Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations

Description of the local microscopic structure in ionic liquids (ILs) is a prerequisite to obtain a comprehensive understanding of the influence of the nature of ions on the properties of ILs. The local structure is mainly determined by the spatial arrangement of the nearest neighboring ions. Therefore, the main interaction patterns in ILs, such as cation–anion H-bond-like motifs, cation–cation alkyl tail aggregation, and ring stacking, were considered within the framework of the nearest-neighbor approach with respect to each particular interaction site. We employed classical molecular dynamics (MD) simulations to study in detail the spatial, radial, and orientational relative distribution of ions in a set of imidazolium-based ILs, in which the 1-butyl-3-methylimidazolium (C₄mim⁺) cation is coupled with the acetate (OAc^–), chloride (Cl^–), tetrafluoroborate (BF₄^–), hexafluorophosphate (PF₆^–), trifluoromethanesulfonate (TfO^–), or bis­(trifluoromethanesulfonyl)­amide (TFSA^–) anion. It was established that several structural properties are strongly anion-specific, while some can be treated as universally applicable to ILs, regardless of the nature of the anion. Namely, strongly basic anions, such as OAc^– and Cl^–, prefer to be located in the imidazolium ring plane next to the C–H^2/4–5 sites. By contrast, the other four bulky and weakly coordinating anions tend to occupy positions above/below the plane. Similarly, the H-bond-like interactions involving the H² site are found to be particularly enhanced in comparison with the ones at H^4–5 in the case of asymmetric and/or more basic anions (C₄mimOAc, C₄mimCl, C₄mimTfO, and C₄mimTFSA), in accordance with recent spectroscopic and theoretical findings. Other IL-specific details related to the multiple H-bond-like binding and cation stacking issues are also discussed in this paper. The secondary H-bonding of anions with the alkyl hydrogen atoms of cations as well as the cation–cation alkyl chain aggregation turned out to be poorly sensitive to the nature of the anion

A new potential model for acetonitrile: Insight into the local structure organization

International audienceThorough understanding of the microscopic organization and dynamics of individual constituents is a crucial step in the description and the prediction the properties of electrolyte solutions based on dipolar aprotic solvents such as acetonitrile. For this aim, a new potential (force field) model for acetonitrile was developed on the basis of comprehensive approach comprising quantum chemical calculations, ab initio molecular dynamics simulations and empirical parameterization. The developed potential model is able to reproduce the experimental thermodynamic and dynamic properties of neat acetonitrile in the range of temperatures between 228.15 and 348.15 K. The local structure of neat liquid acetonitrile then was analyzed in a framework of the nearest neighbor approach. It was shown that the distance standard deviations relative to the average distance between the nearest neighbors have a non-linear behavior that was traced back to the changes in the mutual orientation between acetonitrile molecules. The closest neighbors have a dominant antiparallel dipoles orientation with respect to a reference acetonitrile molecule, while for the further nearest neighbors perpendicular and parallel mutual orientation is observed. The nearest neighbors approach in combination with angular distribution functions was used for the estimation of the Kirkwood factor. Our results show that in order to reproduce the corresponding experimental values derived in the framework of the Onsager-Kirkwood-Fröhlich theory, it is necessary to take into account the mutual orientation of the 5–6 nearest neighbors. Although the atomic charges, on N and the methyl group hydrogen atoms, are negative, the values of the N ⋯ H distance and the N ⋯ H–C (methyl group), are compatible with a weak hydrogen bond between the two atoms

New Force Field Model for Propylene Glycol: Insight to Local Structure and Dynamics

In this work we developed a new force field model (FFM) for propylene glycol (PG) based on the OPLS all-atom potential. The OPLS potential was refined using quantum chemical calculations, taking into account the densities and self-diffusion coefficients. The validation of this new FFM was carried out based on a wide range of physicochemical properties, such as density, enthalpy of vaporization, self-diffusion coefficients, isothermal compressibility, surface tension, and shear viscosity. The molecular dynamics (MD) simulations were performed over a large range of temperatures (293.15–373.15 K). The comparison with other force field models, such as OPLS, CHARMM27, and GAFF, revealed a large improvement of the results, allowing a better agreement with experimental data. Specific structural properties (radial distribution functions, hydrogen bonding and spatial distribution functions) were then analyzed in order to support the adequacy of the proposed FFM. Pure propylene glycol forms a continuous phase, displaying no microstructures. It is shown that the developed FFM gives rise to suitable results not only for pure propylene glycol but also for mixtures by testing its behavior for a 50 mol % aqueous propylene glycol solution. Furthermore, it is demonstrated that the addition of water to the PG phase produces a homogeneous solution and that the hydration interactions prevail over the propylene glycol self-association interactions