Unveiling Noncovalent Interactions in Imidazolium, Pyrrolidinium, or Quaternary Ammonium Cation and Acetate Anion Based Protic Ionic Liquids: Structure and Spectral Characteristics.

In the present work protic ionic liquids (PILs) composed of imidazolium-, quaternary ammonium-, or pyrrolidinium-dications and acetate (OAc) anion have been modeled as the dication-anion complexes through the M06-2x based density functional theory. It has been shown that cation-anion interaction energies are larger for the PILs containing the quaternary ammonium cation, which can be attributed to strong hydrogen bonding from the terminal ammonium protons. Underlying N-H···O and C-H···O hydrogen bonding, electrostatic, and van der Waals interactions are unraveled using the natural bond orbital analyses in conjunction with the quantum theory of atoms in molecules (QTAIM) and noncovalent interaction index reduced density gradient methods. The ramifications of noncovalent binding to H NMR and vibrational spectra are presented. The calculations further demonstrate a linear correlation of the kinetic energy density parameter G( r) in QTAIM analysis with the characteristic frequency shift of -NH stretching in the dication-anion complexes. Moreover, the chemical shifts (δ) in H NMR spectra from theory reveal larger deshielding; the corresponding δ value correlates well with proton affinities and cation-anion binding energies as well. Effect of solvent (DMSO) on structure, binding energies, and H NMR are presented. The shifts of the characteristic carbonyl and the terminal ammonium stretching vibrations accompanying the dication-anion complexes from gas phase calculations are in consonance with the self-consistent reaction field theory.