Dynamic Studies Of Some Hydrogen Abstraction Reactions

Owing to recognition of the adverse impact of chlorofluorocarbons (CFCs) on stratospheric ozone and greenhouse warming, the use of CFCs has been limited. Looking for acceptable alternatives to replace CFCs has been the international questions. Hydrochlorofluorocarbons (HCFCs) are an important class of CFCs replacement, since they have the C-H bond that can react with the atoms or radicals to reduce their lifetime in the atmosphere. However, HCFCs still have the responsibility for depletion of the ozone layer in the stratosphere and the greenhouse effect. Thus, to estimate the atmospheric lifetimes of such species, accurate data for the rate constants as well as their temperature dependencies are needed. The main aim of this thesis is to provide accurate results for the reaction path, the relation between temperature and rate constants, and the branching ratios. The following reactions are studied: CH₃CHF2 + F→products CH₃CH₂CH₂Cl + OH→products CH₃CHClCH₃ + OH→productsFirstly, the geometries and frequencies of the stationary points (reactants, complexes, products, and transition states) are calculated at the levels, such as, MP2, BMK, and B3LYP; 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, BMC-CCSD, G3(MP2), MCQCISD-MPWPW91, and MCG3-MPWPW91. 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 investigation on the reaction CH₃CHF2 + F→products indicates that: Three reaction channels, one forα-hydrogen abstraction and two forβ-hydrogen abstractions, have been identified. By canonical variational transition state theory (CVT) incorporating the small-curvature tunneling (SCT) the rate constants calculated at the G3(MP2)//MP2 agree well with the experimental values. The three-parameter expression of this reaction is k=3.92×10^-16T1.58exp(264/T) cm3molecule-1s-1.(2) The theoretical investigation on the reactions CH₃CH₂CH₂Cl + OH→products (R1) and CH₃CHClCH₃ + OH→products (R2) indicates that: The small-curvature tunneling effect plays an important role in a lower temperature range for all the reaction channels with the positive barrier heights, while the variational effect is small and negligible for some reaction channels. For reactions OH radicals with CH₃CH₂CH₂Cl (R1) and CH₃CHClCH₃ (R2), the channels of H-abstraction from -CH₂- and -CHCl- groups are the major reaction channels, respectively, at lower temperatures. The agreement between calculated and experimental rate constants is seen to be remarkably good. On the other hand, the values of enthalpies of formation for CH₃CHCH₂Cl, CH₂CH₂CH₂Cl, and CH₃CHClCH₂ species are calculated at the BMC-CCSD//B3LYP/6-311G(d,p) level by isodesmic reactions. Finally, the total rate constants are fitted by two modes, i.e., three-parameter and four-parameter expressions over a wide temperature range of 200²000 K. Four-parameter expression is recommended because it gives smaller fitting errors and provides a better low-temperature asymptotic behavior.