Probing molecular interactions underlying imidazolium and pyridinium based ionic liquids

Abstract

Electronic structures of 1-hexyl- and 1-octyl- substituted 3-methylimidazolium (mim), -pyridinium (py), 4-methylpyridinium (mpy) cations and halide (Cl− or Br−) ion-pairs have been obtained from the M06-2x/6-31++G(d,p) density functional theory. The imidazolium cation binds strongly to the anion than py or mpy. Theoretical calculations demonstrated that Cl− ion-pairs possess stronger and more directional hydrogen bonding than those containing Br− ions bringing about the distinct structural features in the ion-pairs which are explained in terms of non-covalent interactions reduced density gradient analysis. Calculated vibrational frequencies have shown that the methine C1–H1 vibration of the cation shifts to lower wavenumber (red shift) in the Cl− ion-pairs while the shift in the opposite direction was predicted for the Br− ion-pairs The direction of frequency shift of characteristics stretching vibration has been explained through the change in dipole moment of ion-pair with respect to the stretching coordinate of the corresponding covalent bond. As opposed to Cl− ion-pairs, the Br− ion-pairs reveal ∂μ∂r<0. Effect of hydration on the structure and energetics of the ion-pair has been simulated by subsequent addition of one to three water molecules. Cation–anion binding and polarizabilities of solvated ion-pairs increase steadily with the number of water molecules. A charge transfer from the anion in hydrated ion-pairs is evident from the natural bond orbital analysis.