Time-dependent Quantum Dynamics Study For Polyatomic Molecule Reaction Collision

Research with quantum theories on the rules of molecular reaction dynamics, which is a main branch of chemical reaction dynamics, is currently one of the most important research topics. With the already ongoing development of quantum scattering theory and the enhancement of computer calculating capacity, enormous progress has been made on the research of polyatomic molecule reaction dynamics, especially on that of the first principles for the state-to-state chemical reaction, which becomes the main task of reaction scattering. Although accurate full quantum calculations can be made currently on reaction systems for four-or-less-atom molecules, what biology and chemistry concerns most are large molecules containing more than four atoms. With the increase of atomic amount and the degrees of freedom, research on dynamics for more-than-four-atom systems becomes more and more difficult, which is why there is great necessity to explore and develop new theory models and calculating methods for polyatomic molecules'chemical reaction. Therefore, some theory models and dimension-reduced calculating methods have been provided.In this paper a recently offered theory model in 1999—semirigid vibrating rotor target (SVRT) model, which is a dimension-reduced model handling polyatomic system reaction, is adopted to study polyatomic molecule reaction. In SVRT model polyatomic molecule whose spacial locomotion can be accurately treated as a regular non-symmetry rotor is dealt with as two different rigid segments which both can vibrate one-dimensionly through the line of their centroid. Since SVRT model can relatively correctly deal with the spacial locomotion, it can exactly demonstrate reaction system's steric dynamics effects, as is a very crucial factor in the research for polyatomic molecule reaction. This model is adaptive to polyatomic molecule one of whose bond is relatively weaker and which can be divided into two segments at the end of the reaction. For polyatomic molecule reaction system, 7 degrees of freedom are necessary to describe it and for atom-polyatom molecule reaction system, only 4 degrees of freedom are much enough.In this paper, the quantum dynamics research on the six-atom molecule reaction system for O(3P)+CH4→OH+CH₃,and F+CH4→HF+CH₃ is explored with SVRT model for the first time. According to the model theory, the reaction polyatomic molecule H-CH₃ is regarded as a semirigid vibrating rotor which is made up of one Hatom and one CH₃ whose geometry structure is fixed. Since CH₃ is dealt as rigid and maintain C3v symmetry in the reaction process, four degrees of freedom are enough to describe the reaction system. In the paper, the time-dependent wave packet method is used to simulate O(3P)+CH4→OH+CH₃ and F+CH4→HF+CH₃ reactions and the J. Espinosa-Garcia with J.C. Garcia-Bernaldez and Cipriano Rangel, Marta Navarrete with J. Espinosa-Garcia provided potential energy surface are adapted to calculate separately the above-mentioned two reaction systems'reaction probability for ground state, vibrating excited state and different rotating excited states, the total cross-section and the rate constant for the ground state.After comparing and analyzing the calculated results, we get the following conclusions: First, each of the two reaction systems has observable reaction probability when it approaches the barrier, which indicates quantum tunnel effects exist obviously. Second, The fact that H-CH₃ molecules'vibrating exciting increase the reaction probability enormously while they decreases the threshold evidently illustrates that the molecule's vibrating energy makes great contribution to collision reaction. Third, the different vibrating states for the molecule have on the reaction probability influences, including that the increase of molecules'vibrating energy makes great contribution to abstract reaction while it has little effects on the reaction threshold and that the initial geometry orientation for the reaction molecule has important influence on the reaction probability. Fourth, the total cross-section of the O(3P)+CH4 reaction systems increases with the enlargement of the translational energy while the rate constant enhances with the rising of the temperature. All in all, in this paper, after the SVRT model research on O(3P)+CH4→OH+CH₃ and F+CH4→HF+CH₃ reaction systems is made, some important micro physics dynamics machineries are disclosed, which provides valuable reference data for combustion chemistry. At the same time, the whole research in the article proves that SVRT model is a kind of accurate and general model handling poly-atom molecule chemical reaction. Theoretically, SVRT model is adaptive to any reaction concerning poly-atom molecule and deserves the application in other poly-atom molecule reaction systems.