Sun Lipeng, Schatz George C
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
J Phys Chem B. 2005 May 5;109(17):8431-8. doi: 10.1021/jp0454568.
A Born-Oppenheimer direct dynamics simulation of the O(+) + CH(4) reaction dynamics at hyperthermal energies has been carried out with the PM3 (ground quartet state) Hamiltonian. Calculations were performed at various collision energies ranging from 0.5 to 10 eV with emphasis on high energy collisions where this reaction is relevant to materials erosion studies in low Earth orbit and geosynchronous Earth orbit. Charge transfer to give CH(4)(+) is the dominant channel arising from O(+) + CH(4) collisions in this energy range, but most of the emphasis in our study is on collisions that lead to reaction. All energetically accessible reaction channels were found, including products containing carbon-oxygen bonds, which is in agreement with the results of recent experiments. After correcting for compensating errors in competing reaction channels, our excitation functions show quantitative agreement with experiment (for which absolute magnitudes of cross sections are available) at high collision energies (several eV). More detailed properties, such as translational and angular distributions, show qualitative agreement. The opacity function reveals a high selectivity for producing OH(+) at high impact parameters, CH(3)(+)/CH(2)(+)/H(2)O(+) at intermediate impact parameters, and H(2)CO(+)/HCO(+)/CO(+) at small impact parameters. Angular distributions for CH(3)(+)/CH(2)(+)/OH(+) are forward scattered at high collision energies which implies the importance of direct reaction mechanisms, while reaction complexes play an important role at lower energies, especially for the H(2)O(+) product. Finally, we find that the nominally spin-forbidden product CH(3)(+) + OH can be produced by a spin-allowed pathway that involves the formation of the triplet excited product CH(3)(+)(ã(3)E). This explains why CH(3)(+) can have a high cross section, even at very low collision energies. The results of this work suggest that the PM3 method may be applied directly to the study of O(+) reactions with small alkane molecules and polymer surfaces.
利用PM3（基态四重态）哈密顿量对超热能下O(+) + CH₄反应动力学进行了玻恩-奥本海默直接动力学模拟。在0.5至10 eV的各种碰撞能量下进行了计算，重点关注高能量碰撞，因为该反应与近地轨道和地球同步轨道中的材料侵蚀研究相关。在这个能量范围内，电荷转移生成CH₄(+)是O(+) + CH₄碰撞产生的主要通道，但我们研究的重点大多是导致反应的碰撞。发现了所有能量上可及的反应通道，包括含有碳-氧键的产物，这与近期实验结果一致。在校正了竞争反应通道中的补偿误差后，我们的激发函数在高碰撞能量（几电子伏特）下与实验（可获得截面的绝对大小）显示出定量一致性。更详细的性质，如平动和角分布，显示出定性一致性。不透明度函数揭示了在高碰撞参数下产生OH(+)、中间碰撞参数下产生CH₃(+)/CH₂(+)/H₂O(+)以及小碰撞参数下产生H₂CO(+)/HCO(+)/CO(+)具有高选择性。CH₃(+)/CH₂(+)/OH(+)的角分布在高碰撞能量下向前散射，这意味着直接反应机制的重要性，而反应复合物在较低能量下起着重要作用，特别是对于H₂O(+)产物。最后，我们发现名义上自旋禁阻的产物CH₃(+) + OH可以通过涉及形成三重态激发产物CH₃(+)(ã³E)的自旋允许途径产生。这解释了为什么即使在非常低的碰撞能量下，CH₃(+)也能有高截面。这项工作的结果表明，PM3方法可直接应用于研究O(+)与小烷烃分子和聚合物表面的反应。
