387. 34 L. Zang, Y. Che and J. S. Moore, Acc. Chem. Res., 2008, 41, 1596–1608. ´das, 35 J. Cornil, D. Beljonne, J. P. Calbert and J. L. Bre Adv. Mater., 2001, 13, 1053–1067. 36 J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem. Commun., 1992, 965–967. 37 J. S. Wilkes and M. J. Zaworotko, Supramol. Chem., 1993, 1, 191–193. ´dua, 38 M. Deetlefs, C. Hardacre, M. Nieuwenhuyzen, A. A. H. Pa O. Sheppard and A. K. Soper, J. Phys. Chem. B, 2006, 110, 12055–12061. 39 A. Triolo, O. Russina, H.-J. Bleif and E. Di Cola, J. Phys. Chem. B, 2007, 111, 4641–4644. 40 X.-W. Li, J. Zhang, B. Dong, L.-Q. Zheng and C.-H. Tung, Colloids Surf., A, 2009, 335, 80–87. 41 M. Deetlefs, C. Hardacre, M. Nieuwenhuyzen, O. Sheppard and A. K. Soper, J. Phys. Chem. B, 2005, 109, 1593–1598. 42 A. I. Frolov, K. Kirchner, T. Kirchner and M. V. Fedorov, Faraday Discuss., 2012, 154, 235–247. 43 M. V. Fedorov and R. M. Lynden-Bell, Phys. Chem. Chem. Phys., 2012, 14, 2552–2556. 44 A. G. Avent, P. A. Chaloner, M. P. Day, K. R. Seddon and T. Welton, J. Chem. Soc., Dalton Trans., 1994, 3405–3413. `, M. Giannone, E. Ragg, G. Fronza, G. Raos 45 A. Mele, G. Romano and V. Marcon, Angew. Chem., Int. Ed., 2006, 45, 1123–1126. ´lez 46 B. Aoun, A. Goldbach, S. Kohara, J.-F. Wax, M. A. Gonza and M.-L. Saboungi, J. Phys. Chem. B, 2010, 114, 12623–12628. ¨ssel, M. Brehm, A. S. Pensado, F. Malberg, M. Ramzan, 47 M. Bru A. Stark and B. Kirchner, Phys. Chem. Chem. Phys., 2012, 14, 13204–13215. 48 M. L. Waters, Curr. Opin. Chem. Biol., 2002, 6, 736–741. 49 N. J. Singh, S. K. Min, D. Y. Kim and K. S. Kim, J. Chem. Theory Comput., 2009, 5, 515–529. 50 C. D. Sherrill, Acc. Chem. Res., 2013, 46, 1020–1028. 51 A. Frontera, P. Gamez, M. Mascal, T. J. Mooibroek and J. Reedijk, Angew. Chem., Int. Ed., 2011, 50, 9564–9583. ˜ onero, C. Garau, C. Rotger and A. Frontera, Angew. 52 D. Quin Chem., Int. Ed., 2002, 3539–3541. 53 K. S. Kim, P. Tarakeshwar and J. Y. Lee, Chem. Rev., 2000, 100, 4145–4186. 54 A. Das, A. D. Jana, S. K. Seth, B. Dey, S. R. Choudhury, T. Kar, S. Mukhopadhyay, N. J. Singh, I.-C. Hwang and K. S. Kim, J. Phys. Chem. B, 2010, 114, 4166–4170. 55 P. Kar, R. Biswas, M. G. B. Drew, A. Frontera and A. Ghosh, Inorg. Chem., 2012, 51, 1837–1851.


View Article Online


Paper

56 S. K. Seth, P. Manna, N. J. Singh, M. Mitra, A. D. Jana, A. Das, S. R. Choudhury, T. Kar, S. Mukhopadhyay and K. S. Kim, Cryst. Eng., 2013, 15, 1285–1288. 57 I. Geronimo, N. J. Singh and K. S. Kim, Phys. Chem. Chem. Phys., 2011, 13, 11841–11845. 58 H. Li, J. A. Boatz and M. S. Gordon, J. Am. Chem. Soc., 2008, 130, 392–393. 59 Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2007, 120, 215–241. 60 J.-D. Chai and M. Head-Gordon, Phys. Chem. Chem. Phys., 2008, 10, 6615–6620. 61 S. Grimme, J. Comput. Chem., 2006, 27, 1787–1799. 62 S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104. 63 S. Grimme, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2011, 1, 211–228. ´. Va ´zquez-Mayagoitia, C. D. Sherrill, E. Apra ` and 64 A B. G. Sumpter, J. Chem. Theory Comput., 2010, 6, 727–734. ´. V. Mayagoitia, B. G. Sumpter and C. D. 65 L. A. Burns, A Sherrill, J. Chem. Phys., 2011, 134, 084107. 66 S. Zahn and B. Kirchner, J. Phys. Chem. A, 2008, 112, 8430–8435. 67 S. Grimme, W. Hujo and B. Kirchner, Phys. Chem. Chem. Phys., 2012, 14, 4875–4883. 68 E. I. Izgorodina, U. L. Bernard and D. R. MacFarlane, J. Phys. Chem. A, 2009, 113, 7064–7072. 69 S. Zahn, D. R. MacFarlane and E. I. Izgorodina, Phys. Chem. Chem. Phys., 2013, 15, 13664–13675. 70 Y. Danten, M. I. Cabaço and M. Besnard, J. Mol. Liq., 2010, 57–66. 71 H. Li, Y. Lu, W. Wu, Y. Liu, C. Peng, H. Liu and W. Zhu, Phys. Chem. Chem. Phys., 2013, 15, 4405–4414. 72 K. Fumino, V. Fossog, K. Wittler, R. Hempelmann and R. Ludwig, Angew. Chem., Int. Ed., 2013, 52, 2368–2372. 73 K. Wendler, F. Dommert, Y. Y. Zhao, R. Berger, C. Holm and L. Delle Site, Faraday Discuss., 2012, 154, 111–132–discussion 189–220–465–471. 74 M. A. Addicoat, S. Fukuoka, A. J. Page and S. Irle, J. Comput. Chem., 2013, 34, 2591–2600. 75 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,


PCCP

76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, ¨ . Farkas, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision D.01, 2009. A. Becke, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38, 3098–3100. C. Lee, W. Yang and R. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785–789. J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2008, 128, 084106. S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553–566. M. O. Sinnokrot, E. F. Valeev and C. D. Sherrill, J. Am. Chem. Soc., 2002, 124, 10887–10893. M. O. Sinnokrot and C. D. Sherrill, J. Phys. Chem. A, 2006, 110, 10656–10668. M. Brehm and B. Kirchner, J. Chem. Inf. Model., 2011, 51, 2007–2023. W. Smith and T. R. Forester, J. Mol. Graphics, 1996, 14, 136–141. M. P. Allen and D. J. Tildesley, Computer simulation of liquids, Oxford University Press, 1987. W. Hoover, Phys. Rev. A: At., Mol., Opt. Phys., 1985, 31, 1695–1697. W. Smith and T. R. Forester, Comput. Phys. Commun., 1994, 79, 52–62. J.-P. Ryckaert, G. Ciccotti and H. J. C. Berendsen, J. Comput. Phys., 1977, 23, 327–341. P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486–516. O. B. Berryman, V. S. Bryantsev, D. P. Stay, D. W. Johnson and B. P. Hay, J. Am. Chem. Soc., 2007, 129, 48–58. C. Hardacre, J. D. Holbrey, S. E. J. McMath, D. T. Bowron and A. K. Soper, J. Chem. Phys., 2003, 118, 273–278. R. M. Lynden-Bell, Phys. Chem. Chem. Phys., 2010, 12, 1733–1740. E. I. Izgorodina, J. Rigby and D. R. MacFarlane, Chem. Commun., 2012, 48, 1493–1495. A. S. Pensado, M. Brehm, J. Thar, A. P. Seitsonen and B. Kirchner, ChemPhysChem, 2012, 13, 1845–1853. ¨thi, J. Chem. Phys., 2001, 114, S. Tsuzuki and H. P. Lu 3949–3957. ń and P. Pulay, Chem. Phys. Lett., 1994, 229, S. Kristya 175–180. E. R. Johnson, R. A. Wolkow and G. A. DiLabio, Chem. Phys. Lett., 2004, 394, 334–338.


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