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

Abstract

Thorough 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 348.15K.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⋯HC (methyl group), are compatible with a weak hydrogen bond between the two atoms.