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To understand the behavior of ionic liquids (ILs) at carbon material, i.e., carbon nanotube (CNT)-containing pores, we simulated different systems and analyzed their structural─in particular their coordination─behavior as well as their velocity distribution. The extension of our analysis tool CONAN presented here allowed us to study the coordination behavior as a function of the distance to the carbon material. Our systems were composed of three different CNTs combined with either the neat IL 1-ethyl-3-methylimidazolium tetrafluoroborate or with their NaBF4salt mixtures. We investigated the impact of the force field charge scaling. As previously detected, the neat IL assumed radial layers within the confinement, with the radial density distribution depending strongly on the pore size. For the salt mixtures, the sodium cation remained in the bulk and was observed only once inside a tube. In all systems, the ions showed an overall decreased coordination behavior for regions in the bulk phase close to the carbon pore and within the confinement. The coordination number was always reduced with scaled charges. For charge scaling, higher dynamics was observed also in confinement. Interestingly, the average velocity of the atoms near the surface inside the confined space was higher than that in the center of the pore.

A GPU-based implementation of the Hybrid Monte Carlo (HMC) algorithm is presented to explore its utility in the chemistry of solidification at the example of liquid to solid argon. We validate our implementation by comparing structural characteristics of argon fluid-like phases from HMC and MD simulations. Examining solidification, both MD and HMC show similar trends. Despite observable differences, MD simulations and HMC agree within the errors during the phase transition. Introducing voids decreases the solidification temperature, aiding in the formation of a well-structured solids. Further, our findings highlight the importance of larger system sizes in simulating solidification processes. Simulations with a temperature dependent potential show ambiguous results for the solidification which may be attributed to the small system sizes. Future work aims to expand HMC capabilities for complex chemical phenomena in phase transitions.

In this work, we present a new theory to treat multicomponent liquids based on quantum-chemically calculated clusters. The starting point is the binary quantum cluster equilibrium theory, which is able to treat binary systems. The theory provides one equation with two unknowns. In order to obtain another linearly independent equation, the conservation of mass is used. However, increasing the number of components leads to more unknowns, and this requires linearly independent equations. We address this challenge by introducing a generalization of the conservation of arbitrary quantities accompanied by a comprehensive mathematical proof. Furthermore, a case study for the application of the new theory to ternary mixtures of chloroform, methanol, and water is presented. Calculated enthalpies of vaporization for the whole composition range are given, and the populations or weights of the different clusters are visualized.

Modeling of complex liquids at solid surfaces and in confinement is gaining attention due to an increase in computer power and advancement of simulation techniques. Therefore, tools to set up structures and for analysis are needed. In this paper, we present CONAN─a Python code designed to facilitate the study of liquids interacting with solid structures, such as walls or pores. Among other things, the program provides the option to generate a variety of different structures, including carbon walls and nanotubes and their boron nitride analogs, as well as the ability to analyze various structural properties of confined and interfacial liquids. In the case of the ionic liquid 1-butyl-3-methylimidazolium acetate in carbon nanotubes of different sizes, we demonstrate the abilities of our tool. The average density within the confinement highly depends on the carbon nanotube size, and it is generally lower than the density of the bulk liquid. The arrangement of the individual species within the tube also depends on size, with radial layers forming within the tubular confinement. The density is largely increased in the respective layers, while it is drastically reduced between the layers.

We present a novel cluster analysis implemented in our open-source software TRAVIS and its application to realistic and complex chemical systems. The underlying algorithm is exclusively based on atom distances. Using a two-dimensional model system, we first introduce different cluster analysis functions and their application to single snapshots and trajectories including periodicity and temporal propagation. Using molecular dynamics simulations of pure water with varying system size, we show that our cluster analysis is size-independent. Furthermore, we observe a similar clustering behavior of pure water in classical and ab initio molecular dynamics simulations, showing that our cluster analysis is universal. In order to emphasize the application to more complex systems and mixtures, we additionally apply the cluster analysis to ab initio molecular dynamics simulations of the [C2C1Im][OAc] ionic liquid and its mixture with water. Using that, we show that our cluster analysis is able to analyze the clustering of the individual components in a mixture as well as the clustering of the ionic liquid with water.

By simulating butan-2-ol dissolved in the chiral ionic liquid 1-ethyl-3-methylimidazolium (S)-alaninate, we investigate the chiral recognition of butan-2-ol in the ionic liquid. The hydrogen bonding between the chiral anion and both enantiomers of butan-2-ol is similar; however, both chiral molecules (anion and alcohol) induce an asymmetry in the achiral cation which leads to a more favorable environment for the alcohol in the heterochiral system as compared to the homochiral system and hence provides an energetic stabilization of the former.

A characteristic feature of ionic liquids is their nanosegregation, resulting in the formation of polar and nonpolar domains. The influence of increasing the alkyl side chain on the morphology of ionic liquids has been the subject of many studies. Typically, the polar network (charged part of the cation and anion) constitutes a continuous subphase that partially breaks to allow the formation of a nonpolar domain with the increase of the alkyl chain. As the nonpolar network expands, the number of tails per aggregate increases until the ionic liquid percolates. In this work, we demonstrate how the complementary software packages TRAVIS and AGGREGATES can be employed in conjunction to gain insights into the size and morphology of the [CnC1Im]Cl family, with n∈ {2, 4, 6, 8, 10, 12}. The combination of the two approaches rounds off the picture of the intricate arrangement and structural features of the alkyl chains.

Ab initio molecular dynamics simulations at elevated temperature are carried out to investigate the microscopic structure of liquid mixtures (deep eutectic solvents) composed of 1:1 and 1:2 choline chloride:ethylene glycol ([Ch]Cl:EG) and 1:2:1 choline chloride:ethylene glycol:water ([Ch]Cl:EG:water). In the present study, we aim to understand the composition effect on the choline chloride:ethylene glycol deep eutectic solvent and whether there is a specific composition in these solvents with marked special behavior at the microscopic level. The role of hydrogen bonds between all components was investigated through distribution functions. The structures are governed by the balance of hydrogen bond networks and the different populations of the EG molecule conformations. In the water-containing system, water competes for association with the anion. Also, the charge distribution analysis, which is consistent with structural analysis, indicates that the results are not impacted by changing composition. In addition, the charge transfer observed between ions, EG, and water molecules appears to be significant.

We present a cluster-weighting approach for calculating the vibrational circular dichroism (VCD) spectra of bulk phase systems. Based on the quantum cluster equilibrium theory, cluster populations are received via self-consistent field calculations that allow a mixing of differently sized oligomers at the same time. The thereby obtained cluster weights are employed for the weighting of individual cluster VCD spectra in order to calculate an overall gas or bulk phase VCD spectrum. As the mixing of different oligomer sizes is possible, different structural motifs and interactions can be included, and explicit solvation, typically necessary for describing hydrogen bonds, is intrinsically taken care of, however, without neglecting monomeric structures and different conformers. We test the sensitivity of Boltzmann and cluster weights with respect to the level of theory employed and show that cluster weights are less sensitive. By a constant increase of the oligomer sizes included in our cluster sets, combined with a continuous truncation of low populated clusters, we are able to improve the agreement between theory and experiment until we reach an overlap which allows a certain assignment of the absolute configuration based on an experimental bulk phase spectrum. Combined with a computationally cost-efficient level of theory, this approach provides a valuable tool for the fast calculation of VCD spectra for bulk phase systems.

Both hydrogen bonding and ionic interactions have an extreme influence on the properties of all kinds of materials. Ionic liquids mostly possess both of them. In the present work, we calculate with the aid of the reactive flux theory the dynamics of hydrogen bonds as well as of the ionic network for different 1-butyl-3-methylimidazolium-based ionic liquids. To apply this method, we carry out molecular dynamics simulations over broad temperature and simulation parameter ranges. Hydrogen bond lifetimes vary with temperature in the range of 10 ps (400 K)–60 ps (300 K) for the more fluid systems to 60 ps (400 K)–2000 ps (300 K) for the more viscous systems. The ion pair dynamics behave differently. While the more fluid systems show ion pairs which possess a factor of 4 longer lifetimes than the hydrogen bonds, the ion pair lifetimes of the more viscous systems actually are a factor of 0.6 smaller and thus faster than the hydrogen bond lifetimes. Although the choice of the hydrogen bond angle is less sensitive, a deviation in the distance criterion of up to 5% from the radial pair distribution functions’ minimum leads to a change of less than 10% in the calculated lifetime. The ion pair dynamics are less sensitive with respect to the variation of the geometrical parameter. Interestingly, for both kinds of dynamics the impact on the activation energy is even less prone to the different cutoff values than the lifetime. In fact, the cutoff for the distance criteria causes the activation energies to remain almost unchanged. We find that the stability of the method is sufficient if the simulation time is at least a factor of 10 longer than the corresponding lifetime.

In this work, we assess the fundamental aspects of mutual miscibility of solvents by studying the mixing of two potential candidates, methanol and n-dodecane, for nonaqueous solvent extraction. To do so, 1H NMR spectroscopy and molecular dynamics simulations are used jointly. The NMR spectra show that good phase separation can be obtained by adding LiCl and that the addition of a popular extractant (tri-n-butyl phosphate) yields the opposite effect. It is also demonstrated that in a specific case the poor phase separation is not due to the migration of n-dodecane into the more polar phase, but due to the transfer of the extractant into it, which is especially relevant when considering industrial applications of solvent extraction. With the aid of molecular dynamics simulations, explanations of this behavior are given. Specifically, an increase of all hydrogen-bond lifetimes is found to be consequent to the addition of LiCl which implies an indirect influence on the methanol liquid structure, by favoring a stronger hydrogen-bond network. Therefore, we found that better phase separation is not directly due to the presence of LiCl, but due to the “hardening” of the hydrogen-bond network.

A thorough analysis of molecular vibrations in the binary system hydrogen chloride/water is presented considering a set of small mixed and pure clusters. In addition to the conventional normal-mode analysis based on the diagonalization of the Hessian, anharmonic frequencies were obtained from the perturbative VPT2 and PT2-VSCF method using hybrid density functional theory. For all normal modes, potential energy curves were modeled by displacing the atoms from the minimum geometry along the normal mode vectors. Three model potentials, a harmonic potential, a Morse potential, and a fourth order polynomial, were applied to fit these curves. From these data, it was possible not only to characterize distinct vibrations as mainly harmonic, anharmonic, or involving higher order terms but also to extract force constants, k, and anharmonicity constants, xe. By investigating all different types of intramolecular vibrations including covalent stretching or bending vibrations and intermolecular vibrations such as librations, we could demonstrate that while vibrational frequencies can be obtained applying scaling factors to harmonic results, useful anharmonicity constants cannot be predicted in such a way and the usage of more elaborate vibrational methods is necessary. For each particular type of molecular vibration, we could however determine a relationship between the wavenumber or wavenumber shift and the anharmonicity constant, which allows us to estimate mode dependent anharmonicity constants for larger clusters in the future.

We present a new method to study position-dependent, anisotropic diffusion tensors inside spherically confined systems—a geometry that is common to many chemical nanoreactors. We use this method to elucidate the surprisingly rich solvent dynamics of confined water. The spatial variation of the strongly anisotropic diffusion predicted by the model agrees with the results of explicit molecular dynamics simulations. The same approach can be directly transferred to the transport of solutes to and from reaction sites located at nanoreactor interfaces. We complement our study by a detailed analysis of water hydrogen bond kinetics, which is intimately coupled to diffusion. Despite the inhomogeneity in structure and translational dynamics inside our nanocages, a single set of well-defined rate constants is sufficient to accurately describe the kinetics of hydrogen bond breaking and formation. We find that once system size effects have been eliminated, the residence times of water molecules inside the coordination shell of a hydrogen bond partner are well correlated to average diffusion constants obtained from the procedure above.

In the framework of the binary quantum cluster equilibrium theory, we introduce a cluster approach to access activity coefficients of binary mixtures. This approach allows derivation of activity coefficients based on quantum chemically calculated clusters. The cluster sets in this work comprise clusters ranging in size from one to six molecules of either a single or two species. For each cluster size up to five conformers if detectable are considered such that important liquid motifs are included. Via self-consistent-field calculations the binary quantum cluster equilibrium theory gives the Gibbs energies and thus the excess Gibbs energies of mixing. Derivation with respect to the particle number allows access to activity coefficients. To achieve an analytical expression, we apply the standard approach of fitting a Redlich–Kister polynomial to the excess Gibbs energy and calculate its derivative, which leads to good results for the binary mixture cases of acetonitrile/benzene and methanol/ethanol, as well as satisfying results for acetone/chloroform. Very good results for vaporization enthalpies are obtained for the pure substances.

Lithium bis(trifluoromethanesulfonyl)imide (LiNTf2) doped ionic liquids (ILs) are investigated herein, as potential electrolytes for lithium-ion batteries, via scaled-charge molecular dynamics simulations. Four model ILs based on the [NTf2]−anion and heterocyclic ammonium cations were studied with varying concentrations, ranging from 0 to 1 M solutions, of the dissolved lithium salt. The pyrrolidinium ([pyrHH]+), piperidinium ([pipHH]+), N-butyl-pyrrolidinium ([pyrH4]+), and N-butyl-N-methyl-pyrrolidinium ([pyr14]+) cations were considered to evaluate the combined effects of increased ring size, as well as the introduction of apolar groups on the nitrogen atom of the cations, on the liquid structure properties of the electrolytes. Among the investigated ILs, [pyr14][NTf2] is the only aprotic IL allowing for a comparison of protic and aprotic ILs. The lithium coordination shell is seen to be quite different in the various IL-based systems; networks of lithium ions bridged by [NTf2]−ions have interesting consequences on the solvation shells and coordination numbers. Aggregate existence and velocity autocorrelation functions are finally evaluated in order to characterize the caging effect of [NTf2]−ions around lithium ions. In conclusion, we find that the lithium mobility and transport are directly proportional to the strength of the interionic interactions within the liquids, whereas the ease of solvation shows opposite trends.

We present an updated version of Peacemaker, a software package for performing neat and binary Quantum Cluster Equilibrium (QCE, bQCE) calculations. Peacemaker2 has been rewritten from scratch and is fast, robust, parallelized, and well-documented. Apart from sketching the implementation and introducing several improvements to bQCE theory, we calculate excess enthalpies of mixing of two selected amide–water binary mixtures to give an illustrative example of an application.