Theoretical Studies Of Structure And Catalysis Mechanism Of Ionic Liquids

As a new class of material, ionic liquids have many especial physical properties,such as negligible vapor pressure, low melting point, high thermal andelectrochemical stability, high ionic conductivity and strong solvent capacity and soon. They have been widely used in organic synthesis, material chemistry,electrochemistry, histological chemistry, analytical chemistry and so on asenvironmentally friendly solvents and catalyst, and exhibited a good developforeground in energy source, environment and life science. Theoretical studies ofstructure and catalysis mechanism of ionic liquids have been benefit to understand thephysical properties of ionic liquids, and provide some theoretical advises for designand application of ionic liquids.In this dissertation, we studies structure of some ionic liquids with densityfunctional theory calculations, analyzed interaction between antion and cation,discussed the relations between structure and properties. We also present a theoreticalstudy of the reaction mechanism of the Michael addition reaction catalyzed by ionicliquids, reveal the role of the ionic liquid played in the reaction, and explained someproblem in orginal reaction about ionic liquids.The main contents as follow:1. We have carried out a system theoretical study for the geometrical structuresand electronic properties of [EMIM]BF₄ and studied the interaction between cationand anion at B3LYP/6-31++G(d,p) level. It is found that in the ion pair, there are fourprimary cation-anion interaction sites: Front, Back, Methside and Ethside. Seven moststable configurations all have multiple hydrogen bonds and charge transfer from anionto cation. Natural bond orbital analyses suggest that there is a close correlationbetween the positive charge on C2 and the relative stability of the ion pair: the morepositive charge on C2, the more stable the ion pair. The orbital interaction from theanion to cation occurs mainly through the LP_F→σ_C-H^* interactions.2. We have carried out a detailed theoretical study for the geometrical structures and electronic properties of the cation and the ion pair of the cycletetramethylguanidinium nitrate ionic liquid. It is found that: (1) in the cation, thetriazolium ring presents almost a plane and the NMe₂ group is not coplanar with thering. The central C2 atom carries high positive charge, which is expected to act as themost active site for the electrostatic attacking of an anion. (2) In the ion pair, threemost stable configurations all have multiple hydrogen bonds. However, the calculatedresults show that the electrostatic interaction between cation and anion plays a crucialrole for stabilizing the ion pair. There is a close correlation between the charge on C2and the relative stability of the ion pair: the more positive charge on C2, the morestable the ion pair. Natural bond orbital and frontier molecular orbital analyses revealthat the charge transfer from the anion to cation occurs mainly through theLPo→σ_C-H^*,LPo→π_C2-N3^* or LPo→π_C2-N1^* interactions.3. By performing DFT calculations, we have studied the detailed reactionmechanism of the Michael addition of acetylacetone to methyl vinyl ketone in theabsence and presence of the ionic liquid 1-butyl-3-methylimidazolium hydroxide([bmIm]OH). In the absence of ionic liquids, the Michael addition takes place with abarrier of 28.65 kcal mol^-1. While in the catalyzed Michael addition reaction, theanion enhances nucleophilic ability of C₇ of acetylacetone as the OH^- anion capturesH₁ to form an acetylacetone anion-H₂O complex, and the cation improves theelectrophilic ability of C₁ of methyl vinyl ketone by forming intermolecularhydrogen-bonds. Both the remarkable effects promote this reaction take place moreeasily and the corresponding barriers of the rate-determining steps are reduced to13.41 kcal mol^-1 along the ketone form pathway and 9.74 kcal mol^-1 along the enolform pathway. Our study provides a detailed reaction mechanism of Michael additioncatalyzed by basic ionic liquid [bmIm]OH, clearly reveal the catalytic role of ionicliquid in important chemical reaction and explains the experimental finding well.4. We have studied the detailed reaction mechanism of [BMIM]⁺ withbenzaldehyde and glucose in the absence and presence of the base by DFTcalculations. In the absence of the base, the energy barriers of the reaction of [BMIM]⁺ with benzaldehyde and glucose are 54.53 and 66.19 kcal mol^-1, respectively.In the catalyzed reaction by base, C2-H can be captured more easily by base andnucleophilic ability has been remarkable enhanced. Reactions of [BMIM]⁺ withbenzaldehyde and glucose can take place more easily with lower barriers. To avoidunexpected side reactions, C2-H can be substituent by alkyl to reduce nucleophilicability of C2. Our studies explain the side reaction of ionic liquids in organic reaction,and provide some theoretical advice on application of ionic liquids.In this dissertation, the valuable results have provided reliable verification andtheoretical guide for further study and application of ionic liquids.