Theoretical Studies On The Influences Of H-bond Interactions On The Alkene Isomerization And Proton Transfer Reaction In The CO₂Capture

Intra-and intermolecular H-bond interactions exist in various chemical and biological systems. They have significance and important roles in enhancing the stability of the molecules and controlling the relevant reactions. For example, the intramolecular proton transfer (PT) through the intramolecular H-bond interactions is the key steps in the alkene and keto-alcohol isomerization reactions. On the other hand, the intermolecular PTs assisted by many solvent molecules (such as water molecules) via the intermolecular H-bond interactions also play an important role in the CO2capture process by ionic liquids and other reactions. Therefore, systematical investigations of the influence mechanisms of H-bond interactions on the above reactions can not only realize the predictability and controllability for the reaction, but also can deepen the understanding of the intramolecular and intermolecular PTs.In this study, the intramolecular PT isomerization behavior in propylene, pentene and their derivatives has been systematically investigated firstly at the molecular level employing the density functional theory (DFT), atoms in molecules (AIM), and natural bond orbital (NBO) theory. The factors influencing the intramolecular PT have been studied via the changes of the end-group atoms. The influence mechanisms of the microscopic water molecules via the H-bond interactions on the PT have also been discussed. Secondly, the reaction mechanisms for the CO2capture by three types of ionic liquids, i.e.,1,2-dimethyl-(3-aminoethyl) imidazolium tetrafluo-roborate ([aEMMIM][BF4]), trihexyl (tetradecyl) phosphonium prolinate and methioninate ([P66614][R](R=Met, Pro)), and1-ethyl-3-methylimidazolium glycine ([EMIM][Gly]) have been investigated. The influence mechanisms of the intramolecular H-bonds between cation and anion in ionic liquids on the intermolecular PT behavior in the rate-controlling step have been explored. Additionally, the influence mechanisms of intermolecular H-bonds associated with microscopic water molecules on the intermolecular PT in the CO2capture process by ionic liquid [EMIM][Gly] have been further investigated. The main investigations are divided into five chapters as follows:Firstly, the research backgrounds for the present study have been overviewed briefly as well as the computational methods widely used in quantum chemical calculations to lay the theoretical basis for the following studies. At the same time, the research contents carried out here and the significance of the present study have been also outlined.In the second chapter, the intramolecular PT reactions in propylene, pentene, and their derivatives have been systematically investigated so as to obtain the useful insights into the influence of the changes of the end-group atoms and the intermolecular H-bonds of water molecule on the intramolecular PT reaction. It was found that the heteroatom substitutions can promote the intramolecular PT for propylene and pentene although the influence mechanisms are multiple. The presence of one water molecule has a different effect on the intramolecular PT. Namely, water molecule acts as a positive catalyst for the propylene derivatives. On the contrary, it is a negative catalyst for the pentene derivatives. In addition, there is a good correlation between the reaction barriers and the charges on the transferring proton regardless of the presence of water molecule or not, which can be used to predict the reactivity of the intramolecular PT in the analogous systems.In the third chapter, the reaction mechanisms for CO2capture by amine-functionalized ionic liquid [aEMMIM][BF4] and [P66614][R](R=Met, Pro) ionic liquids have been systematically investigated. The coupling interaction characters between cation and anions of ionic liquids and the reaction mechanism of CO2capture have been clarified through the constructions of the reaction potential energy profile of the whole reaction. Moreover, the IR spectra of the ionic liquids before and after CO2capture have been analysized and assigned, where the reliability of the B3LYP method has been verified. It was found that the high stability of the ionic liquids should be attributed to the multiple H-bonds between anions and cations of the ionic liquids. Here, the H-bonds should be dominated by the electrostatic interactions in [aEMMIM][BF4]. While in [P66614][R](R=Met, Pro), strong H-bonds possessing the partial covalent characters have been observed. The intermolecular PT is the rate-controlling step in the CO2capture process by ionic liquids. For [aEMMIM][BF4], it can capture CO2through its cation fragment, where the anion can promote the reaction through intramolecular H-bond interactions. On the contrary, it is the anion fragment that captures CO2for [P66614][Met] and [P66614][Pro]. Moreover, the potential applications of trihexyl (tetradecyl) phosphonium threoninate ionic liquid in the capture of CO2have been predicted theoretically. Hopefully, the present results can promote the relevant experimental process.On the basis of the relevant experimental reports, in the forth chapter, we have designed and investigated the CO2capture by ionic liquid [EMIM][Gly]. The reaction mechanism for the CO2capture has been clarified through the investigations of the thermodynamical and kinetic behavior of the whole reaction process. As a result, it was found that the rate-controlling step is the intermolecular PT between the N atom of amino group of anion in ionic liquid and O atom of CO2, resulting in the formation of carbonyl acid structure. The presence of an explicit water molecule can significantly promote the proceeding of the CO2capture through the formation of the intermolecular H-bond interactions. The cation of the ionic liquid can enhance the stability of the initial intermediate although it slightly depresses the proceeding of the reaction. Thermodynamically, the CO2capture process by ionic liquid [EMIM][Gly] is a non-spontaneous and endothermic reaction in the standard state, where high temperature and low pressure are favorable for the proceeding of the reaction. Expectedly, the present results can provide the theoretical guidance and scientific basis for the experimental design and exploration of the novel and high efficient CO2capture materials on the basis of the imidazolium-amino acid ionic liquids.Finally, the important conclusions and innovation points in the present study have been summarized and emphasized so as to provide important references and theoretical guidance for the future relevant studies.