Glycine Ionic Liquid Coupling Mechanism And The Nature Of Research

In recent years, there have been a great enthusiasm for room temperature ionic liquids (RTIL) and their design and use have evolved into a blossoming branch of chemistry. Ionic liquids (ILs) are perceived to be a novel and "green" solvents because they are nonvolatile and their quite remarkable properties, including a negligibly small vapor pressure, high thermal stability, and high ionic conductivity. As a result, ILs are attracting considerable attention as reaction solvents, extraction solvents, and electrolyte materials. Moreover, ILs are now expected to be "designed solvents" because their physical properties could be tailored by adjusting the structures and species of cations and/or anions for a given end use. Therefore, much of ILs has been used in wide fields, for example, organic synthesis and catalytic reaction, electrochemistry, biochemistry, and material engineering.In this study, novel ionic liquids formed by 1-ethyl-3-methyl-imidazolium cation [emim]⁺ and glycine anion [Gly]^- have been investigated theoretically. The difference of these ionic liquids from the others is that the anion is [Gly]^- which is the basic component of protein and indispensable in the natural living body. This is the first time for the investigation of this question theoretically. As a kind of new ionic solvents, the ionic liquids (ILs) have different interal environments compared with the traditional molecular solvents. A systematic study of the interaction in the ILs can get a better understanding of their structure-activity relationship. In the present work, theoretical investigations of the coupling interactions in the ILs have been carried out employing B3LYP/6-31+G^* and B3LYP/6-311++G^** level of theory. The primary innovations are related as follows.Firstly, the structural characteristics of the ionic liquids formed by 1-ethyl-3-methyl-imidazolium cation [emim]⁺ and glycine anion [Gly]^- have been investigated. The interaction modes are most favorable when the carbonyl O atom of [Gly]^- interacts with the C2-H of the imidazolium ring and the C-H of the methyl group of [emim]⁺ through the formation of double intermolecular H-bonds. All of the stable geometries are characterized by the intermolecular H-bonds, which is further supported by the IR absorption peaks. Consistent with the relative order of interaction energies, the most stable complexes have been determined. In the IR spectra, the different intensities occurring in different regions can provide some help in the identification of various complexes. The formation of the ion pairs mainly influences the vibrations of the imidazolium C-H groups and the C=O of [Gly]^-, and other modes of the ions retain their individuality and practically do not mix.Secondly, the nature of the intermolecular H-bond in the ionic liquids has been investigated employing the natural bond orbital (NBO) and atom in molecular (AIM) methods. The NBO analysis demonstrated that the stabilization energy is due to the nâ†'σ_C-H^* orbital interaction. In the formed intermolecular H-bonds, electron transfers occur mainly from the lone pairs of 0 atom of [Gly]^- to the C-H antibonding orbital of [emim]⁺, resulting in the elongation and red-shift of the C-H stretching frequency. Moreover, an almost linear correlation between the elongation of the C-H bond and the change of the electron population in the correspondingσ^*(C-H) orbital has also been observed. The origin of the high stability of the amino acid ionic liquids observed experimentally may be relevant to the nonexistence of the proton-transferred products (neutral pairs) together with the larger energy needed for separation of the ionic pairs in the gas phase.Finally, the effects of the H⁺, Li⁺, Na⁺ and K⁺ cations on the ionic liquids have been investigated. The complexes are more stable when the H⁺, Li⁺, Na⁺ and K⁺ cations interact with the 022 of the [Gly]^-. The intermolecular H-bonds in the ionic liquids have been weakened, resulting in the blue-shifts of the C-H stretching frequency. The order of the interaction is H⁺>Li⁺>Na⁺>K⁺. The interaction between H⁺ and [emim][Gly] is mainly covalent, and the other interactions between Li⁺, Na⁺ and K⁺ and [emim][Gly] are mainly electrostatic. The proton transfer could not occur among those complexes. The formation of the ion pairs mainly influences the vibrations of the imidazolium C-H groups and the C=O of [Gly]^-, and other modes of the ions have also been influenced in some extent.