Ionic Liquids IIIA - ACS Publications - American Chemical Society

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Modeling Ionic Liquids of the 1-Alkyl-3methylimidazolium Family Using an All-Atom Force Field 1

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Downloaded by PENNSYLVANIA STATE UNIV on May 31, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0901.ch010

J.N. Canongia Lopes , J. Deschamps , and Α . A. H . Pádua

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Centro de Química Estrutural, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Laboratoire de Thermodynamique des Solutions et des Polymères, Université Blaise Pascal, 24 Av. des La ndes, 63177 Aubière, France 2

A new force field for the molecular modeling of ionic liquids of the dialkylimidazolium cation family based on the OPLS— AA/AMBER framework is presented and discussed. Ab initio calculations were performed to obtain several terms in the force field not yet defined in the literature. These include torsion energy profiles and distributions of atomic charges that blend smoothly with the OPLS-AA specification for alkyl chains. Validation was carried out comparing simulated and experimental data on fourteen different salts, comprising three types of anion and five lengths of alkyl chain, both in the crystalline and liquid phases. The present model can be regarded as a step towards a general force field for ionic liquids of the imidazolium cation family that was built in a systematic way, is easily integrated with OPLS-AA/AMBER and is transferable between different combinations of cation-anion.

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© 2005 American Chemical Society

In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Ionic liquids (ILs) have attracted much attention from the scientific community in recent years, their status as "green" or "designer" solvents explaining the large amount of studies concerning their possible industrial use as reaction or extraction media (i). Unfortunately, the number of studies regarding their thermophysical characterization is by no means so extensive and the amount and scope of molecular modeling work is also limited (2-7). In this communication we introduce a new force field for ionic liquids ( 7) and review some of the currently available models Q-6). The comparison between force fields will focus on three main issues, i) Internal consistency. Many of the existing IL models borrow parameters from different, and not always compatible, sources. In this work ab-initio calculations are used extensively to provide essential data for the development of an internally consistent force field. This includes molecular geometry optimization and the description of electron densiy using extended basis sets, leading to the evaluation of force field parameters such as torsion energy profiles and point charges on the interaction centers, ii) Transferability. The parameterization of IL models should not be restricted to one molecular species since one wants to deal with families of similar compounds, generally the cations, that can be combined with different counter-ions. The proposed IL force field concentrates on the parameterization of parts of the molecules that are common to an entire family of cations and adopts a strategy to add specific substituents. The parameterization of the cations and anions is performed independently to ensure their interchangeability. iii) Compatibility. The model builds on the OPLS-AA force field (5). This means that any molecule or residue already defined in that database can be combined with the proposed force field. The choice of the OPLS-AA force field as a framework also benefits from its articulation with structural parameters from the even larger AMBER force field (0).

Force field development One of the most extensively studied IL family has been that of the l-alkyl-3methyl-imidazolium cation with counter-ions such as PF ", BET or CI" (Table I). The force fields used in most of the models discussed in this work take the functional form given in Table II. The different terms of the functional are discussed in the section of the manuscript given in the same table. The parameterization of a force field requires the knowledge of two sets of molecular data corresponding to bonded and non-bonded interactions. The first set comprise internal molecular parameters that can be estimated from quantum 6


136 mechanical calculations on isolated molecules and/or gas-phase spectroscopic data whereas the last set also includes external or intermolecular parameters to be inferred from auxiliary diffraction and/or condensed phase thermodynamic data. The nomenclature adopted in this work is given in Figure 1


Table I Ionic liquid models discussed in this work Authors (ref) Cations

HPLB (2) mmim* emim a",PF " +

Anions

6

ABS (3) emim* bmim* A1CV, WT

SBM (4) bmim

ΊΞά (5)^ brnim*

MSB (6) bmim

PF "

FF a.

angles

Σ [V (1 + cos(ç>)) + V (1 - cos(2p)) + V 0 + cos(3ç>))] + ^ b. x

2

3

dihedrals

atomi

atom j

a. Molecular geometry: bond lengths and angles The molecular geometry parameters are generally obtained from low-level ab-initio calculations (HF, B3LYP) using a simple basis set (6-31G) describing a single molecule. All the models under discussion used a similar method to estimate the parameters describing the imidazolium ring and the published results are comparable with experimental data obtained by diffraction studies as shown in Table III. Two conclusions can be drawn directly from the table: i) the ring geometry is not strongly affected by the environment of the imidazolium molecule since the ring geometry in two completely different crystals, [emimlfVOCU] and [ddmim][PF ], (10,11) are comparable between them and similar to the ab-initio values for the isolated imidazolium cation; and ii) the 6


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distortion of the ring caused by different alkyl substituents is so small that the use of a symmetrical ring geometry as in a mmim* cation represents a good approximation

Figure 1. Nomenclature of the 1 -alkyl -3-methyl-imidazolium cations. In the side chains a=alkyl, m=methyl, e -ethyl, b=butyl, h=hexyl dd-dodecyl. t

Most bond and angle parameters used in this work were taken from or based on the OPLS-AA and AMBER force fields (7-0,16), which are largely compatible with respect to the intramolecular terms. Whenever we found significant differences between our ab-initio geometries and OPLS-AA or AMBER parameters, notably in equilibrium distances and angles, we proposed new values, reported in Table IV. We did not modify force constants from what is specified in AMBER and OPLS-AA, since these features have a relatively small effect on thermodynamic properties. The main departure from the OPLS-AA force field at this point regards our use of constrained bond lengths for all G-H stretching mo des present in the cations. Margulis et al. (6) also used OPLS-AA/AMBER parameters to define the internal force field of the l-butyl-3-methyl imidazolium cation but chose to model the ring directly with imidazole parameters (neutral and asymmetric ring). The same kind of remark applies to the work of de Andrade et al.(3): although the CHARMM forcefield contains parametric data concerning the protonated histidine ring, the authors chose parameters that can be understood as averages of the two (asymmetrical) halves of the neutral histidine ring. Unlike the imidazolium ring, the imidazole ring residue is not symmetrical and the distortions of the structure around the carbon connected to the two (non-equivalent) nitrogen atoms are noticeable. The use of imidazole bond distances and angles to define an imidazolium cation can lead to an inaccurate parameterization.


138 To establish the molecular geometry of the PF " and NCV anions (taken as octahedral and planar triangular structures, respectively) it is sufficient to know the P-F and N^-O interatomic distances. These were taken from ab-initio data (13,14) and are also presented in Table III. The corresponding distances obtained from x-ray diffraction studies in selected IL crystals compare favorably with the ab-initio results. 6


Table ID. Comparison between experimental x-ray (XR) geometries and singlemolecule ab-initio (AI) calculations emim XR(10)

ddmim XR(ll)

1.311(4) 1.311(4) 1.357(5) 1.360(6) 1.334(8) 1.452(4) 1.468(4) 1.500(9)

1.322(3) 1.326(3) 1.373(3) 1.374(3) 1.334(8) 1.468(3) 1.477(3) -

CW-NA-Q

109.6(3) 107.1(3) 107.6(4) 108.0(3) 107.6(2) 125.9(3)

CR-NA-C,

125.2(3) 126.5(3)

emim Aim

+a2

emim* AI (4)

ami m* Aid)

bonds (A j" N -C« A

NA-C Cw-Cw NA-C, C.-Q

angles (deg)

C

W

A

^ A -

-

1.314 1.315 1.378 1.378 1.342 1.466 1.478 1.520

109.8 108.1 108.0 106.9 106.8 -

109.9 107.0 107.2 107.9 108.0 125.6

-

125.9 126.4 126.1

-

1.315 1.378 1.341 1.466

-

b

N -Q-N NA-QV-Q, A

-

Q

-

125.4(3) cl

PFf XR(12)

pF

Ionic Liquids IIIA - ACS Publications - American Chemical Society

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