Time-dependent Quantum Dynamics Study For Cl+CH₄ Reaction

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—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 Cl+CH₄→HCl+CH₃ is for the first time explored with SVRT model. According to the model theory, the reaction polyatomic molecule H-CH₃ is regarded as a semirigid vibrating rotor which is made up of one H atom 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 Cl+CH4→HCl+CH₃ reactions and the Joaquín provided potential energy surface is adapted to calculate separately the above-mentioned three 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, and the total cross-section as well as the rate constant for the Cl+CH₄ reaction's the first excited state.After comparing and analyzing the calculated results, we get the following conClusions: First, each of the three 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 each of the three reaction systems increases with the enlargement of the translational energy while the rate constant enhances with the rising of the temperature. For D+CH₄ reaction, the vibrating exciting increases enormously the total cross-section while the reaction threshold lowers, as is consistent with the regulations of the reaction probability changing. Moreover, the fact that the rate constant is far higher in the vibrating excited state than in the ground state indicates that vibrating exciting benefits the reaction process. Fifth, In Cl+CH₄ reaction, quantum resonance structure appears prominently in the reaction probability curve, as is similar in characteristics to H+H₂, H+CH₄ abstract reaction. However, in the total cross-section the prominent resonance structure disappears due to the fact that total cross-section is the sum of the reaction probabilities in different J and during the process of the summing the vibrating structure counteracts each other.All in all, in this paper, after the SVRT model research on Cl+CH₄→HCl+CH₃ reaction systems is made, some important micro physics dynamics machineries is 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.