Abstract:

Gas phase ion pairs of the amino acid ionic liquid 1-ethyl-3-methyl-imidazolium asparagine ([Emim][Asn]) was investigated with density functional theory at B3LYP/6-311+G(d,p) level. Five geometries of the [Emim][Asn] complex were optimized and their geometrical parameters are discussed in detail. Theoretical results indicate that H-bond interactions of [Emim] [Asn] are very strong, -373.96 to -326.28 kJ·mol -1 with zero point energy (ZPE) correction, which is mainly attributed to the interaction between lone pairs of the carbonyl O atom in [Asn]- and the antibonding orbital of C—H in [Emim]+. Interaction H-bond energies, IR spectra, and natural population analysis (NPA) were presented and analyzed，declaring that both the red shift of the C—H stretching frequencies in the imidazolium cation and the charge transferred between cation and anion were in roughly direct proportion to the interaction energies.Atoms inmolecules (AIM) analyses indicate that theH-bond between [Emim]+ and [Asn]- is primarily ionic character. A preliminary analysis of cation-anion interactions provides some initial hints as to the structural factors that contribute to the experimental glass transition temperature Tg.

Key words: Natural population analysis, Amino acid ionic liquid, Density functional theory, Intermolecular H-bond, Interaction energy, IR spectra, Atoms in molecules theory