Full-Dimensional Potential Energy Surfaces And Dynamics Studies Of Polyatomic Molecular Reactions

In this thesis,based on high level ab initio methods,we constructed the full-dimensional potential energy surfaces(PESs)for the H+H2O2 and OH+HO2 reaction systems using fundamental invariant neural network(FI-NN)fitting method.Quasi-classical trajectories(QCT)calculations and ring polymer molecular dynamics(RPMD)calculations are performed for the H+H2O2 two-channel reaction.Based on the latest PES for the Cl+CH4 reaction constructed by Chen et al.,the RPMD rate coefficients of the H atom abstraction reaction are calculated and the kinetic isotope effects(KIEs)were explored.The H+H2O2 reaction plays a critical role in the H2/O2 combustion system.We constructed an accurate full-dimensional ground-state PES based on a high level ab initio method and newly developed FI-NN fitting approach.Extensive QCT calculations were carried out on the new PES at the collision energy of 15 kcal/mol.The product translational energy distributions,angular distributions and rovibrational state distributions of the H+H2O2?H2+HO2 and H+H2O2?OH+H2O product channels were obtained.In addition,we used three theoretical methods including RPMD to compute the chemical reaction rate coefficients of the two reaction channels between 200 K and 1000 K.The comparison made among the calculated results and the available experimental values shows good agreement,demonstrating the accuracy of this FI-NN PES and the applicability of the RPMD to produce accurate rate coefficients of the polyatomic reaction.The OH+HO2 reaction is always considered to be the end step of chain reaction in the combustion model since H2O and O2 are relatively stable products.Due to the multi-reference character of diradical reactants,we employed both CASPT2 and UCCSD(T)-F12 ab initio methods and constructed the low-lying triplet ground-state(3A)and singlet excited-state(1A)full-dimensional PESs for the OH+HO2 reaction,which lays a foundation for the further dynamics investigation for this reaction system.Cl+CH4 is a typical system for studying the basic theory of polyatomic chemical reactions.Based on the new ab initio full-dimensional PES constructed by Chen et al.,we calculated the RPMD rate coefficients for the Cl+CH4?HCl+CH3 reaction in the temperature region of[300,1000]K.The RPMD rate coefficients are in excellent accordance with the previous experimental ones.Furthermore,the kinetic isotope effects of this H atom abstraction reaction are discussed.We find that the KIEs are more remarkable at low temperatures.