Hydrogen bonding investigation in 1-ethyl-3-methylimidazolium based ionic liquids from density functional theory and atoms-in-molecules methods

Because of special importance of hydrogen bond in the non-bonded interaction potential between two ionic liquid's parts is decided to investigate hydrogen bonding in these compounds. Two overviews are chosen to explain the hydrogen bonding in five different popular imidazolium based ionic liquids; EMI-TFSI, EMI-ES, EMI-Lactate, EMI-NO3 and EMI-PF6. Based on van der Waals atomic radii viewpoint, three types of hydrogen bond are determined in these ionic liquids that are C-H center dot center dot center dot N, C-H center dot center dot center dot O and C-H center dot center dot center dot F respectively. Based on our results, the maximum number of hydrogen bond exists in EMI-PF6 while the strongest hydrogen bond is related to the EMI-Lactate ionic liquid. Electron density analysis of hydrogen bond critical points indicated that the type and position of hydrogen bond are in agreement with hydrogen bond length values. From this standpoint, the EMI-Lactate has the largest amount of bond critical points, 0.034 amu, while the EMI-PF6 has the maximum number of them. These observations are in agreement with van der Waals atomic radii viewpoint. In addition to hydrogen bond strength, the radius of the hydrogen bonding is also important in non-bonded interaction potential. For this purpose, distance dependence of bond critical point (rho(BCP)), as a measure of the strength of hydrogen bonds, is examined. Detailed analysis showed that rho(BCP) is an exponential decaying function of distance, as the hydrogen bond disappeared above 2.5 angstrom. It is consistent with van der Waals atomic radii standpoint. Finally polynomial inverse third order equation is obtained for non-bonded interaction potential as a function of intermolecular distance. Based on the outcomes, these potentials are long-range which leads to a dissociation distance above 8 angstrom in these ionic liquids. These finding non-bonded interaction potentials can be used as initial parameters for force filed definition. (C) 2012 Elsevier B.V. All rights reserved.