Number Of Molecules Of Atmospheric Chemical Reaction Mechanism Of Quantum Chemistry Study

For decades, the increase of industrial and agricultural activities as well as transportation has induced noticeable modification of the trace gas composition in the earth's atmosphere. In particular, there has been a steady increase of the concentration of atmospheric nitrogen oxides, principally referring to nitric oxide (NO), nitrogen dioxide (NO₂) and nitrous oxide (N₂O). These compounds are actively involved in both the tropospheric and stratospheric chemistry and contribute to environmental problems such as photochemical smog, acid rain, global warming and particularly, stratospheric ozone depletion. NO_x (NO_x=NO+NO₂) are generated at the earth's surface by combustion and biological processes. Therefore, human activities have a direct impact on the annual emissions of NO_x. An important source for NO_x emission during combustion is the oxidation of nitrogen-containing compounds. Among them, the chemistry of organic-nitro compounds has received special attention from experimental and theoretical chemists for the significant role they play in propellent ignition, combustion and air pollution. Information on their dissociation mechanism and kinetics is critical for understanding their extremely complex reactions in the atmosphere.This work mainly investigated four systems of molecules in atmosphere, including Peroxyacetyl nitrate (PAN, CH₃C(O)OONO₂), Chlorine nitrate (ClONO₂), N-methyl-nitramine (CH₃NHNO₂) and chloromethylperoxy radical (CH₂ClO₂)+HO₂ in detail, in an attempt to elucidate the reaction mechanisms and kinetics in gas phase. The following results are obtained; (1) The complex potential energy surface for the unimolecular isomerization and decomposition of PAN, including 11 isomers, 45 interconversion transition states and 17 major dissociation products, was theoretically probed at the G2MP2//B3LYP/6-311G(2d,2p) level of theory. The geometries and relative energies for various stationary points were determined. Based on the calculated G2MP2 potential energy surface, the possible unimolecular decomposition mechanism of PAN was proposed. It is shown that the most feasible decomposition channels of PAN are those leading to ²CH₃C(O)OO + NO₂,²CH₃C(O)O + ²NO₃, and ²CH₃ + CO₂ + ²NO₃, respectively. Among them, the formations of the products of the first two channels are produced by the homolytic O-N and O-O bond ruptures of PAN with the bond dissociation energies of 32.3 and 33.9 kcal/mol, respectively, while the last one is initiated by the concerted C-C and O-O bond fissions via a barrier of 36.5 kcal/mol. Our results suggest that besides the first two decomposition pathways which have been reported by the literature, the last concerted bond fission dissociation channel via a well defined transition state is also feasible, which has been confirmed by using CBS-Q and CBS-QB3 methods.