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44056-AC6
Quantitative Quantum Dynamics Study of Polyatomic Reactions
We recently made new improved method to study quantum polyatomic reaction dynamics based on the framework of semi-rigid vibrating rotor target model (SVRT) model. The SVRT model is a reduced dimensional model and captures the essential stereodynamics of the reaction system. However, the SVRT model give reaction probabilities and therefore cross sections that can be shifted in the energy-dependence. This is due to the reduced dimension nature of the SVRT model in which the change of zero point energies of neglected degrees-of-freedom are not corrected. By introducing an adiabatic energy correction, which in the simplest form is just a zero point energy shift, we can make a simple energy shift based on the zero point energies of all degrees-of-freedom to get the correct reaction energy threshold.
We used this approach to study several reaction systems. Our calculation for the H + CH4 reaction on the Jordan-Gilbert surface shows that the amount of energy shift needed is negligible. For the O + CH4 reaction, there is a relatively large energy shift. By employing this energy correction, the new shifted energy-dependence of the reaction probability for the O + CH4 reaction becomes in good agreement with the experimental measurement. This work is currently in progress. In addition, we also used this improved SVRT model to study a new reaction, the reaction of H + NH3 -> H2 + NH2 on an ab initio potential energy surface. Our calculation shows that our calculated reaction rate constant is smaller than that from the transition state theory. This result is consistent with the previous study of H + CH4 reaction and adds further evidence to our previous findings that the transition state rate constant can systematically over-estimate reaction rate constant for polyatomic reactions with many degrees-of-freedom. More investigations of reaction systems are currently in progress.
