Interactions in 1-ethyl-3-methyl imidazolium tetracyanoborate ion pair: Spectroscopic and density functional study

Density Functional Theory is used to investigate a weakly coordinating room-temperature ionic liquid, 1-ethyl-3-methyl imidazolium tetracyanoborate ([Emim]+[TCB]−). Four locally stable conformers of the ion pair were located. Atoms-in-molecules (AIMs) and electron density analysis indicated the existence of several hydrogen bonds. Further investigation through the Natural Bond Orbital (NBO) and Natural Energy Decomposition Analysis (NEDA) calculations provided insight into the origin of interactions in the [Emim]+[TCB]− ion pair. Strength of molecular interactions in the ionic liquid was correlated with frequency shifts of the characteristic vibrations of the ion pair. Harmonic vibrations of the ion pair were also compared with the experimental Raman and Infrared spectra. Vibrational frequencies were assigned by visualizing displacements of atoms around their equilibrium positions and through Potential Energy Distribution (PED) analysis.

► [EMIM]+ cation adopts a non-planer conformation in the most stable form.
► Weaker inter-ionic interactions in [EMIM]+[TCB]− compared to [EMIM]+[TCB]−.
► Experimental IR and Raman spectra match calculated spectra.
► Several hydrogen bond interactions between [EMIM]+[TCB]− ion pairs found via AIM.
► NBO analysis decomposes interactions between [EMIM]+[TCB]− ion pairs.