| The study and determination of reaction rate constants has always been one of the main research fields in chemistry. It is one of the most active subjects to predict the rate constants theoretically. In this thesis, ab initio and density functional theory combined with the direct dynamics methods have been used to study the following chemical reactions: CCl3CH2OH + Cl→products CCl3CH2OH + OH→products CH3CF2Cl + F→productsConcerning the adverse impact of chlorofluorocarbons (CFCs) on ozone and global warming, an international effort has been made to replace CFCs with environmentally friendly alternatives. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are considered to be two important classes of CFCs substitutes over the past decade. Recently, halogenated alcohols have been suggested as alternatives to CFCs and HCFCs, therefore their atmospheric concentrations are expected to increase. They contain at least one C–H bond, which makes it readily react with the atmospheric constituents, such as F, Cl atoms and OH radicals to reduce their lifetime in the atmosphere. The main object of the current thesis is to provide accurate results of the reaction mechnism and the temperature dependence of rate constants. The theoretical results may provide useful information for further experimental studies.Firstly, the geometries and frequencies of the stationary points (reactants, complexes, products and transition states) are calculated at several levels, such as, B3LYP, MP2 and MPW1K levels; then, the minimum energy path (MEP) is calculated at the same level by intrinsic reaction coordinate (IRC) theory to confirm that the transition state really connects the minimums along the reaction path. The first and second energy derivatives at geometries along the MEP are obtained to calculate the curvature of the reaction path and the generalized vibrational frequencies; thirdly, the potential profile is refined at the levels, such as, G3(MP2), MC-QCISD and CCSD(T)+CF. All of these calculations are performed by Gaussian 03 and Gaussian 09 program. By means of Polyrate 9.7 program, the rate constants are calculated by conventional transition state theory (TST), canonical variational transition state theory (CVT) and canonical variational transition state theory with small–curvature tunneling correction (CVT/SCT). The main results are summarized as follows:(1) The theoretical study on the reactions CCl3CH2OH + Cl→products (3.1) and CCl3CH2OH + OH→products (3.2) indicates that: For each reaction, two abstraction channels, i.e., methylene– and hydroxyl–hydrogen abstraction, are located. Each reaction channel firstly forms a pre–transition complex, then, through a transition state to reach the products. The enthalpies of formation of CCl3CH2OH, CCl3CHOH and CCl3CH2O species are -69.61±1.2, -23.09±1 and -14.89±1 kcal mol-1, respectively, which are calculated by several levels via isodesmic reactions. The rate constant calculations are carried out by using the variational transition–state theory (VTST) at the MC–QCISD//B3LYP/6–311G(d,p) level over a wide temperature range of 200 K–2000 K. The total rate constants of the title reactions agree well with the available experimental values. It is shown that for the reaction of CCl3CH2OH + Cl, methylene–H–abstraction channel giving CCl3CHOH + HCl as products prevails at lower temperatures, while product CCl3CH2O + HCl occupies a small part at high temperatures. In case of the reaction of CCl3CH2OH + OH, methylene–H–abstraction pathway dominates the reaction over a whole temperature range. The four–parameter expressions (in cm3molecule-1s-1) for the title reactions within 200 K–2000 K are k1=4.04×10-13(T/300)2.44exp[512.0(T+287.9)/(T2+287.92)] (R3.1) and k2=2.43×10-13(T/300)2.71exp[–126.4(T+418.6)/(T2+418.62)] (R3.2).(2) The theoretical study on the reaction CH3CF2Cl + F→products (4) indicates that: From the mechanistic point of view, two reaction pathways are available: CH2H′CF2Cl + F→CH2CF2Cl + H′F (R4a)→CH H′CF2Cl + HF (R4b)The rate constants calculated by the variational transition–state theory (VTST) at the G3(MP2)//MPW1K/ 6-311+G(d,p) level are observed to agree well with the available experimental value. In the lower temperatures, channel R4a will be the major reaction channel. With the temperature increasing, the contribution of channel R4b will increase and exceed that of channel R4a. In the higher temperatures, channel R4b will become the major reaction channel.
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