Electronic structure and spectral characteristics of alkyl substituted imidazolium based dication-X2 (X = Br, BF4, PF6 and CF3SO3) complexes from theory

Abstract

Electronic structure, cation-anion binding and spectral characteristics of the alkyl substituted imidazolium dication and Br−, BF4−, PF6− or CF3SO3− anion complexes have been obtained employing the M06-2X based density functional theory. The dication-anion complexes serve as model systems for describing dicationic ionic liquids and are facilitated by electrostatic, bifurcated hydrogen bonding (C-H⋯O) and anion---π interactions. The lowest energy conformer possesses relatively strong interactions from reactive vinylic protons on the imidazolium cation whereas the largely destabilized conformers reveal increasing contributions to binding from the interactions arising from by ethyl/butyl substituents at the expense of strong hydrogen bonding from vinylic protons on the imidazolium moiety. Underlying dication-anion binding has been probed through quantum theory of atoms in molecules (QTAIM) approach and the natural bond orbital analyses. It has been predicted that binding energies in the lowest energy complexes of the methyl substituted imidazolium complex emerge with a hierarchy: Br− > BF4− > CF3SO3− > PF6−, which is qualitatively different from that of butyl substituted complexes; the latter revealing stronger binding with CF3SO3− anion. The reactive vinylic proton engenders largely downfield δH signals (beyond 8.8 ppm) in their 1H NMR spectra. It has been further shown that hydrogen bond energies of the reactive proton (H1) estimated from the QTAIM theory correlate well with the 1H NMR chemical shifts measured from the experiments. The ramifications of dication-anion binding to vibrational spectra are also presented.