Research On Theoretical Methods Of Adiabatic/Nonadiabatic Dynamics For Elementary Reactions In Gas Combustion

The gas combustion is a process with highly complex chemical reaction network and reaction type.With the improvement of computational hardware and the rapid development of high-precision electronic structure methods,theoretical calculations and simulations are gradually widely used in the areas of elementary reaction dynamics which include the establish of potential energy surface(PES)with highly accurate electronic structure methods,the calculations of the temperature/pressure dependent rate constants,and the probe of reaction dynamics from atom level.On the one hand,transition state theory(TST)is a commonly used method for the electronic adiabatic reactions that involve only a single electronic state(usually the ground state),but classical transition state theory cannot consi der complex factors,such as the anharmonic effects,barrierless reactions,multiple transition states in series,and multiple reaction channels,etc.On the other hand,the luminescence phenomenon during combustion is generally caused by chemiluminescence(transition from a high-energy electronic excited state to a low-energy electronic state),and these excited substances in the combustion reaction network have a great influence on microscopic reaction kinetics and macroscopic flame structure.However,the study of the electronic nonadiabatic reactions and the construction of reaction model that involves excited species is extremely challenging.A key issue in chemical kinetics reaction dynamics is advancing the theoretical framework to handle reactions beyond the domain of textbook theories and developing relevant tools to expand their application.The present thesis has carried out detailed kinetics studies on typical electronic adiabatic/nonadiabatic reactions,and constructed corresponding scheme for typical reactions,meanwhile software/tools have also been developed.The reaction,OH+CO?CO₂+H,is the second most important reaction in combustion chemistry that involves a complex-forming step.In the present study,the reaction-path variational transition state theory(RP-VTST)and the canonical unified statistical theory(CUS)have been used to study this particular reaction.Water molecules have great influence on the gas combustion process.The effect of hydrate OH(H₂O)radical on the CO oxidation path,reaction barrier,and reaction rate is studied.The results show that the rate constant would decrease when take the OH(H₂O)complex as reactant for the CO oxidation.The present study provides a clear solution that can be used in the calculation of high-pressure limit rate constants for processes with multiple reaction paths,and also provides a methodology to study the effect of water molecules in gas-phase reactions.Acetylene molecules(C₂H₂)are important in the fields of combustion chemistry,atmospheric chemistry,catalytic chemistry,and photochemistry etc.The reaction between acetylene molecule and different atoms(H,C,O,F,Cl etc)has been extensively studied.Chlorine atom and chlorinated compounds chlorides are well recognized to inhibit hydrocarbon combustion processes and promote the formation of soot in flames.The reaction of Cl atom with C₂H₂ molecules has been extensively studied,and this reaction is a typical barrierless radical-molecular reaction with two transition states of different type in series.Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory(VRC-VTST)and RP-VTST;then we combine the results with the CUS theory.We have obtained highly accurate high-pressure limit rate constants and also provided clear a scheme for studying similar radical-molecule barrierless association reactions.The electronic nonadiabatic reactions that involved in combustion reaction networks often have complex reaction channels,which make them unsuitable for quick test for method development.Therefore,in the present thesis,the S–H bond fission of thiophenol is selected as a protype for nonadiabatic dynamics study.First,fourfold way/model space diabatization scheme is used for the construction of diabatic potential energy matrix.Then,the anchor points reactive potential(APRP)method is used to get the three-state full-dimensional analytical potential energy surface and the APRP PESs is then used in the nonadiabatic dynamics of thiophenol.This provides a methodology for constructing analytical potential energy surfaces and conducting nonadiabatic dynamics.The study of nonadiabatic dynamics with analytical potential energy surface requires specific procedure for each interested system.As the construction of analytical potential energy surface is rather completed,especially for the nonadiabatic process involved in the combustion network that usually have multiple channels.Direct dynamics that obtains potentials,gradients,and nonadiabatic couplings(NAC)on-the-fly using electronic structure packages during the propagation of trajectories,can avoid the construction of analytical potential energy surface and greatly facilitate the studies of nonadiabatic reaction dynamics for complex system.In the present thesis,we have implemented the coherent switches with decay of mixing(CSDM)method(which is based on semiclassical Ehrenfest but can properly deal with decoherence)into the SHARC package.The algorithm has been improved for the implementation to make CSDM method applicable for arbitrary couplings.Meanwhile,the total angular momentum and overall translation are found to be not conserved when using direct nonadiabatic dynamics algorithms and we have solved this problem by introducing a projection operation to the original NAC to remove the overall translational and rotational components.In the present thesis,theoretical methodology that can be used for adiabatic/nonadiabatic reactions in gas combustion have been construction based on detailed study of typical reaction system.The study of two typical adiabatic reaction,OH+CO and Cl+C₂H₂,shows that the combination of RP-VTST,VRC-VTST,and CUS can be used to calculate actuate high-pressure limit rate constants for barrierless reactions and reactions with multiple reaction paths and transition states.A research scheme for nonadiabatic reactions has been developed from the construction of three-state full-dimensional analytical potential energy surface for thiophenol.And the development of CSDM algorithm and its implementation into direct nonadiabatic dynamics software would greatly facilitates the study of nonadiabatic process in gas combustion.