Theoretical Study of Model Atom+Polyatom Reactions in the Gas Phase: Adiabatic and Nonadiabatic Dynamics

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Direct dynamics quasiclassical trajectory (QCT) calculations are performed to study the dynamics of model atom + polyatom reactions. In the first part of this thesis, we examine adiabatic dynamics of H/O reaction with simple alkanes, CD4 and C2H6. Overall good agreement with state-of-the-art experiments shows that the molecular motion is largely confined to the ground electronic potential energy surface (PES). For the H + CD4 abstraction reaction, we consider several PESs, including an analytical surface EG, and the B3LYP/6-31G** surface. The best agreement with experiment over the 1.21―2.36 eV collision energy range is found on the B3LYP surface. In particular, the CD3 products are broadly sideways scattered at 1.2 eV and shift to backward directions as energy increases, indicating the increasing importance of the stripping mechanism. Regions of the surface away from the minimum energy path are found to play an important role in high-energy dynamics We further study the hyperthermal O(3P) collisions with C2H6 for understanding materials erosion of spacecraft in low Earth orbit. By running QCT trajectories on the MSINDO and B3LYP/6-31G** surfaces, we examine the dynamics associated with the three primary reaction channels, H abstraction, H elimination and C-C breakage, and achieve generally good agreement with experiment as to product translational and angular distributions and the product relative yields at 90 kcal mol-1. In the second part, our focus is on chemical reactions where the Born-Oppenheimer approximation breaks down. Specifically we study intersystem crossing effects in the O(3P) + C2H4 reaction at a low collision energy of 0.56 eV, and at a hyperthermal energy of 3.0 eV. We use a simplified version of the trajectory surface hopping method with surface transitions only allowed at the triplet―singlet crossing points along the B3LYP/6-31G** trajectory. The transition probability is evaluated according to the Landau-Zener formula and we use an average spin-orbit coupling throughout our calculations. Our results show reasonable agreement with the low energy experiment as to the product branching ratios, product energy and angular distributions. Dynamics at hyperthermal energy differs from low energy dynamics due to decreased intersystem crossing and novel reaction pathways that are open at high energy

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  • 08/06/2018
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