Theoretical Studies Of Potential Energy Surfaces And Quantum Dynamics For N+CH,N+C₂ And H₂/Co Reaction Systems

Molecular reaction dynamics,which investigates the molecular level mechanism of chemical elementary processes,is an integral part of modern chemistry and a bridge between macroscopic and microscopic dynamics.In the past decades,with the rapid developments of experimental technology,theoretical method,and computer technology,much progress has been achieved in this field.With close interplay between experiment and theory,detailed information about dynamics can be obtained to get better understandings of the mechanisms not only for the gas-phase elementary reactions but also for interface reactions.In this thesis,we focus on the nitrogen-bearing atom-radical reactions and gas-surface reactions.To get an in-depth understanding of microscopic mechanisms of these reactions,quantum dynamic calculations for the N+CH and N+C2 reactions were preformed based on newly constructed global potential energy surfaces(PESs).Moreover,the quantum dynamics for dissociative chemisorption of H2 on Co(0001)and Ag(111)surfaces were also investigated intensively.The main achievements of this dissertation are summarized as follows.1.PESs and quantum dynamics for nitrogen-bearing atom-radical reactionsNitrogen molecule is one of the most abundant species in interstellar clouds.However,the recently measured N2 abundance is much lower than the estimated value from astrochemical models,due to the lack of accurate rate constants of the reactions related to the N2 formation.In this work,we studied state-to-state quantum dynamics for N+CH and N+C2 reactions and predicted their low-temperature rate constants.For the N+CH reaction,we developed accurate global PESs of two lowest-lying triplet states of HCN(13A' and 13A")by spline interpolations to more than 37000 points at the MRCI + Q/AV5Z level,and quantum dynamics of the N(4Su)+ CH(X2?r)reaction was then investigated using Chebyshev real wave packet method.The calculated rate constants are in excellent agreement with the latest experimental results,which show a positive temperature dependence,in contrast to earlier experimental work.The reaction probabilities are dominated by numerous oscillations due to long-lived resonances.The CN product is highly excited in both vibrational and rotational degrees of freedom.The forward-backward symmetric differential cross sections are consistent with a complex-forming mechanism.For the N+C2 reaction,we constructed a global PES of the a4A" state for C2N by fitting to more than 13000 ab initio points.State-to-state quantum dynamics was preformed using Chebyshev real wave packet method.The reaction is barrierless and has two deep wells corresponding to the linear CCN and CNC species.The resonances supported by these deep wells manifest in reaction probabilities as numerous oscillations.The rotational distributions of the CN product are highly inverted,while the vibrational distribution decreases with the increace of vibrational quantum number.The DCSs were found to be dominated by scattering in both forward and backward directions with a forward bias,providing evidence for the non-statistic behavior of this complex-forming reaction.The theoretical rate constants increase as a function of temperature over the range from 10 to 300 K,which are in good agreement with the latest experiment,especially around room temperature.These studies will not only improve our understanding of detailed microscopic mechanism of atom-radical reactions in low temperatures,but also provide useful information for further experimental and theoretical studies.2.Quantum dynamics for the dissociative chemisorption of H2 on metal surfacesThe dissociative chemisorption of H2 on the metal surfaces is not only a typical gas-surface elementary process,but also an essential step in many heterogeneous catalytic processes in the industry.To improve the reactivity effectively,it is of great importance to have in-depth understanding of its reaction dynamics.As the simplest gas-surface reaction,numerous work have been performed for H2 on Cu,Ni and Au surfaces.However,there has been few experimental and theoretical studies on the dynamics for H2 on Co surface.In this work,we constructed a six-demensional potential energy surface for this system using DFT calculations and permutation invariant polynomial-neural network method.Six-dimensional quantum dynamics calculations on this PES were performed for studying the dissociative chemisorption of H2.Vibrational energy was found to promote the reaction but much less effectively than the same amount of the translational energy,consistent with Polanyi's rules and the recently proposed sudden vector projection model.In addition,we extensively calculated four-dimensional fixed-site dissociation probabilities in two prototypical dissociative chemisorption processes,D2 on Ag(111)and H2 on Co(0001).It was shown that the reactivity is not only controlled by the height of the barrier,but also by the topography of the PES in the strong interacting region.The 4D site-averaged reaction probabilities cannot reproduce the 6D results very well,especially for the H2/Co(0001)system,due to the significant steering effects found in both systems.These studies will provide theoretical basis for further experimental studies and facilitate more dynamical research for other gas-surface reactions.