Published: November 14, 2011 r2011 American Chemical Society 15280 dx.doi.org/10.1021/jp206974h | J. Phys. Chem. B 2011, 115, 15280–15288 ARTICLE pubs.acs.org/JPCB How Hydrogen Bonds Influence the Mobility of Imidazolium-Based Ionic Liquids. A Combined Theoretical and Experimental Study of 1-n-Butyl-3-methylimidazolium Bromide Miriam Kohagen, † Martin Brehm, † Yves Lingscheid, ‡ Ralf Giernoth, ‡ Joshua Sangoro, § Friedrich Kremer, § Sergej Naumov, § Ciprian Iacob, § J€ org K € arger, § Rustem Valiullin, § and Barbara Kirchner* ,† † Wilhelm-Ostwald-Institut f€ ur Physikalische und Theoretische Chemie, Universit € at Leipzig, Linn estr. 2, D-04103 Leipzig, Germany ‡ Department f€ ur Chemie der Universit € at zu K€ oln, Greinstr. 4, D-50939 K€ oln, Germany § Institut f€ ur Experimentelle Physik I, Universit € at Leipzig, Linn estr. 5, D-04103 Leipzig, Germany b S Supporting Information ’ INTRODUCTION Hydrogen bonding in imidazolium-based ionic liquids is a highly controversial issue. 1À9 Ionic liquids—being liquid sub- stances composed entirely of ions—still show many unexplained phenomena. 10 In some investigations, hydrogen bonding seems to be important, in others not. 11,12 It sometimes enhances cer- tain transport properties but opposite effects have also been reported. 2,3,7,13 One example is the increase of viscosity (melting point) when hydrogen bonding is eliminated by replacing the acidic proton in imidazolium-based ionic liquids with a methyl group. 14,15 Nishikawa and co-workers could show by NMR spectroscopy that the origin of this effect does not stem from the reduction of butyl group motion but from the amount of stable anion interaction sites. 16,17 Additionally, it is suggested that viscosity increase by C(2) methylation can be interpreted accord- ingly. In contrast, lower viscosities were observed in acidic AlCl 3 melts where larger anions give rise to lower hydrogen bonding. 2 Previously, as an experiment in the virtual laboratory, we explored the tremendous influence of different quantum chemically derived charges applied in molecular dynamics simu- lations on structural properties, i.e., in computer simulations. 18 The system under investigation in both studies is the ionic liquid 1-n-butyl-3-methylimidazolium bromide ([C 4 C 1 im][Br]). While two models (natural bond orbital (NBO) 19 and restraint electro- static potential (RESP) 20 ) agree well, the third set of charges (shared electron number (SEN) 21 ) provided erroneous results with respect to the experimentally 4,22À24 as well as theoretically 25 expected structure (cf. Figure 1 in which the spatial distribution functions are displayed 18 ). In this article we focus on the dynamic properties of the different charge sets. We will provide further systematically altered charge sets in order to explore the influence of hydrogen bonding. We will also prove the validity of the charge set that is usually applied in simulations 18 by comparing it to experiment. In Received: July 21, 2011 Revised: November 9, 2011 ABSTRACT: The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner.