Interaction between cation and anion in ion-pairs of [EPy] [BF₄] and [EPy] [PF₆] in gas and liquid phases

The geometry structure optimization and frequency analysis of ion pairs of ionic liquids 1-ethylpyridinium tetrafluoroborate ([EPy] [BF₄]) and 1-ethylpyridinium hexafluorophosphate ([EPy] [PF₆]) were studied using ab initio method and density functional theory at HF/6-31G and B3LYP/6-311 + G (d, p) level. Structure and interaction of ionic liquids in liquid phase were investigated by conductor-like polarizable conti-nuum model (CPCM). The most stable structures in gas and liquid phases and infrared spectra of two ionic liquids were obtained. Hydrogen bonding interaction between anion and cation was confirmed by geometry parameters of two ion pairs. The simulated infrared spectra agreed well with the experimental spectra. Natural bond orbital (NBO) analysis was performed to analyze atomic charge distribution and charges transfer in [EPy]⁺ cation and ionic liquids. The results show there are electrostatic interaction and hydrogen bonding interaction between cation and anion of ionic liquids. By means of geometry parameters, interaction energy and NBO analysis, because of neutralization of surrounding charges, interaction in liquid phase becomes smaller.