Hoo + Hoo Gas Reaction Mechanism And Kinetics Of Theoretical Research

The hydroperoxy radical (HOO) is not only the simplest and most important peroxy radical in the chemistry of Earth's atmosphere from troposphere to mesosphere, but also a key transient intermediate in the combustion of hydrocarbon fuels, atmospheric photolysis cycles, and biochemical processes. In particular, the gas-phase bimolecular self-reaction of HOO plays an important role in atmospheric chemistry, the oxidation reactions with ozone and oxygen, and biochemical systems. This reaction has been shown to be dominated by formation of hydrogen peroxide (HOOH) and oxygen molecule (O2) and is a key process controlling steady-state concentrations of HOX radicals in the troposphere, as well as the main source of HOOH in the atmosphere.Our objective investigation in this thesis is to provide theoretically basics to reveal the detailed mechanisms and dynamics for the hydrogen abstraction reaction of the biradical HOO without and with water molecules. Using Moller-Plesset perturbation theory (MP2) and Coupled Cluster Theory(CCT), the structures and frequencies of the reactants, products, intermediates and the transition states along the reaction pathways have been obtaind as well the potential energy surface information. Besides, we use the Polyrate 8.2 and VKLab programs to calculate the rate constants by means of the conventional transition state theory (TST) and canonical variational transition state theory (CVT) without and with small-curvature tunneling correction (SCT). The main contents and conclusions in our work are listed as follows:In the 3rd chapter, a theoretical study of the mechanism and the kinetics for the hydrogen abstraction self-reaction of HOO has been presented at the CCSD(T)/6-311++G(3d,2p)//CCSD/ 6-31+G(d,p) level of theory.The triplet reaction channel a is the dominating reaction channel and Path (2) is found to be the most favorable pathway. The rate constants of Path (2) for the triplet reaction channel are evaluated by means of theκTST,κCVT andκCVT/SCT using Polyrate 8.2 procedure. The fitted three-parameter expression for Path (2) isκCVT/SCT=1.4×10-37T7.0exp(-5012.3/T) cm3·molecule-1·s-1 in the temperature range of 200-2500 K.In the 4th chapter, the mechanism and kinetics of a water-assisted self-reaction of HOO have been studied by quantum chemistry methods and variational transition state theory. The geometries and frequencies of all the stationary points, including the reactants, intermediates, transition states and products as well as the selected points along the singlet and triplet potential energy surfaces for the title reaction are calculated at the MP2/6-31+G(d,p) level of theory. The single point energies are refined at the CCSD(T)/6-311++G(3d,2p) level of theory.The calculated results show that the title reaction has 2 reaction channels and 7 pathways. Path (1W) and (4W) were proved to be the most favorable pathway of channel (aw) and (bw), respectively. Furthermore, the favorable pathways for both Path(1W) and (4W) were obtained through hydrogen abstraction H of HOO by the 0(1) atom end of HOO·HOH complex.Comparing with the barriers for the title reaction without water molecule, the barrier of Path (1W) for formation of (H2O2·HOH+3O2) reduces by 4.92-9.62 kcal·mol-1,respectively, while the barriers of Path (4W)-(7W) for formation of (H2O·HOH+1O2) reduces by 1.09-14.90 kcal·mol-1 for different pathways. In comparison with the CVT/SCT rate constants of Path(1)without water molecule at the CCSD(T)/6-311++G(3d,2p)//MP2/6-31+G(d,p) level of theory, the CVT/SCT rate constants of Path(1W) from CW to PWl at the same level is enhanced by 1.88-10.60 times within the temperature range of 200-600 K. The theoretically calculated CVT/SCT rate constants of the favorable path of Path(1W) from CW to PW1 at the CCSD(T)/6-31++G(3d,2p)/MP2/6-31+G(d,p) level of theory are in good agreement with the available experimental values at the experimentally measured temperatures.The total rate constantsκCVT/SCT and the ratios of the rate constants (κCVT/SCT/κaCVT/SCT) in the presence of water are good agreement with the experimental values. The variational effect is not important for the title reaction in the presence of a water molecule within the temperature range of 200-600 K and the small-curvature tunneling plays an important role particularly in the low temperature range. A water-assisted reaction mechanism for the title reaction is expected to be reasonable based on the dual level kinetic study.In the 5th chapter, the mechanism of two water-assisted self-reaction of HOO have been studied by CCSD(T)/6-311++G(3d,2p)//MP2/6-31+G(d,p) level of theory. The calculated results show that the title reaction has 2 dominating reaction channels and 2 pathways. Path(11W) is proved to be the most favorable pathway and its apparent activation energy is-0.06 kcal·mol-1.Comparing with the barriers for the HOO+HOO reaction without H2O at the level of CCSD(T) /6-311++G(3d,2p)//MP2/6-31+G(d,p), the barrier of Path (11W) for formation of (H2O2·2HOH+3O2) reduces by 0.19 kcal·mol-1.Besides, comparing with the barriers of Path (1W) for the HOO+HOO reaction with a water molecule at the CCSD(T)/6-311++G(3d,2p)//MP2/6-31+G(d,p) level, the barrier of Path(11W) is 4.73 kcal·mol-1.