Theoretical Investigations On The Reactions Mechanisms And Kinetics Of Halogenated Methyl, Haloalkane And Hydrofluoroether With Several Radicals

Halogenated Methyl (such as CH₂F, CH₂Cl and CH₂Br) is the important intermediates derives from the combustion of halogenated methyl or waste containing halogen, which plays a significant role in the combustion chemistry and atmospheric chemistry.Hydrochlorofluorocarbons (HCFCs) and hydrofluoroethers (HFEs) have the similar physical chemistry property of CFCs. They have been the first and third generation alternatuvel compounds to CFCs and are used in various industrial applications. It is necessary to know the atmospheric lifetime in order to better assess the atmospheric and environmental impact of HCFCs and HFEs. Therefore, detailed investigations on the mechanisms and kinetics of those reactions are very important to control atmospheric pollutions.Using ab initio and density function theory (DFT) chemistry methods, we studied the the detailed mechanisms and pathways of halogenated methyl (CH₂Br,CHBrCl and CH₂I), haloalkanes((CH₂ClF and C2H5I), and hydrofluoroethers(CF3OCHFCF3) with active radicals and atoms reactions. Furthermore, based on the potential energy surface (PES) obtained, kinetics properties for series reactions of haloalkane and hydrofluoroethers. Theoretical prediction of the rate constants, the temperature dependence of branching ratios are provided for further studying and using these reactions experimentally. The important and valuable results in this thesis are summarized as follows:1. Theoretical studies on the mechanisms of CH₂Br/CHBrCl+NO2 reactions: (1) For the CH₂Br + NO2 reaction, the C atom of CH₂Br radical can barrierlessly attack the O atom of NO2 to form the initial radical-molecular adduct H₂BrCONO-trans and H₂BrCONO-cis. Two primary products P1 (CH₂O + BrNO) and P2 (CHBrO + HNO) are obtained. (2) For the CHBrCl + NO2 reaction, three kinds of primary products P1 (CHClO + BrNO), P2 (CHBrO + ClNO) and P3 (CBrClO + HNO) should be observed. Among these products, P1 is the most favorable product while P2 and P3 are second and third feasible products, respectively. (3) The PES features of the CH₂Br + NO2 and CHBrCl + NO2 reactions are similar. The triplet pathways have much less competitive abilities for both reactions and can thus be neglected.2. For the theoretical studies on the mechanisms CH₂I + NO2 reaction: (1) With the different atom (N or O atom) in NO2 approaching to C atom, different initial intermediates are formed, H₂ICNO2, H₂ICONO-trans, and H₂ICONO-cis, respectively. (2) Starting from H₂ICONO-trans, two primary products CH₂O + INO and CHIO + HNO, and one secondary product CH₂O + I + NO should be observed. CH₂O + INO may be the most feasible product with a largest yield, and CHIO + HNO may be the second favorable products. Our results agree well with the experimental observation for CH₂I + NO2 reaction. (3) The PES features of the CH₂I + NO2 and CH₂Br + NO2 reactions are similar. The triplet pathways have much less competitive abilities for both reactions and can thus be neglected. From electronegativity and barrier height analysis, we predicted that reaction CH₂I + NO2 is faster than CH₂Br + NO2, which is in accord with the experimental results. 3. The reaction Cl + CH₂FCl→products have been studied by dual-level direct dynamics methods. The potential energy surface information is obtained at the QCISD(T)/6-311++G(d, p)//MP2/6-311G(d, p) level. Three reaction channels are identified, i.e., H-abstraction, Cl-abstraction, and F-abstraction. The calculated potential barriers show that major pathway is H-abstraction channel leading to the products, CHFCl + HCl. For each individual reaction channel, the theoretical rate constants in the temperature region of 220-3000 K are calculated by the canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT). The calculated total rate constants of these reactions are in good agreement with the corresponding experimental values. Three-parameter rate constant expressions of the whole reaction is given as follows: (in unit of cm3 molecule-1 s-1) k(T) = 1.48×10-17T2.04exp(-913.91/T).4. The multichannel reactions C2H5I + Cl are studied by a dual-level direct kinetics method. The potential energy surface information is obtained at the MP2 level with 6-311++G(d, p) basis set for C, H and Cl and aug-cc-pVTZ-PP basis set for I atom. The higher-level energies for the stationary points and extra points along the minimum energy path are refined at the CCSD(T) and QCISD(T) levels. For the title reaction, four reaction channels are identified. The rate constants and the branching ratios for three hydrogen abstraction reaction channels and the corresponding deuterium substitution reaction channels are calculated by the ICVT incorporating SCT correction. The contribution of H-abstraction from -CH₂- group channel is important below 1200 K. However, H-abstraction from -CH3 group channel is competitive at higher temperatures. The three parameter expression for the total reaction within 220-1500 K is k(T) = 2.33×10-16 T 1.83 exp(-185.01/T) cm3 molecule-1 s-1. The deuterium KIE is significant in the low-temperature range, but is less important in the high-temperature region.5. The geometries, frequencies of all the stationary points, the minimum energy paths (MEPs), and reaction mechanisms of CF3CHFOCF3 + OH and CF3CHFOCF3 + Cl reactions. Standard enthalpies of formation of CF3CHFOCF3 and CF3CFOCF3 radical are estimated theoretically using group-balanced isodesmic reactions (R7.3a-R7.4c). The rate constants of the hydrogen abstraction reactions calculated by improved canonical variational transition state theory with small-curvature tunneling (ICVT/SCT) correction are consistent with the available experimental values. The rate constant calculations show that the rate constants of CF3CHFOCF3 + Cl reaction have positive temperature dependence. For CF3CHFOCF3 + OH reaction, at lower temperature (220-250 K), the rate constants have negative temperature effect. The three-parameter Arrhenius expressions are as follows (in units of cm3 molecule-1 s-1): k1 = 2.87×10-21 T 2.80 exp (-1328.60/T) k2 = 3.26×10-16 T 1.65exp (-4642.76/T).