الفهرس 
Chapter 1: introductionOnly 14 pages are availabe for public view
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Abstract
In this thesis, we have studied theoretically the thermochemsitry and kinetics of unimolecular thermal degradations and bimolecular oxidation of isobutanol as model biofuel in the gas phase (298 K) using density functional theory (DFT) at (BMK/6-31+g(d,p), BMK, BB1K level) and ( MP2, multilevel CBS-QB3) ab initio calculations. The results were compared with n-butanol and 2-butanol. The thesis consists of three chapters as follows Chapter 1 presents types of energy resources, renewable and non renewable energy resource. Due to the need for renewable and clean energy sources, we briefly gave a short background on biofuels which represent one of the promising energy resources. Our attention in this work was paid to one type of biofuels, namely bioalcohol. Lower alcohol such as bioethanol is the most widely motor fuel used in Brazil and in the United States is suffering from a number of drawbacks. Therefore, research has been directed to higher alcohol such as biobutanol. Isobutanol has been used in food industry, solvent as well as in pharmaceutical and chemical industry for a long time. Isobutanol has high energy density, low vapor pressure, high octane hydrocarbon that burns in a combustion engine such as conventional gasoline without affecting the performance. Isobutanol is also readily converted to butenes that can be used directly to produce hydrocarbon-based fuels and others materials. So, we reviewed shortly its ways of production. Studying the combustion chemistry of these biofuels is needed to understanding of the process and minimization of the production of toxic intermediates such as aldehydes Therefore, we have performed quantum chemical calculations to predict the thermal decomposition and oxidation pathways of isobutanol. Chapter 2 includes a short background on quantum chemical calculations and a detail description of the procedures used throughout this work. Geometry optimizations have been performed at the BMK/6- 31+G(d,p) levels of theory. For all stationary points we have carried out frequency calculations at BMK/6-31+G(d,p) to characterize their nature as minima or transition states and to correct energies for zero-point energy and thermal contributions. Energies were further refined at the BMK/6- 311++G(2d,2p)//BMK/6-31+G(d,p) and BB1K/6-311++(2d,2p)//BMK/6- 31+G(d,p)level of theory. Ab initio multilevel CBS-QB3 procedures also have been used to obtain more accurate results and judge the performance of the DFT method. Chapter 3 presents results and discussion. Our findings can be summarized as follows: BB1K seems superior to BMK for both thermal decomposition and oxidation pathways as declare from recalculation carried out on 1-butanol and comparison with benchmark method present. Enthalpies of formation from isodesmic equations at BMK and atomization energies at the CBS-QB3 level give the most accurate results when compared with the available experimental data. CBS-QB3 bond dissociation energies (BDEs) calculation for isobutanol show the following : • Cα-Cβ and O-H bonds as the weakest and strongest bonds, respectively. • The C-O and Cα-H bonds have comparable strength. Summary - 110 - • The difference in energy between the two Cα-H bonds is modest (0.45 kcal/mol). • The C-H bond strength of isobutanol follows the order Cα-H8 < Cα- H9 < Cβ-H < Cγ-H. Therefore, abstraction of H from Cα is the most preferable pathway in the course of oxidation of isobutanol Thermal unimolecular decomposition of isobutanol can proceed via two main pathways, namely simple and complex decompositions. The first category represents simple bond scission, while the second group includes bonds are breaking and forming simultaneously. Seven complex decomposition and seven simple decomposition have been investigated. Among the seven complex fissions, four involve formation of isobutene, carbene, methyl cyclopropane, and butanal via dehydration reactions. The other three channels occur through migration of a hydrogen atom from carbon or oxygen atoms to other carbons. Isobutanol decompose mainly by H abstraction This is because it has two easily abstractable Cα-H bond. Therefore, biomolecular oxidation of isobutanol using •OH have been investigated. H-abstraction from different positions has been considered. Six transitions state have been located for H-abstraction reaction. Some reactions pass through prereactive intermediates. All reactions have post-reactive complexes before the formation of the products. Pre-reactive complexes are adducts exist between reactants and transition states, while, post-reactive intermediates are located between transition states and products. They represent adducts between product radicals and water. Pre- and post-reactive are intermediates account for the presence of negative activation energy in some exothermic reactions. We calculated the minimum energy path (MEP) connecting reactants to products through intrinsic reaction coordinate (IRC) in mass-weighted cartesian coordinates. The potential energy diagrams for all of the investigated channels were constructed at BMK, BB1K and CBS-QB3 levels of theory. Finally, rate constant calculations for all channels at different temperatures were computed using the conventional transition state theory (TST). All channels of thermal decomposition are endothermic and the least endothermic channel is the formation isobutene and water. While, formation of hydroxyl methyl and isopropyl radicals are considered as the most favorable channel for simple isobutanol decomposition. All channels of oxidation of isobutanol by OH radicals are exothermic. Based on the calculated energy barriers for oxidation channels, the order for H-abstraction reactions is β < α < γ < OH at BB1K/6- 311++G(2d,2p)//BMK/6-31+G(d,p). H-abstraction from the Cα and Cβ positions was found to be the most favored kinetically and thermodynamically, with the former being the most exothermic as shown from the branching ratios. Preferable abstraction from Cα-H was attributed to the weakness of C-H bond, while the feasibility of abstraction from Cβ was explained in term of hydrogen bond stabilization of transition state.