Theoretical Study Of Reactive Collision Dynamics For H₂X（X=Au, Li, S）Reaction Systems

Molecular reaction dynamics is a discipline which studies chemical reaction at the micro level, and it can help to understand the mechanism of chemical reaction. The potential energy surface is an important foundation for study of reaction dynamics, and the quality of the potential energy surface directly decides the accuracy of the results of the molecular reaction dynamics. Theoretical study of reactive collision dynamics for several H2X reaction systems has been carried out in this work. First, we study the hydrogenation reactions of Au atom (Au+ H2, Au+HD, Au+D2) and discuss the reaction mechanism; the potential energy surfaces of H+LiH and S(’D)+H2 reactions are constructed and the reaction dynamics are studied based on the new potential energy surfaces. The specific work includes the following four parts:(1) The time-dependent quantum wave packet (TDWP) and quasiclassical trajectory calculations (QCT) are carried out for the Au+H2→AuH+H reaction on a global potential energy surface, and the results of the reaction probabilities, integral cross sections (ICSs) and differential cross sections (DCSs) are obtained. For reaction probabilities, there are a mass of sharp oscillations at low collision energy, which can be attributed to the resonances supported by the potential well. Due to the endothermicity of the title reaction, the total ICS shows a threshold about 1.53 eV. In order to further investigate the reactive mechanism, the lifetime of complex is calculated by QCT method. At the low collision energy, most intermediate complexes are long-lived, which implies that the reaction is governed by indirect reactive mechanism. With the collision energy increasing, the direct reactive mechanism occupies the dominant position. Due to the change of the reactive mechanism, the angular distribution shifts toward the forward direction with collision energy increasing. The isotopic variant, Au+D2→ AuD+D reaction, is also calculated by TDWP method. The calculated reaction probabilities and ICSs show that the isotope effect reduces the reactivity. (2) The TDWP and QCT calculations are carried out for the Au+HD reaction. The reaction probabilities, ICSs and DCSs of the two reactive channels (AuH+D/AuD+H) are calculated. The ICS results indicate that the dominant reactive channel changes from AuD+H to AuH+D with the collision energy increasing. As known from the distributions of complex lifetime, this change mainly results from the difference of direct reactive mechanism in the two channels. With collision energy increasing, the growth of direct reactive collisions in AuH+D channel is much larger than that in AuD+H channel. It is explained by the fact that, in direct reactive mechanism with a small impact parameter, the D atom with a larger mass can get away easily from the potential well, and the H atom is likely to be captured by Au atom. To compare vector property of the two reactive channels, study on stereodynamics for the title reaction is also carried out by QCT method. Due to more direct reactive collisions, product rotational polarization of AuH+D channel is more obvious than that of AuD+H channel.(3) Based on high-level energies, a new accurate PES is obtained for the ground electronic state of LiH2 system using the neural network method. The energy points are calculated at the multireference configuration interaction level with aug-cc-pVXZ (x=Q,5) basis sets, and these energies are extrapolated to the complete basis set limit. The neural network method and hierarchical construction scheme are applied in the fitting process and the root mean square error of the fitting result is very small (0.004eV). The dissociation energies and equilibrium distances for LiH(X1∑+) and H2(X’Eg+) obtained from the new PES are in good agreement with the experimental data. On the new PES, the time-dependent wave packet studies for the H(2S)+ LiH(X1∑+)→Li(2S)+H2(X1∑g+) reaction have been carried out. In this reaction, no threshold is found due to the absence of the energy barrier on the minimum energy path. The calculated ICSs are high at low collision energy and will decrease with the increase of the collision energy. The product molecule H2 tends to be forward scattering due to direct reactive collisions, which becomes more evident at higher collision energies.(4) We constructed a new accurate PES for the electronic ground state (’A’) of H2S, based on 21,300 ab initio energy points. The ab initio energies are obtained from multireference configuration interaction calculations with a Davidson correction using basis sets of quadruple zeta quality. The neural network method is applied to fit the PES, and the root mean square error of fitting is small (1.68 meV). TDWP studies for the S(’D)+H2(X1∑g+)→H(2S)+ SH(X2П) reaction on the new PES are conducted to study the reaction dynamics. The calculated ICSs decrease with increasing collision energy and remain fairly constant within the high collision energy range. Both forward and backward scatterings can be observed as expected for a barrierless reaction with a deep well on the PES. The calculated ICSs and DCSs are in good agreement with the experimental results.