DOI: 10.1002/cphc.200900144 What Far-Infrared Spectra Can Contribute to the Development of Force Fields for Ionic Liquids Used in Molecular Dynamics Simulations Thorsten Kçddermann, [a, b] Koichi Fumino, [a] Ralf Ludwig,* [a, c] JosØ N. Canongia Lopes,* [d] and Agílio A. H. Pµdua* [e] 1. Introduction Ionic liquids (ILs) are organic salts with melting points below 100 8C. They represent a new class of non-molecular liquid ma- terials with unique properties. [1–3] The field of ionic liquids offers phenomenal opportunities for new advances in science and technology. The increased interest in these new materials can be attributed to the diversity of possible applications, either as solvents for reactions and material processing or as extraction media and working fluids in mechanical applica- tions. Any use of an ionic liquid entails knowledge of its physi- cal properties and solvent behavior. Some of their unique properties include their extremely low volatility, low melting point, high ionic conductivity as well as thermal and electro- chemical stability. The unique variability of the ions often allows the properties of interest to be imparted, so that ILs are denoted as designer solvents. [4, 5] For the rational design of ionic liquids it is important to un- derstand their nature and interactions. In principle, molecular dynamics (MD) simulation is a powerful tool to obtain structur- al, dynamical and thermodynamic properties of such kind of liquids. The crucial point is the derivation of a reliable force field that can describe the interactions between ions in an ap- propriate way. For H-bonded liquids such as water, alcohols or amides this is usually done by fitting charges and Lennard- Jones parameters to experimentally well-known properties. Force fields are parameterized to be in agreement with pair correlation functions from neutron or X-ray structure factors, self-diffusion coefficients from NMR and heats of vaporization from vapor pressure measurements. [6, 7] Until recently, most of these properties were not available for ionic liquids. Moreover, they are not easy to determine with sufficient accuracy, and they are effectively known only for a few members of some ionic liquid families. One of the first and most prominent force fields used for ILs was developed by two of us [J. N. Canongia Lopes, A. A. H. Pµdua, (CLaP)] using Lennard-Jones parameters taken from the OPLS (see below) force field describing similar organic (and neutral) molecules as a starting point. The miss- ing parameters (mainly atomic point charges and dihedral angles) were parameterized using data obtained using ab initio and MD calculations. Due to the aforementioned scarcity of data, the force field could only be validated by comparison of the MD simulation results with the corresponding crystalline structures and liquid densities of selected ionic liquids. A series of four articles [8–11] represent the CLaP force field at this stage and establish a general protocol for the molecular simulation of common ionic liquids within the framework of statistical me- chanics. It must be stressed that the force field has been devel- oped in the spirit of the OPLS model and is thus primarily ori- ented towards the calculation of structural and equilibrium ther- modynamic properties of liquid phases. Moreover, the force field was built in a stepwise manner that allows the construction of models for entire families of ionic liquids and allows the mutual interchange of their anions or cations without further re- parameterization. However, the wide coverage of such a proto- col comes at the expense of its accuracy for specific cases. For the ionic liquids of the 1-alkyl-3-methylimidazolium bis- triflamide family, [C n mim]ACHTUNGTRENNUNG[NTf 2 ], the measured density and the MD density differ by about 3 % when using the CLaP force field. It is more difficult to assess the ability of this force field to predict transport properties. Calculation of diffusion coeffi- cients through equilibrium molecular dynamics methods ap- pears to lead to values that are one order of magnitude lower than experimental ones. It has been shown that inclusion of polarizable charges leads to faster dynamics when using equili- brium methods, [12] resulting in a drop of about 1/3 in viscosity and a three-fold increase in the ion diffusion coefficients. But other studies [13–15] have shown that the use of nonequilibrium methods can lead to viscosities and diffusion coefficients that agree very well with experiment even for fixed-charge models, and in some of these calculations ions were parameterized through the CLaP model. Regarding transport properties, the roles of the model or of the simulation method in attaining quantitative predictions are not yet completely resolved. Final- ly, the estimated enthalpies of vaporization are too large (by 20 and 50 %) when compared to the available experimental data. [16–20] In this case, one can assume that some of the inter- [a] Dr. T. Kçddermann, Dr. K. Fumino, Prof. Dr. R. Ludwig Abteilung Physikalische Chemie, Institut für Chemie Universität Rostock, Dr.-Lorenz-Weg 1, 18051 Rostock (Germany) Fax:ACHTUNGTRENNUNG(+49) 381 498 6524 E-mail : ralf.ludwig@uni-rostock.de [b] Dr. T. Kçddermann Algorithmen und Wissenschaftliches Rechnen, Fraunhofer Institut Schloss Birlinghoven, 53754 St. Augustin (Germany) [c] Prof. Dr. R. Ludwig Leibniz-Institut für Katalyse an der Universität Rostock A.-Einstein-Str. 29a, 18059 Rostock (Germany) [d] Prof. Dr. J. N. Canongia Lopes Centro de Química Estrutural Instituto Superior TØcnico, 1049 001 Lisboa (Portugal) Fax:ACHTUNGTRENNUNG(+351) 218 464455 E-mail : jnlopes@ist.utl.pt [e] Prof. Dr. A. A. H. Pµdua Laboratoire de Thermodynamique des Solutions et des PolymØres UniversitØ Blaise Pascal/CNRS, Clermont-Ferrand (France) Fax:ACHTUNGTRENNUNG(+33) 473 405328 E-mail : agilio.padua@univ-bpclermont.fr ChemPhysChem 2009, 10, 1181 – 1186  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1181