Theoretical Studies Of Carboxyl-Functionalized Ionic Liquids

Ionic liquids (ILs) are unique solvents that consist entirely of ions. These low-melting organic salts typically contain a large asymmetric cation and a weakly coordinating anion (e.g. BF4-, PF6-, or Tf₂N-) so that efficient packing of the ions in a crystal lattice is disfavored and the coulombic interactions between the cations and the anions are of limited strength. A feature of ionic liquids with weakly coordinating anions is that metal salts are poorly soluble in them, which is a disadvantage for applications like the electrodeposition of metals. A solution to this problem is to attach functional groups to the cationic core. This has recently been illustrated for carboxylate-functionalized ILs. However, up until now the systematic reserches with quantum chemistry calculation and experimental methods for carboxylate-functionalized ILs are quite rare. In this paper, a series of carboxylate-functionalized ILs formed by imidazolium cation and bis(trifluoromethylsulfonyl)imide anions have been experimentally synthetised and theoretically calculated. The main points of this dissertation are summarized as follows:1. The interaction mechanisms between the cation and anion in protonated betaine bis(trifluoromethylsulfonyl)imide ionic liquids [Hbet][Tf₂N] were investigated at the B3LYP/6-311+G(d, p) level. Stable isomers of [Hbet][Tf₂N] were optimized and geometrical parameters of them have been discussed in details. Interaction energy, natural population analysis (NPA), natural bond orbital analysis (NBO) and atoms in the molecule calculation (AIM) have been performed to give a deeper insight on the chemical nature of H-bond.2. [HbetC1Im][Tf₂N] IL was synthesized with two-steps methods. CuO was selected for studies, which can dissolve in [HbetC1Im][Tf₂N], and should form metal complex. Additionally, carboxylate-functionalized ILs with a longer alkyl chain, [HbetC4Im][Tf₂N], was synthesized, and by B3LYP/6-311+G(d, p) we explored the effect of the alkyl side-chain length on the properties of ILs.3. Considering the computational time, the B3LYP/6-31+G(d) method is selected to calculate the [HbetC_nIm][Tf₂N] (n=1-4) systems. Geometrical parameters, interaction energies, natural population analysis (NPA), natural bond orbital analysis (NBO) and atoms in the molecule calculation (AIM) have been performed to give a deeper insight on the chemical nature of H-bond. The preliminary analysis of cation-anion interactions provides some initial hints as to the structural factors contributed to physical properties of the ionic liquids and their selective extraction of metal ions. 4. In order to obtain the stongly coordinating ILs, we synthesized the N,N-bis(carboxymethyl)imide bis(trifluoromethylsulfonyl)imide ([AAIm][Tf₂N]) ILs. Additionally, the optimized structures, energies, natural population analysis (NPA), and the effect of functional groups are presented and analyzed by B3LYP/6-31+G(d).In this dissertation, we take a first step in this direction on the physical properties and microscopic mechanisms of carboxylate-functionalized ILs by combining the experiments and theoretical computations. The attention will be focused on the synthesization, geometrical and electronic structures, and their variations across different carboxylate group series, which are the foundation for clearly understanding the application of carboxylate-functionalized ILs on metal extractions.