Ab Initio Studies Of Confomations And Properties Of Amino Acids And Fire Suppression Reaction Kinetics Of CF3H

Along with the rapid development of computational methods and computer technology, theoretical computation has become more and more important in science research. On the one hand, using various computational software, people have realized high level accuracy study to chemical substances and chemical reaction on computer at a less cost; On the other hand, on the basis of quantum chemistry theory, people can do a new reasonable design on the structure and function of chemical substances as well as reaction to guide further investigation by experiment. Theoretical computation has become a powerful theoretical tool to solve many questions in science research.This dissertation is to study configurations and properties of amino acids and fire suppression mechanism of CF3H using the DFT and ab initio method. There are four parts in the dissertation: The first part (chapter 1) is about the introduction of computational methods and the basic theory of dynamic computation used in the dissertation; The second part including three chapters (from chapter 2 to chapter 4) focuses on the theoretical studies of configurations and properties of amino acids both in gas and in aqueous; The third part (chapter 5) is the study on the dynamics of the reaction. The last part (chapter 6) is the summarization for the whole dissertation. More detailed information of the dissertation is listed below.In the first chapter, we introduce computational methods and the basic theory of dynamic computation used in the dissertation. First of all, we look back the development of quantum chemistry and then present the basic principles of quantum chemistry including Configuration Interaction, Coupled Cluster, Many-Body Perturbation Theory, Density Functional Theory and Time-dependent DFT; At the same time, we also introduce the solvation model, Atoms in Molecules and Natural Bond Orbital theory as well as Canonical transition-state theory and RRKM theory used to compute chemical reaction rate constant. In chapter 2, we first simply introduce the twenty amino acids which are made up of the basic structure of protein. Because of the phenol group in tyrosine which can be used as a chromophore to detect the intramolecular interacting of protein, so it is very important to study the configurations and properties of tyrosine for further research the big protein molecules. We systematically explore the conformational potential energy surfaces of aromatic amino acids tyrosine and calculate the conformational properties and conformation distribution using the high-level ab initio methods and DFT theory. We assess the order of quality of different methods and basis sets according to our results computed. In addition, we discuss the electronic abstraction spectrum of neutral and protonated tyrosine applying time-dependent DFT theory, explaining the experimental UV spectra of aromatic amino acids tyrosine.In chapter 3, we study the conformations and properties of aliphatic hyroxyamino acids thronine in the gas phase, explaining the experimental spectrum. The amino and hydroxy hydrogen atoms in threonine take part in the formations of various hydrogen bonds, and this is largely responsible for its biological significance. H-bonding is a type of both important and complex interaction. It is weaker than covalent bond but it is very important to describe the molecular behavior and it decides the physical and chemical property of matter, and especially in biomolecule many physiological processes are embodied by H-bonding. We reveal the intramolecular interaction of threonine using the ab initio and DFT calculations as well as AIM and NBO theory.Starting from chapter 4, we begin to focus on conformations and properties of amino acids in aqueous solution. Full structural assignments of the conformers of neutral and zwitterionic theonine and the related properties as well as conformational distribution in solution are presented using Onsager and CPCM salvation model. Also, the Combined Discrete/SCRF method is employed to show the neighboring water molecule's influence on the energy order of neutral and zwitterionic cysteine in gas phase and in solution.The above three chapters concern amino acids only. In chapter 5, we study the fire suppression mechanism of CF3H which is an important model system for promising Halon alternative. The potential energy surfaces (PES) for the reactions (CF3H unimolecular dissociation reaction and reactions of CF3H with free radical OH and H) involved in the fire suppression process of CF3H were investigated at the B3LYP/6-311++G^** and QCISD(T)/6-311++G^** levels and by the G3B3 theory. All the possible stationary and first-order saddle points along the reaction paths were verified by the vibrational analysis. The calculations account for all the product channels. Canonical transition-state theory with Wigner tunneling correction was used to predict the rate constants for the temperature range of 298-2500 K without any artificial adjustment, and the theoretical rate constants are compared with the available experimental data. The H abstraction of CF3H by OH is found to be the main reaction channel for the CF3H fire extinguishing reactions while the CF3H unimolecular dissociation reaction plays a negligible role.In chapter 6, there is the summarization for the whole dissertation.