Cation and anion substitution effects on the ultrafast dynamics of interionic interaction in imidazolium based ionic liquids

Room-temperature Ionic Liquids(ILs) have numerous unique properties that differ from those of conventional molecular solvents.Although the unique properties of ILs have been suggested to origin from their microscopic interionic interaction,detailed dynamics of interionic interaction of ILs has not been fully understood.Here,with the Femtosecond Optical Heterodyne-Detected Raman Induced Kerr Effect Spectroscopy(fs-OHD-RIKES),we measured the ultrafast dynamics of the interionic interaction of three typical imidazolium based ILs,1-butyl-3-methylimidazolium tetrafluoroborate([bmim][BF4]),1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]),and 1-decyl-3-methylimidazolium tetrafluoroborate([dmim][BF4]).We observed several periods of subpicosecond oscillation in their fs-OHD-RIKES signals.Through decomposing their fs-OHD-RIKES signals into four Brownian oscillators in time domain,we explored the cation and anion substitution effects on the ultrafast dynamics of interionic interaction of ILs.We found that the cation substitution affected all the low frequency motions we observed,while the anion substitution only affected the two higher low frequency motions.

Computational studies of the structure and cation-anion interactions in 1-ethyl-3-methylimidazolium lactate ionic liquid

Quantum chemical calculations of the structures and cation-anion interaction of 1-ethyl-3-methylimidazolium lactate ([Emim][LAC]) ion pair at the B3LYP/6-31++G** theoretical level were performed. The relevant geometrical characteristics, energy properties, intermolecular H-bonds (H-bonds), and calculated IR vibrations with respect to isolated ions were systematically discussed. The natural bond orbital (NBO) and atoms in molecule (AIM) analyses were also employed to understand the nature of the interactions between cation and anion. The five most stable geometries were verified by analyzing the relative energies and interaction energies. It was found that the most of the C-H···O intermolecular H-bonds interactions in five stable conformers have some covalent character in nature. The elongation and red shift in IR spectrum of C-H bonds which involve in H-bonds is proved by electron transfers from the lone pairs of the carbonyl O atom of [LAC] to the C-H antibonding orbital of the [Emim]+. The interaction modes are more favorable when the carbonyl O atoms of [LAC] interact with the C2-H of the imidazolium ring and the C-H of the ethyl group through the formation of triple H-bonds.