Electronic structure, vibrational spectra and 1H NMR chemical shifts of the ion pair composites within imidazolium functionalized geminal dicationic ionic liquids from density functional theory

Abstract

Electronic structure, vibrational and 1H NMR spectra of geminal dication based ionic liquids composed of the functionalized imidazolium cation and bis(trifluoromethanesulfonyl)imide (Tf2N‾) anion have been obtained through the M06-2x based density functional theory. Underlying C–H⋯O, C–H⋯F, anion-π and van der Waals interactions in such complexes are unveiled through natural bond orbital and noncovalent reduced density gradient analyses. Calculated infrared spectra have shown that dication-anion complexes are accompanied by frequency downshifts of the hydrogen bonded SO3 and CF3 vibrations, whereas the uncoupled and intense C–H stretching emerges with its shift in the opposite direction (blue shift or frequency up shift) compared to free Tf2N‾ anion. Further the C-H vibrational frequency increases steadily with diminutive kinetic energy density component in the quantum theory of atoms in molecules (QTAIM) analyses. Besides, the large down-field signal of the (reactive) methine proton on the imidazolium in 1H NMR spectra correlates well with the electron density at the bond critical point in QTAIM analyses.