Theoretical Studies On Structure And Catalytic Mechanism Of Several Ionic Liquids

In the past two decade, Ionic liquids (ILs), a new class of liquids that are entirely composed of ions, have been one of the most heated research topics in the academic and industrial fields. This is mainly attributed to their fascinating "green" characteristic, such as high thermal and chemical stability, nonflammability, nonvolatility and good reusability, which make ILs be widely recognized as environmentally friendly medium. Moreover, the designable characteristic is another important factor that is responsible for the unprecedented prosperity of ILs. Cations and anions constituting ILs can be optionally varied so that ILs can be designed technically to possess unique properties for special applications. So far, the applications of ILs have traversed many fields, including organic synthesis and catalysis, chemical separation, electrochemistry, and biochemistry, where they were widely used as solvents, reaction mediums, catalysts, and functional materials.Compared with the experimental researches on ILs, their theoretical studies are relatively laggard. Researchers paid much attention to investigating the structures, properties and structure-property relationships of ILs through the quantum chemistry calculations and molecular dynamics (MD) simulations. However, most of the theoretical studies in this area focused on imidazolium-based ILs, and the corresponding concern for other type of ILs, such as pyridinium- and aminophenol-based ILs, is relatively less. This situation is unfavorable to thoroughly knowing the microstructures of ILs, building up the perfect structure-property relationships of ILs, and developing new kind of task-specific ILs. Furthermore, theoretical studies of how ILs control desired reactions are very limited in contrast with their increasing applications as catalysts. This lack will prevent the understanding for the catalytic mechanism of ILs and the wide use of new IL catalysts with high performance. These indicate uniformly the further need for the theoretical researches on ILs.In this dissertation, a series of theoretical studies have been carried out for several kinds of ILs with excellent performance. On the one hand, by performing density functional theory (DFT) calculations and MD simulations, we have shown the microstructure details of typical pyridinium-based ILs and new dicationic ILs, and investigated the corresponding structure-property relationships. On the other hand, Our specific attention is focused on addressing the microscopic details on how ILs influence chemical reactivity and selectivity based on DFT calculations. The important and valuable results in this dissertation can be summarized as follows:1. DFT calculations have been carried out to investigate the ion pair structures of ILs [BPy]+[BF4]- and [(MIm)C3(MIm)]2+[Br]-2, the representatives of pyridinium-based ILs and geminal dicationic ILs. The stable geometries and electronic properties for the ion pairs are shown, and the intrinsic interactions between cations and anions are discussed. The present results may establish a basis for understanding the structures and properties of the two kinds of ILs.We studied the geometries of [BPy]+[BF4]- ion pair by mean of DFT method. The calculations show the relative stabilities between the different geometries of the isolated ion pair, from which the preferential localizations of [BF4]- anion around [BPy]+ cation are obtained. Moreover, the H-bond interactions between cation and anion are investigated, and the distribution and strength of H-bonds are shown. It is found that the relative stability for ion pair configurations is synergically determined by the electrostatic attractions and the H-bond interactions between the ions of opposite charge.Subsequently, DFT calculations were performed to investigate the geometrical and electronic structures of the dication and the ion pair in IL [(MIm)C3(MIm)]2+[Br]-2. The geometrical structures and relative stabilities for the dication and the ion pair are shown. It is found that there exist theπ…πstacking effect between two imidazole rings in the most stable ion pair structure, which play an important role for stabilizing the ion pair. Moreover, based on the natural bond orbital (NBO) analyses, the electronic delocalization in the dication and ion pair is discussed in detail, and the relative acidity of the H atoms on the imidazole ring is measured. The intrinsic interaction between the dication and Br- anions in the most stable conformer was studied by using frontier molecular orbital (FMO) analyses. The calculations show the interactions between the dication and Br- anions mainly occur between theσ* C-H andπ* orbitals of the dication moiety and one pAO of Br-anions, and the excessive electrons on Br- anions are transferred to the dication moiety mainly through the a-type interactions between one of the Br- pAOs and theσ* C-H orbitals. The great charge transfer and interaction energy between Br- anions and the dications offer support for the higher melting point and thermal stability of the dicationic ILs.The corresponding results have been published in J. Phys. Chem. A (2010,114, 3990-3996) and J. Mol. Struct. (THEOCHEM) (2009,900,37-43).2. MD simulations have been performed to deeply study the microstructure of IL [BPy]+[BF4]- and the influence of H2O on the microstructure and dynamic properties of IL [BPy]+[BF4]-. The calculated results bring about a better understanding of the microstructure of pyridinium-based ILs, and reveal the structure-property relationship for the binary system of pyridinium-based ILs with H2O to some extent.Based on the above DFT calculations, we choose reliable force field and computational modes to investigate the microstructure details of [BPy]+[BF4]- IL by performing MD simulation. The results show that [BPy]+[BF4]- IL represents strong long-range ordered structure with cations and anions alternately arranging. In the first coordination shell surrounding the cation, the favorable sites occupied by the anions from MD simulations are in good agreement with the findings from the DFT calculations. While the cations mainly lie in the next space that does not be occupied by anions in the first coordinate shell. Unlike the imidazolium-based ILs, T-shaped orientation plays a key role for the interaction between two pyridine rings. The H-bond details between the cations and anions is further shown by analyzing the site-site radial distribution functions (RDF) between the H atoms of [BPy]- cations and the negatively charged F atoms of [BF4]- anions. It is found that the H-bonds between the F atoms and the H atoms on the pyridine rings are stronger than those between the F atoms and the butyl chain H atoms, which is consistent to those observed in the DFT calculations. However, the strengths of the C-H…F H-bonds obtained from MD simulations are weaker than those found from the DFT calculations.MD simulation on the mixtures of [BPy]+[BF4]- IL with H2O was also performed to investigate how H2O influences the microstructure and dynamics properties of [BPy]+ [BF4]- IL. RDFs and spatial distribution functions analyses show that the presence of H2O does not change the basic structural characteristics of [BPy]+[BF4]-IL, as well as the trend of the H-bond strength between cations and anions. H2O molecules with small size are mainly located in the first coordination shell around the given cation, and their distribution areas surrounding the cation are similar to, but not as wide as, these high probability areas of anions around cations obtained above. Anions are distributed in the second coordination shell around the given cation. The interactions of H2O molecules with the anions are much stronger. With increasing the H2O content, the number of H2O molecules around the cations and anions increases, and the number for the ions of opposite charge decreases, which results in the reduction of the interaction between cations and anions. Moreover, the rotation of the cation and the diffusive motion of cations and anions accelerate with the increase of the H2O content, indicating the presence of H2O can efficiently reduce the viscosity of [BPy]+[BF4]-IL.The corresponding results have been published in J. Phys. Chem. A (2010,114, 3990-3996).3. We have launched a theoretical project of primary researches on the important organic synthesis reactions catalyzed by imidazolium-based ILs, such as Diels-Alder (D-A) reaction, Markovnikov addition and the cycloaddtion of CO2 with propylene oxide, by performing DFT calculations. These theoretical researches revealed the catalytic mechanism of imidazolium-based ILs, make clear the roles of cations and anions in imidazolium-based ILs played in the reactions, and mastered the key factor controlling the reactions.(1) We consider the D-A reaction of cyclopentadiene with methacrolein in the presence of diethylimidazolium salts as the first prototype of our systemic studies about important organic synthesis reactions catalyzed by imidazolium-based ILs. The calculations show the mechanism details of the D-A reaction with and without the dialkylimidazolium cation and the corresponding potential energy surface (PES) profiles. It was found that the diethylimidazolium cation acts as a Lewis acid centre to catalyze the D-A reaction, which decreases the barrier and increases the asynchronicity of the D-A reaction, but does not change the PES profile of the reaction compared to the non catalyzed processes. The effect of diethylimidazolium cation on the D-A reaction has been rationalized via performing the FMO analysis and NBO analysis.The corresponding results have been published in Inter. J. Quant. Chem. (2007, 107(9),1875-1885).(2) The Markovnikov addition of imidazole to vinyl acetate in the presence of the basic ionic liquid [BMIm]+[OH]-has been chosen as the second prototype of our systemic studies. Our DFT calculations have showed clearly the catalytic mechanism details of [BMIm]+[OH]-controlling the Markovnikov addition as a catalyst. It is found that the different vinyl acetate conformations result in two different reaction pathways (stepwise and concerted). The Markovnikov addition in the presence of [BMIm]+[OH]-is highly exothermic and exergonic. Both the cation and anion of [BMIm]+[OH]-play important roles in the Markovnikov addition, which decrease the barrier and increase the selectivity of Markovnikov addition. [BMIm]+stabilizes transition state via its coulombic attraction to C7 atom, while OH- deprives N1-proton of imidazole to strengthen its nucleophilic ability.The corresponding results have been published in J. Phys. Chem. A (2007,111, 4535-4541).(3) We studied the cycloaddition of CO2 with propylene oxide to synthesize five-membered cyclic carbonates in the absence and presence of [R1MIm]+[Cl]-(Ri=Ethyl, Butyl, and Hexyl) by performing DFT calculations. In the absence of [RiMIm]+[Cl]- the reaction proceeds via two possible channels (each of them involves one elementary step) and the corresponding barriers are found to be as high as 59.71 and 55.10 kcal mol-1. In the presence of [R1MIm]+[Cl]-there exist five possible reaction channels (each of them involves two or three elementary steps) and the barriers of the rate-determining steps are reduced to 27.93-38.05 kcal mol-1, clearly indicating that [R1MIm]+[Cl]- promotes the reaction via modifying the reaction mechanism and thereby remarkably decreases the barrier. The notable catalytic activity of [RiMIm]+[Cl]- may originate from the cooperative actions of the cation and anion, which stabilize the intermediates and transition states through the H-bond interaction and make the ring opening easier via the nucleophilic attack to propylene oxide, respectively.The corresponding results have been published in J. Phys. Chem. A (2007,111, 8036-8043).4. The Michael addition of cyclohexanone with trans-β-nitrostyrene catalyzed by pyrrolidine-imidazolium bromide, which represents a prototype of Chiral IL-promoted asymmetric syntheses, has been investigated by performing DFT calculations. We show the details of the mechanism and energetics, the influence of the acid additive on the reactivity, the functional role of the Chiral IL in the asymmetric addition, and the origin of good diastereoselectivity and high enantioselectivity of the Michael adduct. From the theoretical results, we provide a reasonable explanation for the experimental observations and reveal a valuable clue for the further Chiral IL design with high catalytic efficiency.It is found that the reaction proceeds via two stages, i.e. the initial enamine formation and the subsequent Michael addition. The calculations show that the presence of the acid additive changes the imine formation mechanism and lowers the reaction barrier, as well as, more importantly, makes the reaction become highly thermodynamically favored. It is also suggested that both the anion and cation of the Chiral IL synergically facilitate the reaction, which act as the proton acceptor in the imine-enamine tautomerism and the stabilizer of the negative charge in the C-C bond formation process, respectively. By comparison of the C-C bond forming processes in the absence and presence of Br- anion, we found that the intermolecular H-bonds between the imidazolium cation and the nitro group of trans-β-nitrostyrene and the steric hindrance of the imidazolium cation moiety on the Si-face of enamine dominate the stereoselectivity of the Michael addition. The corresponding results have been published in Chirality (Early View).