Theoretical Studies Of Potential Energy Surfaces And Quantum Dynamics Of Small Molecules:from Gas Phase To Metal Surfaces

Molecular reaction dynamics is one of the forefronts of the chemical research and the bridge between microscopic and macroscopic reaction dynamics. The atomic and molecular motions are subject to the quantum mechanics, in principle, one can first separate the electron motion and nuclear motion in the reactive system, and then calculate the potential energy at different geometries within quantum chemistry method to construct a potential energy surface (PES) for the nuclear motion. Finally, all information of quantum reactive dynamics can be obtained by solving the Schrodinger equation for nuclear motion. Potential energy surface is not only the cornerstone of chemical reaction dynamics, but also one of the core issues in theoretical and computational chemistry. The construction of PES plays a key role in the studies of molecular properties and reactions and becomes the basis of understanding the mechanism of chemical reaction at the atomic and quantum state level. The quantum dynamical calculations can be carried out once the PES is constructed. The quantum state resolved reaction dynamics allows us to investigate the chemical reaction at the atomic and molecular level, so as to provide us the most detailed information for the mechanism of the chemical reaction and a deep understanding of the nature of the chemical reaction. In this thesis, we have firstly studied several typical elementary reactions in gas phase, including new diabatic PESs for ClH2and BrH2reactive system, new diabatic PESs and state-to-state photodissociation dynamics for H2O. The dissociative chemisorption of H2O on Cu(111) surface has been studied subsequently, which is a very important elementary reaction in many catalytic processes on metal surfaces. The related dynamical mechanisms have been obtained in detail.The Cl+H2(?)H+HC1and Br+H2(?)H+HBr reactions are both important elementary reactions. For Cl(2P3/2,2P1/2)+H2reaction, the reactivity of Cl(2P1/2), which is controled by the non-adiabatic transition, has been controversial for years. Several years ago, the measured cross sections for Cl(2P1/2)+H2reaction were found to be larger than Cl(2P3/2)+H2reaction, which was contrary to later calculated results. On the other side, for H+HBr abstraction reaction, the calculated rate constants based on previous PES were found to be larger than the experimental data by a factor of2. In addition, the contribution of the non-adiabatic transition reaction is still an open issue. For both systems, the lowest three electronic states and the corresponding SO coupling have been computed using highly accurate ab initio methods and very large basis sets, followed by being transformed to the fully coupled6×6diabatic potential energy matrix in the diabatic representation. For the Cl(2P3/2,2P1/2)+H2reaction, the quantum dynamical calculations have verified the recent experimental results. It is confirmed that the Non-Born-Oppenheimer behavior which leads to a large reactivity of Cl(2P1/2), only dominates at very low collision energies, while at higher collision energies, this reaction can be effectively described in the Born-Oppenheimer approximation. For the H+HBr abstraction reaction, the calculated rate constants using our adiabatic potential were found to be agreed with the measurements quite well in a wide range of temperature, much better than those obtained from previous PESs. Moreover, comparing to previous semi-empirical diabatic potentials, our newly ab initio diabatic potentials have a better description for the quenching or non-adiabatic dynamics in Br(2P3/2,2P1/2)+H2reaction. It is shown that Br(2P1/2)+H2(v) can effectively quench to Br(2P3/2)+H2(v+1), while the non-adiabatic effect is quite small, in other words, the non-adiabatic product HBr+H is negligible. Further quantum dynamical calculations are underway and we expect to simulate more experiment studies for this reaction.The photodissociation process via B state of H2O is also a prototypical non-adiabatic process. Very recently, Xueming Yang’s group investigated the photodissociation dynamics of H2O at the five strongest peak energies in B band of the absorption spectrum by high-resolution experiments, the most detailed differential cross sections (DCSs) were obtained, but these results were not clearly clarified without theoretical work. We have constructed the new B-X diabatic PESs based on high level ab initio calculations and have developed a Chebyshev real wavepacket method along with the computer program to study the non-diabatic state-to-state photodissociation dynamics via B state. The DCSs have been obtained quantum mechanically for the first time for a triatom, and the overall agreement with experimental results is good. The remarkable difference between the DCSs for OH(X) and OH(A) found in the experiments, has been reproduced in our calculations. It is clearly illustrated that the two products encounter different dissociation pathways, resulting in a significant influence on the final dissociation picture. It is expected that this study will inspire the related experimental and theoretical researches on H2O photodissociation in the future.The dissociative chemisorption of H2O on the transition metal surface is not only a very important example in the gas-surface reaction, but also an obligatory step in many heterogeneous catalytic process in the industry. In order to ultimately control and/or enhance the catalytic process, it is crucial to have a better understanding of this reaction dynamics. So far, to our best knowledge, there has been few experimental or theoretical studies on the dynamics of this process. As a first attempt, the influence of the vibrational excitation of H2O on the reactivity has been inspected. To this end, we have proposed a reduced model which involves all the internal coordinates of H2O, and have constructed a global six dimensional PES. Subsequently, we have developed a transition state wavepacket method propagated by Chebyshev iteration, which has been employed to calculate several initial state selected reaction probabilities of this process. It is found that the dissociation of H2O and CH4on metal surfaces are quite different, only the symmetric stretching vibrational mode of CH4leads to significant enhancement relative to the same amount of translational energy, while the symmetric stretching, bending, anti-symmetric stretching vibrational modes of H2O all enhances the reactivity prominently. It is expected that our theoretical prediction will inspire more state-resolved dynamical experiments to study this important system.