Theoretical Studies of Potential Energy Surfaces and Dynamics for the HCl+OH and O+C2H2 Reaction Systems

Author:Zuo Jun Xiang

Supervisor:xie dai qian


Degree Year:2019





Molecular reaction dynamics is one of the most advanced studies in theoretical and computational chemistry,which provides a very useful tool for studying and predicting the properties of reaction systems in many fields,such as atmospheric chemistry and combustion chemistry.It is used to study how elementary reactions occur at the microscopic level and reveal the nature of elementary chemical reactions.In recent years,great progress has been made in the theoretical and experimental study of chemical reaction dynamics,which greatly improves our understanding of the microscopic mechanisms of some important elementary reactions.The quantitative theoretical prediction of simple atomic and diatomic reaction systems and some triatomic reaction systems is available,but the potential energy surfaces(PESs)and dynamical properties for more complex polyatomic systems are still difficult.In this paper,the reliable accurate full-dimensional global PESs have been constructed for HCl+ OH and O + C2H2 reaction systems using high-level theoretical approaches and advanced fit methods.The reaction nature and dynamics mechanism of those reactions have been investigated in depth by employing quasi-classical trajectory(QCT)and quantum dynamics(QD)calculations.Several interesting dynamic phenomena and significant results were obtainedThe reaction of HCl+ OH →Cl H2O is an important elementary reaction in atmospheric chemistry and its precise rate coefficients are particularly important in atmospheric models.This reaction is a typical heavy-light-heavy reaction system,how to accurately characterize the quantum effects such as tunneling,recrossing,and zero point energy(ZPE)remains a challenge in theoretical study.Thermal rate coefficients at temperatures between 200 and 1000 K were calculated for the reaction on a multi-reference configuration interaction(MRCI)PES,which was the most accurate PES at that time,using a full-dimensional approximate quantum mechanical method,named ring polymer molecular dynamics(RPMD).The results show that the reaction is strongly influenced by quantum effects,such as tunneling and recrossing,at low temperature.Agreement with available experimental rate coefficients is achieved on the trend and both have shown a strong non-Arrhenius behavior at low temperature The theory-experiment discrepancy is attributed to the remaining errors in the potential energy surface,which is known to slightly overestimate the reaction barrierIn order to accurately describe the dynamical properties of related reaction,we report here a new and more accurate full-dimensional global PES for the ground electronic state of CIH2O system,which was developed by fitting 15 777 points obtained using an explicitly correlated unrestricted coupled-cluster method with single,double,and perturbative triple excitations(UCCSD(T)-F12b).The fit is carried out using the permutation invariant polynomial-neural network(PIP-NN)method.Reaction path analysis shows that the new PES can better describe the reaction path information,especially the reaction barrier,than the MRCI PES.The results of the quantum scattering calculations indicate that slight differences in the PES can lead to significant differences in reaction dynamics.The thermal rate coefficient of HCl + OH reaction is re-determined on the accurate coupled cluster PES by RPMD method and the primary kinetic isotopic effect is also investigated.It is shown that quantum effects,such as tunneling and ZPE,are of critical importance for the HCl+ OH reaction at low temperatures,while the heavier deuterium substitution renders tunneling less facile in the DCl+ OH reaction.The calculated RPMD rate coefficients are in excellent agreement with experimental data for the HCl + OH reaction in the entire temperature range.However,the RPMD rate coefficients for the DCl+ OH reaction agree with some,but not all,experimental values.On the basis of the self-consistency of the theoretical model and the problem of isotope exchange in the experimental studies of the deuterated reactant,it is concluded that most isotopic experimental measurements contain errors and our results provide theoretical foundation for further experimental measurement.Acetylene(C2H2)is the major intermediate of most hydrocarbon flames and one of the precursors in soot formation,which plays a key role in the generation and growth of PAHs.The O+ C2H2 reaction is the first step in the significant acetylene oxidation.Obtaining accurate kinetic data and understanding the reaction dynamics of this reaction are of great importance.To this end,a full-dimensional global PES for the ground triplet state of the O + C2H2 reaction is constructed based on approximately 85000 ab initio points calculated at the level of UCCSD(T)-F12b/VTZ-F12.The PES is also fit using the PIP-NN approach.The key topographic features of the PES,including multiple potential wells and saddle points along different reaction pathways,are well represented by this fit PES.The kinetics and dynamics of the O+C2H2 reaction are investigated using the QCT method.The calculated rate coefficients are in good agreement with experimental data over a wide temperature range(298-3000 K).The product branch ratio has also been determined.We have also analyzed the reaction flux through different reaction pathways to provide insights into the detailed microscopic reaction mechanisms and dynamics characteristics.