Barbara Kirchner

Pnicogen Bonds: A New Molecular Linker?

In the past decades a lot of progress has been achieved inthe design of miniaturized systems. For instance, the manu-facture of computer chips employing optical technologies,for example, phototemplating, is well established. However,optical procedures are limited to approximately 50 nm. Al-ternatively, nanostructures may be formed by means of selfassembly systems. Besides the arrangement of the molecularbuilding blocks by hydrogen bonds or inorganic metal–ligand bonds, unexpected strong chalcogen–chalcogen inter-actions or halogen bonds have captured interest as connec-tors.

Are there stable ion-pairs in room-temperature ionic liquids? Molecular dynamics simulations of 1-n-butyl-3-methylimidazolium hexafluorophosphate.

Molecular dynamics simulations with an all-atom model were carried out to study the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF(6)]. Analysis was carried out to characterize a number of structural and dynamic properties. It is found that the hydrogen bonds are weaker than expected, as indicated by their short lifetimes, which is due to the fast rotational motion of anions. Transport properties such as ion diffusion coefficients and ionic conductivity were also measured on the basis of long trajectories, and good agreement was obtained with experimental results. The phenomenon that electrical conductivity of ionic liquids deviates from the Nernst-Einstein relation was well reproduced in our work. On the basis of our analysis, we suggest that this deviation results from the correlated motion of cations and anions over time scales up to nanoseconds. In contrast, we find no evidence for long-lived ion-pairs migrating together.

Estimating the Hydrogen Bond Energy

First, different approaches to detect hydrogen bonds and to evaluate their energies are introduced newly or are extended. Supermolecular interaction energies of 256 dimers, each containing one single hydrogen bond, were correlated to various descriptors by a fit function depending both on the donor and acceptor atoms of the hydrogen bond. On the one hand, descriptors were orbital-based parameters as the two-center or three-center shared electron number, products of ionization potentials and shared electron numbers, and the natural bond orbital interaction energy. On the other hand, integral descriptors examined were the acceptor-proton distance, the hydrogen bond angle, and the IR frequency shift of the donor-proton stretching vibration. Whereas an interaction energy dependence on 1/r(3.8) was established, no correlation was found for the angle. Second, the fit functions are applied to hydrogen bonds in polypeptides, amino acid dimers, and water cluster, thus their reliability is demonstrated. Employing the fit functions to assign intramolecular hydrogen bond energies in polypeptides, several side chain CH...O and CH...N hydrogen bonds were detected on the fly. Also, the fit functions describe rather well intermolecular hydrogen bonds in amino acid dimers and the cooperativity of hydrogen bond energies in water clusters.

The Structure of Imidazolium-Based Ionic Liquids: Insights From Ion-Pair Interactions

A large number of ab-initio (B3LYP/6–31++G(d,p)) computed ion-pair structures have been examined in order to determine if such calculations are capable of offering insight into the physical properties of the liquid state, particularly viscosity and melting point. Ion pairings based around the 1-butyl-3-methylimidazolium (C4C1im) cations and a range of anions (Cl, BF4, and N(Tf)2 where N(Tf)2 is bis(trifluoromethylsulfonly)imide) were chosen because of the range of viscosities exhibited by the corresponding ionic liquids. We have used these results to build up a ‘picture’ of the ionic liquid structure which is consistent with molecular dynamics simulations and experimental evidence. However, further work is required to established if such an analysis could be predictive. Nevertheless, we establish clear relationships relating ion-pair association energy, a derived ‘connectivity index’, and the diversity of structures with viscosity and melting point. Our calculations indicate that ions in C4C1imCl form a strong, highly connected and regular array thus rationalizing the high viscosity and melting point. In contrast the ion-pairs of C4C1imN(Tf)2 form a weakly interacting, highly disordered, and low connectivity network consistent with the low viscosity and melting point. C4C1imBF4 lies midway between these two extremes.

Intermolecular Forces in an Ionic Liquid ([Mmim][Cl]) versus Those in a Typical Salt (NaCl)

Understanding chemical bonding and intermolecular forces is one of the major topics in chemistry. In general, chemical compounds are divided into classes based on their properties. The class of ionic liquids (ILs) has been known since the beginning of the last century. Owing to their tuneable properties and low vapor pressure, ILs have become a hot research area with a wide range of applications in recent years. A promising route towards better understanding ionic liquids is to determine the dominating intermolecular forces and compare them to those in an example such as NaCl, which is solid at room temperature and, in other words, a compound representative of its class. One must be careful in probing for general properties of ILs, because the search for generality has led to many myths about ionic liquids. Often properties of ionic liquids are explained by the chemical nature of the particular substance and are not a general feature. As an example for imidazolium-based ionic liquids, we chose 1,3-dimethylimidazolium chloride ([Mmim][Cl]). If one considers the theoretically predicted total interaction energies from ab initio calculations between a typical cation and an anion at the equilibrium distance in a typical IL, these energies range from 300 to 400 kJmol , in agreement with Ref. [13] (for details on the methods see the Supporting Information). Calculating the same energy for NaCl gives a value of 545.0 kJmol . This strongly points toward correlating these energies with melting points of the corresponding bulk system. It is obivous from Figure 1 that there is no correlation between the predicted energies and the melting points. A simple model for estimating the melting points of ILs suggests that most likely the liquid behavior of ILs can be attributed to large, unsymmetrical ions with high conformational flexibility. Recent studies reveal complex structures having microheterogenous polar and nonpolar domains for imidazolium-based systems with extended side chains; this phenomenon is not observed when the side chains are shorter. It has been inferred that other forces besides pure Coulombic interactions must play a role in ionic liquids. Thus, we decompose the total interaction energy of one ion pair of [Mmim][Cl] and one ion pair of NaCl by the symmetry-adapted perturbation theory (SAPT) method into different contributions in analogy to a multipole expansion (Figures 2 and 3). Note that the equilibrium distance is set to zero in order to provide comparability. For the NaCl pair (red diamonds in Figure 2) the dispersion term is negligible, whereas this contribution is comparable in magnitude to the induction term for the two conformers of the ionic liquid pair [Mmim][Cl] (blue and green diamonds in Figure 2). The main contribution to the total energy stems from the electrostatic interaction for all species, (circles in Figure 2) in agreement with Ref. [13]. For NaCl the total energy consists of only electrostatic, exchange, and induction contributions (see Figure 3; the curve with red squares almost exactly matches the curve with diamonds). In Figure 3 we can make another interesting observation concerning the minima: Whereas the NaCl pair features the minima for all curves exactly at the equilibrium distance (at zero, see black dotted vertical line), this is, surprisingly, not the case for the [Mmim][Cl] pairs (see black Figure 1. Melting points plotted against the interaction energies between one cation and one anion for several different ILs. A ball-andstick model of each IL is also given in the figure. 1: [Emim][AlCl4] , 2 : [Mmim][Cl], 3 : [Emim][BF4], 4 : [Emim][Cl], 5 : [Emim][DCA], 6 : [Emim][SCN]. Emim=1-ethyl-3-methylimidazolium ion, DCA=dicyanamide.

Cooperativity in ionic liquids

Cooperativity in ionic liquids is investigated by means of static quantum chemical calculations. Larger clusters of the dimethylimidazolium cation paired with a chloride anion are calculated within density functional theory combined with gradient corrected functionals. Tests of the monomer unit show that density functional theory performs reasonably well. Linear chain and ring aggregates have been considered and geometries are found to be comparable with liquid phase structures. Cooperative effects occur when the total energy of the oligomer differs from a simple sum of monomer energies. Cooperative effects have been found in the structural motifs examined. A systematic study of linear chains of increasing length (up to nine monomer units) has shown that cooperativity plays a more important role than expected and is stronger than in water. The Cl...H distance of the chloride to the most acidic proton increases with an increasing number of monomer units. The average bond distance approaches 218.9 pm asymptotically. The dipole moment grows almost linearly and the dipole moment per monomer unit reaches the asymptotic value of 16.3 D. The charge on the chloride atoms decreases with an increasing chain length. In order to detect local hydrogen bonding in the clusters a new parametrization of the shared-electron number method is introduced. We find decreasing hydrogen bond energies with an increasing cluster size for both the first hydrogen bond to the most acidic proton and the average hydrogen bond.

On the physical origin of the cation–anion intermediate bond in ionic liquids Part I. Placing a (weak) hydrogen bond between two charges

The intermediate bond forces in ionic liquids are investigated from static quantum chemical calculations at various methods and two basis sets. The experimentally observed red-shift of the donor-proton bond stretching frequency due to a bond elongation is confirmed by all methods. Comparing Hartree-Fock to second-order Møller-Plesset perturbation theory, the Hartree-Fock method gives in many cases an erroneous description of the geometries. Furthermore, the Hartree-Fock interaction energies can deviate up to 60 kJ mol(-1) from Møller-Plesset perturbation theory indicating the importance of dispersion interaction. While the usual trends of decreasing stability or interaction energies with increasing ion sizes are found, the geometries involving hydrogen atoms do not change this order of total interaction energies. Therefore, the hydrogen bond is not the most important interaction for ion pairs with regard to the total interaction energy. On the other hand, the different established analysis methods give rise to hydrogen bonding in several ion pairs. Charge analysis reveals the hydrogen-bonding character of the ion pair and shows, depending on the type of ions combined and further on the type of conformers considered, that a hydrogen bond can be present. The possibility of hydrogen bonding is also shown by an analysis of the frontier orbitals. Calculating potential energy surfaces and observing from this the change in the donor proton bond indicates that regular hydrogen bonds are possible in ion pairs of ionic liquids. Thereby, the maximum of bond elongation exceeds the one of a usual hydrogen bond by far. The more salt-like hydrogen-bonded ion pair [NH(4)][BF(4)] exhibits a steeper maximum than the more ionic liquid like ion pair [EtNH(3)][BF(4)]. The fact that imidazolium-based ionic liquids as [Emim][Cl] can display two faces, hydrogen bonding and purely ionic bonding, points to a disturbing rather than stabilizing role of hydrogen bonding on the interaction of the counterions in imidazolium-based ionic liquids. While geometry and charge analysis provides attributes of weak (blue-shifted) hydrogen bonds, large bond elongations accompanied by red-shifts are obtained for the ion pairs investigated. This can be understood by the simple fact that these imidazolium-based ionic liquid ion pairs constitute weak hydrogen bonds placed between two delocalized charges.

Why are ionic liquid ions mainly associated in water? A Car-Parrinello study of 1-ethyl-3-methyl-imidazolium chloride water mixture.

In this study we present the results of a first principles molecular dynamics simulation of a single 1-ethyl-3-methyl-imidazolium chloride [C(2)C(1)im][Cl] ion pair dissolved in 60 water molecules. We observe a preference of the in plane chloride coordination with respect to the cation ring plane as compared to the energetic slightly more demanding on top coordination. Evaluation of the different radial distribution functions demonstrates that the structure of the hydration shell around the ion pair differs significantly from bulk water and that no true ion pair dissociation in terms of completely autonomous solvation shells takes place on the timescale of the simulation. In addition, dipole moment distributions of the solvent in distinct solvation shells around different functional parts of the [C(2)C(1)im][Cl] ion pair are calculated from maximally localized Wannier functions. The analysis of these distributions gives evidence for a depolarization of water molecules close to the hydrophobic parts of the cation as well as close to the anion. Examination of the angular distribution of different OH(H(2)O)-X angles in turn shows a linear coordination of chloride accompanied by a tangential orientation of water molecules around the hydrophobic groups, being a typical feature of hydrophobic hydration. Based on these orientational aspects, a structural model for the obvious preference of ion pair association is developed, which justifies the associating behavior of solvated [C(2)C(1)im][Cl] ions in terms of an energetically favorable interface between the solvation shells of the anion and the hydrophobic parts of the cation.

Structure and dynamics of the protic ionic liquid monomethylammonium nitrate ([CH3NH3][NO3]) from ab initio molecular dynamics simulations

The dynamics of the protic ionic liquid monomethylammonium nitrate is investigated by Car-Parrinello molecular dynamics simulations. On average, 1.8 of 3 possible hydrogen bond contacts are formed. Therefore, one hydrogen bond acceptor and one donor site in each ion pair of monomethylammonium nitrate remains free, which is similar to water. Furthermore, like water, monomethylammonium nitrate exhibits a fast fluctuating hydrogen bond network. The comparable hydrogen bond network and dynamics of both liquids might explain the similar impact on reactivity and selectivity found for chemical reactions. However, the hydrogen bond network of monomethylammonium nitrate and water show some structural differences. While the hydrogen bonds in water arrange in parallel fashion, the hydrogen bonds of monomethylammonium nitrate prefer angles of 0 degrees, 90 degrees, and 180 degrees. The ion dynamics of monomethylammonium nitrate indicate that at about 85% of the ion pairs are still connected after 14.5 ps. A closer inspection of the first solvation shell dynamics of one cation reveals that after 11 ps the current ion pair conformation is independent of the initial ion pair conformation because the ion pairs lose their information of the initial ion pair conformation much faster than the time needed to escape from their solvent cage. The ion dynamics of monomethylammonium nitrate can be described by the following model: There are ions rattling in long living cages which are formed by long living ion pairs.

What keeps ionic liquids in flow

The elimination of a hydrogen bond in imidazolium based ionic liquids which results in an increased melting point is investigated by means of static quantum chemical and molecular dynamics simulations.