Theoretical Studies On Structure, Formation And Catalytic Mechanism, And Their Interaction With Solvent Of Several Ionic Liquids

In the past two decades, room temperature ionic liquids (RTILs), composed exclusively of organic cations and inorganic anions, have elicited substantial interest both in academia and in industry. This is mostly because of their unique physicochemical properties, such as low melting point, vanishing vapor pressure, powerful solvent capacity, large liquid range, excellent thermal stability, and high ionic conductivity. In particular, in light of their nonvolatile nature and great catalytic reactivity, RTILs have been very popular as solvents and catalysts in many fields of chemistry, including organic synthesis, separation, and biochemistry. At present, much interest has been focused on applications and the macroscopical properties, such as density, viscosity, conductivity and solubility. While compared with extensive experimental researches on RTILs, the theoretical studies are relatively laggard, and the knowledge at the molecular level is still fragmentary. To design more effective RTILs, predict their properties and even choose more suitable one for a specific application, theoretical studies at the molecular level seem to be extraordinarily necessary. In this dissertation, by performing density functional theory (DFT) calculations, we carried out a series of theoretical studies on kinds of RTILs to gain an insight into their general synthesis mechanisms, electronic structures, catalytic reactivity, and the detailed interaction mechanism between RTILs and other components. The important and valuable results in this dissertation can be summarized as follows:1. To better understand the property of the binary systems composing of imidazolium salt, [emim]+A-(A=Cl-, Br-, BF4-, and PF6-) and methanol, we have investigated in detail the interactions of methanol molecule with anions A-, cation [emim]+, and ion pair [emim]+A-of several ionic liquids (ILs) based on 1-ethyl-3-methylimidazolium cation by performing density functional theory calculations. It is found that H-bonds are universally involved in these systems, which may play an important role for the miscibility of methanol with imidazolium-based ILs. The interaction mechanisms of methanol molecule with anion and cation are found to be different in nature:the former mainly involves LPX-σO-H* interaction, while the latter relates with the decisive orbital overlap of the type of LPO-σO-H*. Based on the present calculations, we have provided some reasonable interpretations for properties of the binary mixtures of ILs and alcohol and revealed valuable information for the interaction details between ILs and alcohols, which is expected to be useful for the design of more efficient ILs to form superior solvent system with alcohol.The corresponding results have been online in Journal of Molecular Modeling, early view, DOI:10.1007/s00894-010-0879-1.2. The Menshutkin reaction between the N-methyl imidazole with chloroethane is reexamined to rationalize the experimental discovery by performing density functional theory calculations. The calculated results show that the reaction proceeds via a SN2 mechanism with a barrier of 119.1 kJ mol-1, which is much lower than that reported in previous literature according to a five-membered transition state mechanism. Moreover, it is found that the barrier is further reduced to 98.1 kJ mol-1 in toluene solution. The present result validates the experimental finding that the Menshutkin reaction for synthesizing N-alkyl imidazolium halide salts proceeds smoothly at lower heating temperature.The corresponding results have been online in Journal of Molecular Modeling, early view, DOI:10.1007/s00894-010-0916-0.3. By using density functional theory calculations, we have performed a study on the synthesis mechanism, electronic structures, and catalytic reactivity of a pyridinium-based ionic liquid,1-ethyl-pyridinium trifluoroacetate ([epy]+[CF3COO]-). It is found that the synthesis of pyridinium salt follows a SN2 mechanism. The electronic structural analyses show that multiple H-bonds are universally involved in the pyridinium-based ionic liquid, especially C* (on the ortho-position of nitrogen atom in pyridinium ring)-H…O H-bond, which may play a decisive role for the stability of the system. The mechanism of how the anion interacts with the cation mainly involes LPo→σ*C*-H interaction, which is associated with the C*-H…O H-bond. This present work have also given clearly the catalytic mechanism of [epy]+[CF3COO]-on the Diels-Alder (D-A) reaction of acrylonitrile and 2-methyl-1,3-butadiene. Both the cation and anion are shown to play important roles for catalizing the D-A reaction, which has been rationalized by the NBO and FMO analyses. [epy]+, as a Lewis acid, can interact with reactant acrylonitrile by C=N…H H-bond to increase the polarity of C=C double bond, while the CF3COO- anion links with the hydrogen atom of methyl group in 2-m ethyl-1,3-butadiene by C-H…O H-bond, which would weaken the electron-donating capability of methyl and then lower the energy barrier of the D-A reaction. Based on the present calculations, we hope this work could provide valuable theoretical basis for the design, development and application of pyridinium-based ionic liquids.The corresponding results have been accepted in The Journal of Physical Chemistry A.4. By performing density functional theory calculations, we systematically studied the Diels-Alder (D-A) reaction between cyclopentadiene and methacrylate catalyzed by alanine methyl ester nitrate ([AME][NO3]), an amino acid-based ionic liquid (AAIL). The uncatalyzed reaction was first calculated in both gas phase and dichloromethane, for comparison, and then the catalytic effect of [AME][NO3] IL on the D-A reaction was mimicked by using one, two, and up to three ion pairs as catalysts. The calculated results show that [AME][NO3] plays a role of Lewis acid to promote the reaction and the catalytic active center is the NH3 group in [AME]+ cation, which forms the effective N-H…O H-bonds with the carbonyl oxygen atom in methacrylate to effectively polarize the C=C double bond. As a result, the energy barrier of reaction is remarkably reduced, and the asynchronicity of reaction is increased. The calculated barrier for the reaction with the presence of two ion pairs is lower than those with the presences of one and three ion pairs, implying that the optimal molar ratio among two reactants and the reaction medium/catalyst [AME][NO3] should be 1:1:2. The present results rationalize the early experimental findings, and provide a useful reference for the rational design of usual D-A reactions in AAILs.The corresponding results have been submitted in The Journal of Organic Chemistry.