Modeling protic dicationic ionic liquids based on quaternary ammonium, imidazolium or pyrrolidinium cations and bis(trifluoromethanesulfonyl)imide anion: Structure and spectral characteristics

Abstract

Protic dicationic ionic liquids (PDILs) have attracted growing attention owing to their applications in domains of electrochemistry, proton conducting materials and other diverse areas. In the present work protic dicationic ionic liquids (PDILs) comprising of quaternary ammonium-, imidazolium- or pyrrolidinium-dications and bis(trifluoromethanesulfonyl)imide (Tf2N‾) anion have been modelled as the dication-(Tf2N)2 complexes. Electronic structure, vibrational and 1H NMR spectra of these complexes have been derived employing the M06-2x density functional theory. Theoretical calculations have shown that the strength of cation-anion binding follows the order: methylpyrrolidinum > quaternary ammonium > butylpyrrolidinium > imidazolium, which can be attributed to number and strength of N-H---O and C-H---O interactions. The dication-(Tf2N)2 complexes emerge with signature in frequency up-shift of the characteristic N-H stretching in their infrared spectra. Underlying molecular interactions are unveiled through natural bond orbital analyses, Quantum theory of atoms in molecules (QTAIM) and noncovalent interaction reduced density gradient method. The calculations have shown that cation-anion binding energies increase linearly with kinetic energy density component G(r) in QTAIM analysis and proton affinities in the PDILs. A correlation between change in free energies accompanying the dication-(Tf2N)2 complexes and proton affinities has also been established.