A subtle difference: In ionic liquids the “weak” dispersion forces have a significant impact on the shape of the potential energy surface, which results in a shallow profile when all of the ...contributions are considered. Such findings are commonly accepted to determine the liquid state.
We carried out classical molecular dynamics simulations with a standard and two quantum chemistry based charge sets to study the ionic liquid 1-n-butyl-3-methylimidazolium bromide, C4C1imBr. We split ...the cation up into different charge groups and found that the total charge and the charge distribution in the imidazolium ring are completely different in the three systems while the total charge of the butyl chain is much better conserved between the methods. For comparison, the spatial distribution functions and the radial distribution functions as well as different time correlation functions were calculated. For the structural properties we obtained a good agreement between the standard and one of the two quantum chemistry based sets, while the results from the second quantum chemistry based set led to a completely different picture. The opposite was observed for the dynamic properties, which agree well between the standard set and the second quantum chemistry based set, whereas the dynamics in the first charge set obtained by quantum chemistry calculations proceeded much too slow, which is not obvious from the total charge. We observed, that the structure of the butyl chain is mostly unaffected by the choice of the charge set. This is an indirect proof for separation into ionic parts and nonpolar domains. A second focus of the article is the investigation of dynamical heterogeneity and the ion cages. Therefore, we analyzed the reorientational dynamics in the three systems and at five different temperatures in system with the standard charge set. Generally speaking, we detected four different time domains. The fastest movement can be found for the continuous hydrogen bond and the nearest neighbor ion pair dynamics. In the second time domain the movement of the butyl chain took place. The third time domain consisted in the increasing movement of the imidazolium ring as well as in the continuous distortion of an ion cage, i.e., the departure of one of the several counterions from the central ion’s first shell, and the intermittent hydrogen bond dynamics. The remaining domain involves the translational displacement of the ions.
Ten C8C1Im+ (1‐methyl‐3‐octylimidazolium)‐based ionic liquids with anions Cl−, Br−, I−, NO3−, BF4−, TfO−, PF6−, Tf2N−, Pf2N−, and FAP− (TfO=trifluoromethylsulfonate, ...Tf2N=bis(trifluoromethylsulfonyl)imide, Pf2N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two C8C1C1Im+ (1,2‐dimethyl‐3‐octylimidazolium)‐based ionic liquids with anions Br− and Tf2N− were investigated by using X‐ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While 1H NMR spectroscopy is found to probe very specifically the strongest hydrogen‐bond interaction between the hydrogen attached to the C2 position and the anion, a comparative XPS study provides first direct experimental evidence for cation–anion charge‐transfer phenomena in ionic liquids as a function of the ionic liquid’s anion. These charge‐transfer effects are found to be surprisingly similar for C8C1Im+ and C8C1C1Im+ salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.
Charges in charged systems: Anion and cation interactions in ionic liquids strongly modify the charges on the ions as proven by XPS, NMR spectroscopy and DFT calculations. The smaller, more basic and, thus, more strongly coordinating the anion, the more pronounced the charge transfer to the cation (see graphic). Hydrogen‐bonding‐type interactions between anion and cation do not significantly influence the amount of charge transferred.
We present a comprehensive density functional study, using the Perdew–Burke–Ernzerhof (PBE) functional, to elucidate the effect of including or neglecting the dispersion correction on the structure ...and dynamics of the ionic liquid 1‐ethyl‐3‐methylimidazolium thiocyanate. We have investigated the structure of the liquid phase and observed that specific interactions between the anions and cations of the ionic liquid were not accurately represented if the dispersion was neglected. The dynamics of the system is more accurately described if the dispersion correction is taken into account and its omission also leads to an incorrect representation of the hydrogen‐bonding dynamics. Finally, the power spectrum is predicted and in good agreement with experimental results. Thus, we conclude that it is possible to represent the structure and dynamics of systems containing ionic liquids accurately using ab initio molecular dynamics and a correction for dispersion.
To PBE or to PBE‐D, that is the question! A comprehensive density functional study, using the Perdew–Burke–Ernzerhof (PBE) functional, to elucidate the effect of including or neglecting the dispersion correction on the structure and dynamics of the ionic liquid 1‐ethyl‐3‐methylimidazolium thiocyanate is presented.
Estimating the Hydrogen Bond Energy Wendler, Katharina; Thar, Jens; Zahn, Stefan ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
09/2010, Letnik:
114, Številka:
35
Journal Article
Recenzirano
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.
Employing first-principles molecular dynamics simulations, we characterize the structural and dynamical hydrogen bonding in the ionic liquid C2C1imSCN. The geometric picture indicates a superior role ...for the most acidic hydrogen bond (at H2) as compared to the two other hydrogen atoms at the rear. Despite the structural picture, the hydrogen bond dynamics at H2 is found to decay faster than the according dynamics at the H4 and H5 proton. Neglecting the directionality provides a dynamics which reflects the geometrical analysis. Two movements are identified. First, a fast (<0.3 ps) hopping of the anion above and below the imidazolium ring and second translational motion of the anion away from the cation in-plane of the imidazolium ring (5−10 ps).
In order to test the validity of the cluster ansatz approach as well as of the continuum model approach and to learn about the solvation shell, we carried out first-principles molecular dynamics ...simulations of the alanine hydration. Our calculations contained one alanine molecule dissolved in 60 water molecules. Dipole moments of individual molecules were derived by means of maximally localized Wannier functions. We observed an average dipole moment of about 16.0 D for alanine and of about 3.3 D for water. In particular, the average water dipole moment in proximity of alanine's COO- group decayed continously with increasing distance, while, surprisingly, close to the CH3 and NH3 + group, the dipole moment first rose before its value dropped. In a cluster ansatz approach, we considered snapshots of alanine surrounded by different water molecule shells. The dipole moments from the cluster approaches utilizing both maximally localized Wannier functions as well as natural population analysis served to approximate the dipole moments of the total trajectory. Sufficient convergence of the cluster ansatz approach is found for either of the two solvent shells around the polar groups and one solvent shell around the apolar groups or two solvent shells around the polar groups surrounded by a dieletric continuum.
Hydrogen Bond Detection Thar, Jens; Kirchner, Barbara
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
03/2006, Letnik:
110, Številka:
12
Journal Article
Recenzirano
In this Article we extend the idea of detecting a hydrogen bond solely on one single quantum chemically determined descriptor. We present an improvement of the method introduced by Reiher et al. ...(Theor. Chim. Acta 2001, 106, 379), who mapped the strength of the hydrogen bond onto an easily accessible quantity, namely, the two-center shared-electron number σHA. First, we show that the linear dependence between the interaction energy from the supermolecular approach and σHA is valid for a test set of about 120 hydrogen-bonded complexes. Furthermore, we demonstrate that a classification according to acceptor atoms of the hydrogen-bonded complexes can give more accurate results. We thus recommend to detect hydrogen bonds with a specific acceptor atom according to our subset linear regression analysis. Case studies on alcohols and isolated base pairs and trimers from RNA and DNA show the utility of the detection criterion. The shared-electron number method yields that the strength of the N1···N3 hydrogen bond is in the range of 30 kJ/mol. Furthermore the A−U pair is indeed stronger bound than the A−T complex if environmental effects are incorporated in the calculations.
In this article we review the behavior of static plane wave basis set calculations in comparison to Gaussian basis set calculations. This was done in the framework of density functional theory for ...description of hydrogen bonds with the water dimer as an example. Furthermore we carried out molecular dynamics simulations enforcing the self-dissociation reaction of the water dimer to study the influence of the basis set onto the reaction. Not surprisingly, we find strongly varying results of the calculated forces for a chosen cutoff along the reaction coordinates. The basis set superposition errors of the dimer interaction energy are analyzed along the free-energy surface, i.e., along the trajectories. Based on the analysis along the trajectories a qualitative and quantitative estimate depending on the particular point of the free-energy surface can be provided. Namely, at the intermolecular O···H distance close to the equilibrium geometry the errors are smaller than at shorter O···H distances. However, the distribution at the equilibrium distance is more unsymmetrical than the distribution at short distances. It is wider, and the standard deviation is larger than at shorter distances where the basis set superposition error is larger.
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