Abstract: Eleven types of alkyl imidazolium tetrafluoroborate ionic liquids (ILs) have been investigated using the density functional theory (DFT) B3LYP method together with basis set 6-311++G(d,p). First, we performed geometry optimization of the ion system {[XIM] [BF₄]_n}^(n-1)- (n=2, 3), which is composed of one alkyl imidazolium cation XIM⁺ and two or three BF₄^- anions. Then the intramolecular interaction energies were calculated for those structures with the lowest energies, and the basis set superposition error was corrected by the counterpoise method. The relationship between the experimental melting points and the interaction energies was also investigated. A linear correlation was found for the alkyl imidazolium tetrafluoroborate compounds studied, which was also consistent with the linear correlation previously found for amino acid cation based ILs. Our work shows the possibility of designing ILs with the help of quantum chemistry in the future.

Key words: Density functional theory, Imidazolium tetrafluoroborate, Interaction energy, Ionic liquid, Melting point