DFT study of 1-butyl-3-methylimidazolium salicylate: a third-generation ionic liquid

A detailed theoretical investigation of the third-generation ionic liquid (IL) 1-butyl-3-methylimidazolium salicylate ([BMIM][Sal]), performed within the framework of density functional theory (DFT), is presented in this paper. The B3LYP-D3, M06-2X, and M06-2X-D3 functionals were used to obtain the equilibrium geometries of the two ions [BMIM](+) and [Sal](-). It is shown that the equilibrium ion geometries obtained with the dispersion-corrected B3LYP functional are very close to the ion geometries obtained with the M06-2X and M06-2X-D3 functionals. Global reactivity was assessed using molecular orbital theory and quantum molecular descriptors. Molecular electrostatic potential (MEP) surfaces and average local ionization energy (ALIE) surfaces were created in order to elucidate the charge distribution and reactivity of the investigated IL. Ion-pair binding energies were calculated with all three functionals, and the results confirmed the presence of a strong electrostatic interaction between the ions, while further insight into the interactions between the two ions was obtained by analyzing noncovalent interactions based on the reduced density gradient (RDG) surface, which revealed a total of nine interactions between the ions. Finally, the aromaticity of each ion was investigated by calculating the nucleus-independent chemical shift (NICS) parameter, which indicated that significant changes in the charge delocalization on each ion occur when the two ions interact.