Theoretical study of chemical reactions of neutral molecules in interstellar space

在这个项目中，从反应动力学的理论观点研究了中性-中性化学反应，这些化学反应在星际云中起着重要的作用。我们的目标是为这种星际化学反应提供重要的物理量，包括反应速率常数和产物分支比。这很简单，因为人们普遍认为，在极低温度和极低密度条件下，实验测量这些量是非常困难的任务。首先，我们对碳原子与乙炔的反应进行了量子反应散射计算，这是已知在星际空间碳链增长中起作用的。我们建立了两类降维量子散射模型，并计算了速率常数和乘积分支比。接下来，我们重点从理论上研究了在星际介质中经常检测到的HNC的产生机制。我们研究了自由基-自由基反应生成HNC的重要性，因为这两种自由基都是在星际环境中发现的。我们在精确的理论水平上进行了相互作用势能面的电子结构计算。结果发现，C2+NH反应是无障碍进行的。然后，我们进行了即时直接动力学计算，以确定反应产物。经过多次测试，我们选择使用混合B3LYP密度泛函方法来节省计算时间。从这些直接动力学计算的结果中，我们首次发现HNC是通过C2+NH反应有效地产生的。详细的分析表明，这是动力学的结果，CCNH中间体的寿命很短。研究还发现，在C2+NH→C+HNC反应中，动量传递是非常有效的。

In this project, neutral-neutral chemical reactions, which are known to play an important role in interstellar clouds, have been studied from a theoretical viewpoint of reaction dynamics. Our goal is to provide important physical quantities including reaction rate constants and product branching rations for such interstellar chemical reactions. This is simply because it is generally accepted that experimental measurements of such quantities are very difficult tasks under very low temperature and very low density conditions.First, we carried out quantum reactive scattering calculations for the reaction of the carbon atom with acetylene, which is known to play a role in carbon-chain growth in interstellar space. We have developed two types of reduced-dimensionality quantum scattering models and calculated rate constants as well as product branching ratios.Next, we focused on theoretical study of the production mechanism of HNC, which has been frequently detected in interstellar medium. We studied the importance of the HNC production from the radical-radical reaction, C_2+NH, since both radicals are found in interstellar environment. We carried out electronic structure calculations of the interaction potential energy surface at accurate levels of theory. As a result, we found that the C_2+NH reaction proceeds without a barrier. We have then carried out on-the-fly direct dynamics calculations in order to identify the reaction products. After several tests, we have chosen to use the hybrid B3LYP density-functional method to save computational time. From results of these direct dynamics calculations, we found that HNC is efficiently produced via the C_2+NH reaction for the first time. Detailed analysis revealed that this is a result of dynamics, where the lifetime of the CCNH intermediate is very short. Also, it was found that momentum transfer occurs quite efficiently in the C_2+NH → C+HNC reaction.