Tachikawa Hiroto
Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan.
J Phys Chem A. 2020 Mar 12;124(10):1903-1910. doi: 10.1021/acs.jpca.9b11122. Epub 2020 Feb 28.
Ammonia cluster cations are a chemical species that has recently attracted considerable research attention as an ion-molecule reaction species in the planetary atmosphere, surface reaction species in materials chemistry, and super-alkali species. Reactions of the radical cation of an ammonia cluster, [(NH)] ( = 2-6), following the ionization of the parent neutral cluster, were investigated using direct ab initio molecular dynamics to elucidate the reactions of the ammonia cluster cation under astrochemical conditions. The calculations showed that two competing reaction channels-proton transfer (PT) channel and complex formation channel-operate after the ionization of neutral clusters. In the PT channel, a proton of NH was transferred to a neighboring ammonia molecule. The PT channel was found in all clusters ( = 2-6). Reaction via the PT channel became faster with increasing cluster size and saturated around = 5-6. In the complex formation channel, a face-to-face complex having a HN-NH structure (with a N-N bond) was formed. This channel was found only in larger clusters ( = 5-6). Time scales of PT and complex formation channels were calculated to be 20-30 and 40-50 fs, respectively. The reaction mechanism was discussed based on the results of theoretical calculations.
氨簇阳离子是一种化学物种，最近作为行星大气中的离子 - 分子反应物种、材料化学中的表面反应物种以及超碱物种吸引了大量的研究关注。利用直接从头算分子动力学研究了氨簇[(NH)ₙ]（n = 2 - 6）母体中性簇电离后其自由基阳离子的反应，以阐明氨簇阳离子在天体化学条件下的反应。计算结果表明，中性簇电离后存在两个相互竞争的反应通道——质子转移（PT）通道和络合物形成通道。在PT通道中，NH₃的一个质子转移到相邻的氨分子上。在所有簇（n = 2 - 6）中均发现了PT通道。通过PT通道的反应随着簇尺寸的增加而加快，并在n = 5 - 6左右达到饱和。在络合物形成通道中，形成了具有HN - NH结构（含N - N键）的面对面络合物。该通道仅在较大的簇（n = 5 - 6）中被发现。计算得出PT通道和络合物形成通道的时间尺度分别为20 - 30飞秒和40 - 50飞秒。基于理论计算结果对反应机理进行了讨论。
