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Mixing weak acids and bases can produce highly complicated binary mixtures, called pseudo‐protic ionic liquids, in which a complex network of effects determines the physicochemical properties that are currently impossible to predict. In this joint computational‐experimental study, we investigated 1‐methylimidazole‐acetic acid mixtures through the whole concentration range. Effects of the varying ionization and excess of either components on the properties, such as density, diffusion coefficients, and overall hydrogen bonding structure were uncovered. A special emphasis was put on understanding the multiple factors that govern the conductivity of the system. In the presence of an excess of acetic acid, the 1‐methylimidazolium acetate ion pairs dissociate more efficiently, resulting in a higher concentration of independently moving, conducting ions. However, the conductivity measurements showed that higher concentrations of acetic acid improve the conductivity beyond this effect, suggesting in addition to standard dilution effects the occurrence of Grotthuss diffusion in high acid‐to‐base ratios. The results here will potentially help designing novel electrolytes and proton conducting systems, which can be exploited in a variety of applications. [ABSTRACT FROM AUTHOR]

Ionic liquids raise interesting but complicated questions for theoretical investigations due to the fact that a number of different inter-molecular interactions, e.g., hydrogen bonding, long-range Coulomb interactions, and dispersion interactions, need to be described properly. Here, we present a detailed study on the ionic liquids ethylammonium nitrate and 1-ethyl-3-methylimidazolium acetate, in which we compare different dispersion corrected density functional approximations to accurate local coupled cluster data in static calculations on ionic liquid clusters. The efficient new composite method B97-3c is tested and has been implemented in CP2K for future studies. Furthermore, tight-binding based approaches which may be used in large scale simulations are assessed. Subsequently, ab initio as well as classical molecular dynamics simulations are conducted and structural analyses are presented in order to shed light on the different short- and long-range structural patterns depending on the method and the system size considered in the simulation. Our results indicate the presence of strong hydrogen bonds in ionic liquids as well as the aggregation of alkyl side chains due to dispersion interactions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present an efficient implementation for the calculation of maximally localized Wannier functions (MLWFs) during parallel Car-Parrinello molecular dynamics simulations. The implementation is based on a block Jacobi method. The calculation of MLWFs results in only a moderate (10%–20%) increase in computer time. Consequently it is possible to calculate MLWFs routinely during Car-Parrinello simulations. The Wannier functions are then applied to derive molecular dipole moments of dimethyl sulfoxide (DMSO) in gas phase and aqueous solution. We observe a large increase of the local dipole moment from 3.97 to 7.39 D. This large solvent effect is caused by strong hydrogen bonding at the DMSO oxygen atom and methyl groups. Decomposing the dipole moment into local contributions from the S-O bond and the methyl groups is used to understand the electrostatic response of DMSO in aqueous solution. A scheme is given to derive charges on individual atoms from the MLWFs using the D-RESP methodology. The charges also display large solvent effects and give insight into the transferability of recent force field models for DMSO. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Wepresent a first principles molecular dynamics study of carbondioxide solvation by protic ionic liquids using ethylammonium nitrateas an example solvent. Microheterogeneity of the alkyl chains andthe extended hydrogen bond network could be observed. Thus, the entirestructure of the investigated protic ionic liquid mixed with CO2closely resembles the one of the pure liquid. Our data indicatesthat CO2most likely creates an energy loss due to enteringthe liquid via the too-small voids. But this is fully compensatedby specific attractive interactions of CO2with the cationand anions of ethylammonium nitrate. This result might serve as anexplanation for the question of why the volume of the ionic liquidis not increasing through CO2uptake. The CO2cluster formation, which shows a structure similar to supercriticalCO2, is guided by the dominance of the nonpolar groupsin the CO2solvation shell. [ABSTRACT FROM AUTHOR]

The solvation of the carbene 1-ethyl-3-methylimidazole-2-ylidene in the ionic liquid 1-ethyl-3-methylimidazolium acetate was investigated by ab initio molecular dynamics simulations in order to reveal the interaction between these two highly important classes of materials: N-heterocyclic carbenes with superb catalytic activity and ionic liquids with advantageous properties as solvents and reaction media. In contrast to previously published data on analogous systems, no hydrogen bond is observed between the hypovalent carbon atom and the most acidic ring hydrogen atoms, as these interaction sites of the imidazolium ring are predominantly occupied by the acetate ions. Keeping the carbene away from the ring hydrogen atoms prevents stabilization of this reactive species, and hence any retarding effect on subsequent reactions, which explains the observed high reactivity of the carbene in acetate-based ionic liquids. Instead, the carbene exhibits a weaker interaction with the methyl group of the imidazolium cation by forming a hitherto unprecedented kind of C⋅⋅⋅HC hydrogen bond. This unexpected finding not only indicates a novel kind of hydrogen bond for carbenes, but also shows that such interaction sites of the imidazolium cation are not limited to the ring hydrogen atoms. Thus, the results give the solute-solvent interactions within ionic liquids a new perspective, and provide a further, albeit weak, site of interaction to tune in order to achieve the desired environment for any dissolved active ingredient. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation of amide pseudorotaxane IA spectra in the harmonic approximation. In particular, we focus on the effect of axle substitution on the hydrogen bonds that are formed between axle and wheel. Two types of pseudorotaxanes are studied: one with the substituent affecting mostly the axle's carbonyl group and one with the effect influencing primarily the amide NH group. Sizeable red shifts are predicted for the carbonyl stretching frequencies, and large red shifts for the NH stretching frequencies. For the wheel amide groups involved in hydrogen bonding merely with their NH hydrogens, a small shift is observed for the carbonyl stretch mode. A clear relation is observed between the NH stretch shifts and individual hydrogen bond energies. This is confirmed by correlations of the shared electron number with the NH stretch shift showing that this quantity can be taken as an indicator for individual hydrogen bond energies. Axle substitution influences the strengths of the individual hydrogen bonds which is again reflected in the NH stretch frequency shifts. A linear relationship of Hammett's substituent parameters with the NH freciuency shifts can be established. [ABSTRACT FROM AUTHOR]

Abstract: It is the aim of this work to elucidate the usefulness and feasibility of the first-principles approach and to extend it to the regime of liquid molecular substances of complex structure. Physical and thermodynamic properties of complicated liquids are investigated by means of Car–Parrinello molecular dynamics (CPMD) and also with static quantum chemical methods. The connection between the dynamic and static approach is given by the quantum cluster equilibrium (QCE) theory. Since the QCE theory is not yet well established, a new implementation in the MD post-processing program PEACEMAKER is presented. It can be shown that it is by far more important to include cooperative effects rather than to concentrate the effort on the inclusion of weak dispersion forces not present in current density functionals. Traditionally, investigations of complicated liquids were also undertaken with the tools of simple liquids, because for some problems the size of the system does not allow for a more accurate description. Although linear-scaling techniques are simplifications from the point of view of quantum chemistry, they might be severe improvements when compared to traditional molecular dynamics simulations. For the interpretation of the liquid state the introduction of local properties is inevitable. New methods are presented for the calculation of local dipole moments and for the estimation of hydrogen bond energies in quantum mechanically nondecomposable systems. The latter also allows for the detection of hydrogen bonds in simulations through a wavefunction-based criterion instead of one which is solely grounded on the geometric structure of the atomic nuclei involved. The article then discusses prominent liquids which show properties that are not yet understood. Another part of the work analyzes the effect of solvent molecules on solutes and their reactions in the solvent. Finaly, neoteric solvents, such as ionic liquids are discussed. [Copyright &y& Elsevier]

A biomimetic complex which mimics the arginine—phosphonate diester interaction of the arginine fork is investigated with respect to structure and energetics of stable configurations. Within this work, we provide knowledge on local minima of the isolated system obtained from first-principles calculations. Non-negligible solvation effects are studied in a microsolvation approach. The interactions which govern the structural patterns of molecular recognition in this tweezer—guest complex can be significantly modulated by the action of hydrogen bond accepting and donating solvent molecules, such as dimethyl sulfoxide or water, which were present in experimental investigations on this system. Different tweezer—guest structures are evaluated with respect to their temperature-dependent thermodynamical properties as products of the first association reaction step of the bisphosphonate tweezer template and the guanidinium moiety. [ABSTRACT FROM AUTHOR]

Examines the hydrophobic interactions of the methyl groups with water. Involvement of the water molecule atoms in the hydrogen bond network; Classification of hydrogen-acceptor interactions; Structures of dimethyl sulfoxide-water cluster.

Here we report the first structural and dynamic investigation on ethylammonium nitrate, a representative protic Ionic liquid, and dimethylsulfoxide. By using joined x/ray and neutron diffraction, we exploit the EPSR approach to extract structural information at atomistic level. EAN/DMSO turns out to be homogeneous at microscopic scales and indications for the existence of a structural leit motiv with stoichiometric composition 2DMSO:1EAN are found. Dielectric spectroscopy is used to access the relaxation map of the DMSO:EAN = 60:40 mixture. No crystallisation is detected and three relaxation processes could be characterised. Overall this study provides new indications of strict analogies between water and ethylammonium nitrate. [ABSTRACT FROM AUTHOR]

Deep eutectic solvents (DESs) are mixtures of ionic compounds and molecular hydrogen bond donors. Due to the many components and their different interacting subgroups, they give rise to a plethora of many different interactions which can be studied by ab initio molecular dynamics simulations, because within this method all the forces are calculated on the fly and no parametrization prior to the calculation is necessary. Since DESs can be applied in gas capture, for example for SO 2 absorption, we performed ab initio molecular dynamics studies of both the pure choline chloride/glycerol DES and the same mixed with SO 2 . We identified the hydrogen bonding and other specific interactions between all components. With addition of SO 2 , we observed a decrease in the anion-OH group interplay, because the chloride anions form complexes with the SO 2 molecules. Furthermore, the SO 2 molecules are incorporated into the hydrophobic network and the interaction between the hydrogen bonds of all OH groups remain constant. This decrease of anion-OH interaction might be responsible for the more fluid state of the SO 2 -DESs mixture than the pure DES. [ABSTRACT FROM AUTHOR]

• α -amino amidines are in structure similar to their a-amino acid analogues. • Dipole moments are highly dependent on state and conformation of a molecule. • α -amino amidines in water are in an equilibrium between neutral and protonated state. • Amidine and carboxylic acid show significant differences in electronic structure. Among hypothetical alternative biochemistries, ammonia based life is one of the more promising ideas to investigate, due to the many similarities of water and ammonia as well as the existence of various ammonia based equivalents to prominent biomolecules. In this scenario α -amino amidines form the analogues to the α -amino acids, which can also be classified as the building blocks of life. Comparison of basic solvation behaviour between α -amino amidines and α -amino acids can provide insight into general chemical properties. To better understand the solvation of these compounds, we conducted ab initio molecular dynamics simulations for alanine, leucine, 2-aminopropionamidine and 2-amino-4-methylpentanamidine dissolved in 128 water molecules. We started simulations both from the neutral as well as from the zwitterionic form. For the α -amino amidines we observed protonation of the amidine group by water. Amino acids do not change spontaneously between the two states in our simulations. The molecular dipole moment is highly influenced by the state and conformation of the molecule and is a key property to trace in the course of the simulation. Structural analyses demonstrate the interactions of the compound with the surrounding solvent, solvation shells and hydrogen bonding. [ABSTRACT FROM AUTHOR]

Review: 85 refs. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations with an all-atom model were carried out to study the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. 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. [ABSTRACT FROM AUTHOR]