Time-dependent Wave Packet Quantum Dynamical Calculations Of Ground State NH+D Reaction

Chemical reactions are at the heart of Chemistry. An overall reaction involves a serial of elementary reactions and together they constitute what is called the reaction mechanism of the reaction. In molecular reaction dynamics field, people directly study elementary reactions at detailed molecular level with the help of various modern experimental and theoretical techniques. Among the numerous theoretical calculation methods, time-dependent wave packet method as a quantum dynamical method has explicit physical concepts and clear physical picture. It possesses the merits of both classical intuition and quantum mechanical accuracy. In this thesis, we use the time-dependent wave packet method to study the ground state NH + D reaction: NH（X³Σ^-）+D（²S）→N（⁴S）+HD（X¹Σ_g⁺）. This reaction belongs to the simplest bimolecular reaction which only contains three atoms. The ground state NH + H reaction plays an important role in the pyrolysis of ammonia（NH₃） at high temperatures, combustion of nitrogen-containing species and other nitrogen hydrides reactions.In the calculations we used two different NH₂-⁴A" potential energy surfaces（PESs）, the double many-body expansion ⁴A" potential energy surface reported by Poveda and Varandas in 2005（DMBE PES） and combined many-body expansion and neural network ⁴A" potential energy surface reported by Zhai and Han in 2011（PES-ZH）, respectively. And we both performed centrifugal sudden（CS） approximation calculation and exact Coriolis coupling（CC） calculation. Moreover, the initial vibrational and rotational quantum states are v₀ = 0 and j₀ = 0, respectively. Finally, we obtained the reaction probabilities of the total angular momentum quantum numbers J = 0–70 and integral reaction cross-sections over the collision energy range from 0.0 to 1.0 e V, and thermally-averaged rate constants in the temperature range 200–2500 K.The calculation results show that the threshold of NH（X³Σ^-）+D（²S）→N（⁴S）+HD（X¹Σ_g⁺） reaction is smaller than the barrier height of the PES. Its reaction probability increases by some oscillations with collision energy increases. The reaction probability of J = 0 is the biggest comparing with other J’s. As J increasing, the threshold rises to higher energy because of the increasingly centrifugal barrier, and the reaction probability also decreases. The integral reaction cross-section increases monotonically with increasing collision energy, and without apparent oscillation phenomenon. The thermally-averaged rate constant increases with temperature increases. Coriolis coupling effect mainly decreases reaction probability of larger J’s, integral cross-section of higher collision energies, and thermally-averaged rate constant in high temperature region, which conforms to the direct over-the-barrier mechanism of the reaction.Dynamical isotope effect analyses show that the thermally-averaged rate constant of NH + D reaction decreases compared with NH + H reaction in the low temperature region. At the same time, the calculations display that the PES-ZH is more accurate for NH₂-⁴A" than DMBE PES. The calculated rate constants on the PES-ZH are larger than the results calculated on the DMBE PES in the temperature range from 200 to 2500 K. The differences between the two PESs reflect in the dynamical calculations of the same reaction. The more accurate calculated CC thermally-averaged rate constant of NH + D reaction at 298 K on PES-ZH is 1.86 × 10¹² cm³ mol^-1 s^-1. We suggest, in the dynamical calculation of reaction NH（X₃Σ^-） + H（²S） → N（⁴S） + H₂（X¹Σ_g⁺） and its various isotope reactions, choosing the PES-ZH first.