Acid-base Bifunctional Catalytic Hydrogen - Migration Of The Theoretical Study Of The Reaction

The hydrogen-transfer isomerization is one of the most basic reactions in many chemical and biological systems, in which the tautomerism of the amine and imine configuration and the rearrangement between enol form and keto form are important types. A lot of experiment and theoretical studies show that some small neutral molecules (such as H2O,CH3OH,NH3,HF etc.) have fine catalytic efficiency and play significant roles in the reactions. At first, the catalytic molecule interact所with reactant to form complex intermediate by hydrogen bonding, and then it can change the path of the hydrogen-transfer by stabilizing transition state. One of the results is the active energy reduces greatly and the reaction becomes easy to occur. Because of the molecule acting both as proton donor and proton accepter in the reaction, it commonly referred as acidic and basic bifunctional catalyst or proton transfer catalyst.The bifunctional catalysis of the water molecule had been proved in the hydrogen-transfer isomerization as early as 1870's. Since then the work on this area has developed gradually which contains the increasing number of catalyst type and reaction system. Moreover, with the precision of theoretical calculation being improved and computer technology continuously progressing, theoretical calculation can not only get the theoretical value closer to the experimental result, but also give reasonable forecast and proper deduction for the difficult reaction which can be done contemporarily.Besides, it is well known that different isomers have different chemical and biological properties and there are rigid demands on the configuration of isomers in certain organic synthesis and biology, and the catalytic molecule may has some impact on the reactant and product in the reaction of hydrogen-transfer isomerization, even can change the stable order of them, so it is necessary to know the structures of isolated molecule and complex molecule combined with catalytic molecule and to compare the relative stabilities of them.Considering the above facets, we start our systemic study on three group organic reaction under the application of acidic and basic bifunctional catalysts by using the B3LYP method in the density functional theory. The main jobs are: optimizing the target molecules'structures and compare their relative stabilities; searching for the transition state of the reaction and analyze the reaction pathways; comparing the different catalytic effects in the different reactions.We hope that our studies can give more reasonable interpretation to the hydrogen-transfer isomerization of this type and find favorable catalyst molecule, and we also hope to provide certain usefull theory information for the later advanced research.The thesis consists of three parts as follows:1. Theoretical study on the hydrogen-transfer isomerization of cyanamide We used the DFT method at the B3LYP/6-311G** level of theory to optimize the configurations of isolated and complex cyanamide and all their isomers and the structures of corresponding transition station. Then we began to discuss the relative stability of isomers and the possible hydrogen-transfer pathways. At last the catalytic effect of the neutral molecule was compared with respect to the reaction's active energy.The result shows that: the energy difference between one pair of isomers is very little; the neutral molecule can make the reaction easy and the order of their catalytic effectiveness is NH₃﹥H₂O﹥HF.2. Theoretical study on the hydrogen-transfer isomerization of 2-hydroxyl imidazoleWe used the DFT method at the B3LYP/6-311G** level of theory to optimize all the configurations of isolated and complex 2-hydroxyl imidazole and the structures of corresponding transition station. Then we began to discuss the relative stability of isomers and the possible hydrogen-transfer pathways. At last the catalytic effect of the H2O was analyzed with respect to the reaction's active energy.The result shows that: the isomer′s keto form is more stable than enol form from both thermodynamics and dynamics; the place and the number of water molecule in the complex have remarkable impacts on the reaction active energy.3. Theoretical study on the hydrogen-transfer isomerization of cyanamide We used the DFT method at the B3LYP/6-311G** level of theory to optimize all the configurations of isolated and complex hymexazol and the structures of corresponding transition station. Then we began to discuss the relative stability of isomers and the possible hydrogen-transfer pathways and the reaction's active energy.The result shows that: the isomer′s keto form is more stable than enol form; and the water molecule in the complex have important function on reducing the reaction active energy.All calculations here are performed on PIV PC computer by using Gaussian 98 program system.