Investigation On The Structure And The Interaction Of Ionic Liquids

As a kind of new ionic solvents, the ionic liquids (ILs) have different internal environment compared with the tranditional molecular solvents. A systematic study of the interaction in the ILs might lead to a better understanding of their structure-activity relationship. In the present work, quantum chemical calculations, molecular dynamics are combined to investigate the interactions in the ILs.The interaction between the anion and the cation in the ILs was firstly focused on using the density functional theory (DFT). Forty structures of different ion pairs were optimized and geometrical parameters of them have been discussed in details. Anions have been gradually placed in different regions around imidazolium cation and the interaction energies between the anion and the cation have been calculated. Theoretical results indicate that there are four active regions in the vicinity of the imidazolium cations, in these regions, the imidazolium cations and the halide anions formed stable ion pairs. Imidazolium cations can interact with one, two or three but no more than three nearest halide anions by forming hydrogen bond. The halide ions are situated in hydrogen bond positions rather than at random.In order to obtain deeper insight on the interaction between the cation and the anion, the natural bond orbital (NBO) and atom in molecular (AIM) were used to study the ion hydrogens in the ion pairs. Based on the calculation results, a better understanding of the origin of the interaction between the imidazolium cations and the anions. The nature of the ion hydrogens are mainly electrostatic, however, the charge transfer and orbital interaction can contribute significantly, thereby making the interaction partly covalent. In addition, the NBO analysis demonstrated that the stabilization energy isdue to the n→σ^* C-H orbital interaction.The presence of water could affect the activity of the ILs dramatically and it is often presented as a contaminant in hydrophilic as well as in hydrophobic ILs, significantly affecting their physical properties. Quantum chemical calculations have been used to investigate the interaction between the water molecules and ILs based on the imidazolium cation with different anions: [Cl^-], [Br^-], [BF₄^-], and [PF₆^-]. The predicted geometries, interaction energies implied that the water molecules interact with the Cl^-, Br^-, BF₄^- anions to form X^-…W (X=Cl or Br, W=H₂O), 2X^-…2W, BF₄^-…W and W…BF₄^-…W complexes. The hydrophobic PF₆^- anion could not form stable complex with the water molecules at the DFT level. Further studies indicate that the cation could also form strong interaction with water molecules. The l-Ethyl-3-methylimidazolium cation (Emim⁺) has been used as a model cation to investigate the interaction between the water molecule and the cation. In addition, the interaction between the ion pairs and water were studied using the 1-ethyl-3-methylimidazolium chloride ([emim][Cl]) as a model ionic liquid. The strengths of the interactions in these categories follow the trend anion-W > cation-W > ion pair-W.The mechanism of how the ILs affect the Diels-Alder is an active area of ongoing research. A microscopic insight in the mechanism of the reaction via quantum chemical calculations was given, evidencing how the ILs affect the energy barrier and promot the reaction. The ILs can lower the dienophile LUMO and HOMO energies, bringing dienophile LUMO energy closer to the diene HOMO. The effect of the positive charge of the imidazolium cation was then estimated. It was surprised to find that the positive charge could decrease the LUMO_dienophile-HOMO_diene energy gap significantly. Based on these results, it could be predicted that the positive charge center of cation could affect the reaction significantly. These results provide opportunities in the design of future ILs catalytic systems for the organic reactions.The consuming of the CO₂ and the SO₂ has become a world-wide problem. The emerging of the ILs provides a new way for the absorption of the CO₂ and SO₂ gases. An all-atom force field is developed using a combination of density functional theory calculations and OPLS force field parameter values for the 1,1,3,3-Tetramethyl -guanidium Lactate (TMG) lactic acid (LAC) ionic liquid (TMGL). Molecular dynamics simulations are then conducted to investigate the solubility of the SO₂ and CO₂ gases in the TMGL. The simulations show strong organization of SO₂ about the cation and the anion of the TMGL, but relatively weak organization of CO₂ about the cation and the anion of the TMGL, which well explained the selectivity of the TMGL toward the SO₂ and CO₂. Based on the calculations, it was found that the active groups such as the -NH2 and -OH has great effect on the absorbing of the SO₂ and CO₂, this inspired us to design two simple amine ILs which include the -NH₂ group. The experiment result demonstrates that the TECAc ionic liquid is a good solvent for CO₂.All in all, the chemical theory, computer simulations and experiment are combined to investigate the interaction relationship in the ILs. The frame of studing on interaction in ionic liquid is established to successfully reveal the structures and interactions in the systems. It could be expected that these methods are applicable to investigate more important phenomena in the ILs.