The Construction Of Potential Energy Surfaces And The Dynamics Studies Of The C₂H_2x+1(x=1,2,3) Systems

Molecular reaction dynamics,as an important research field of modern chemistry,mainly studies the mechanisms of chemical reactions at the atomic and molecular levels.Here,we focus on two topics of molecular reaction dynamics.One is a novel reaction mechanism named as "roaming".It is anunusual pathway passing some significantly distorted geometric structures(loose structures)of high potential energies,which is quite different from the conventional minimum-energy path via transition state.The other is the "supercollision(or super energy transfer)" process in which large portion of energy can be transferred.In this thesis,we take the C2H2x+1(x=1,2,3)reaction systems as examples to study the roaming mechanism and the supercollision process.Based on high level electronic structure calculations(UCCSD(T)-F12/AVTZ)at a large amount of different geometries,we have constructed the full-dimensional ground-state potential energy surfaces(PESs)for the three C2H2X+1(x=1,2,3)reaction systems using the fundamental invariant neural network(FI-NN)fitting method.Furthermore,based on these PESs,extensive quasiclassical trajectory(QCT)calculations have been carried out to study the chemical reaction mechanisms and the super energy transfer process in the three collision systems of H+C2H2X(x=1,2,3).For the C2H3 system,based on high-level ab initio calculations of roughly 116000 energies,an accurate global full-dimensional PES of the ground electronic state has been constructed using the FI-NN fitting method with the root-mean-square error(RMSE)of 9.84 meV.Extensive QCT calculations have been performed on this FI-NN C2H3 PES to investigate the collisions of the H+C2H2 system.On one hand,two novel roaming pathways for the H+C2H2?H2+C2H reaction have been found.According to the specific molecular structure during roaming process,the two roaming pathways are named as the "acetylene-facilitated roaming"(A-roaming)and the "vinylidene-facilitated roaming"(V-roaming)pathways,respectively.On the other hand,for the H+C2H2?H+C2H2 reaction,we observed the supercollision phenomena in a large collision energy range from 20 to 70 kcal/mol.The most probably transferred(total or vibrational)energy via the supercollision process varies nearly linearly with the collision energy in this collision energy range.For the C2H5 system,we have constructed an accurate full-dimensional ground state PES based on the FI-NN fitting to roughly 100000 ab initio energies,with RMSE of 12.86 meV.Based on this FI-NN C2H5 PES,we performed extensive QCT calculations.For the reaction of H+C2H4?H2+C2H3,in addition to the complex-mediated roaming mechanism,a noncomplex forming roaming mechanism,named as "collision-induced" roaming,has been found.For the other reaction of H+C2H4?H+C2H4,we have found that the supercollision process can also exist for the collision energy ranging from 20 to 70 kcal/mol.Although the"frustrated complex-forming" process,in which the incoming H atom penetrates into C2H4 with a small C-H distance but promptly and directly leaves C2H4,can contribute to the supercollision to some extent,the complex-forming mechanism plays a dominant role in the supercollisions.For the C2H7 system,a new global PES has been constructed based on FI-NN fitting to roughly 158000 ab initio energies with the RMSE of 29.48 meV.Based on extensive QCT calculations,it is indicated that for the reaction of H+C2H6? H2+C2H5,the direct abstraction pathway through the transition state is the dominant reaction pathway,while the roaming mechanism is not obvious.Additionally,it is found that in the other reaction of H+C2H6? H+C2H6,the supercollision process is not obvious,either,due to the lack of long-lived complex intermediates in the C2H7 system.The above findings provide theoretical reference for the deep understanding of the roaming mechanism and the supercollision process in the collision involving the hydrocarbon molecules.