Hydrogen chemisorption on polycyclic aromatic hydrocarbons via tunnelling
Hydrogen chemisorption on polycyclic aromatic hydrocarbons via tunnelling
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多环芳烃的隧道效应氢化学吸附
DOI：
10.1111/j.1365-2966.2011.18924.x
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发表时间：
2011
影响因子：
4.8
通讯作者：
T. Goumans
中科院分区：
文献类型：
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作者：
T. Goumans
利用量子力学隧道效应研究了低温下氢原子在多环芳烃上的化学吸附。多环芳烃普遍存在于星际介质中，可以以各种电荷状态以及氢化状态存在。多环芳烃被认为可以通过化学吸附氢原子在光子主导的区域催化H2的形成。氢化的多环芳烃也牵连的红外波段的相对强度在原行星状星云，反射星云和H II地区。氢原子化学吸附到石墨的活化势垒在低到中等温度下高得令人望而却步（约5000 K），以使该反应经典地发生。然而，在多环芳烃上，边缘位置更灵活，可以更容易地容纳进入的氢原子，从而导致更低的势垒。结合进一步的速率增强通过隧道效应，氢化学吸附在PAH边缘可能成为可行的星际介质中的各个区域。我们提出了谐波量子过渡态理论计算，其中包括隧道效应，芘作为一个模型PAH系统。事实上，氢原子在边缘位置上化学吸附的相对较低的经典激活势垒（约2000 K）与强隧穿相结合，在低至40 K的温度下产生了不可忽略的10−16-10−18 cm 3 site−1 s−1量级的速率，具有大的动力学同位素效应kH/kD约64，这是隧穿的特征。在这个温度下，PAH核心的化学吸附速度要慢几个数量级，即使是最轻的氢同位素也要慢10−22.5 cm 3 site−1 s−1。通过隧穿将H原子添加到PAH边缘位点可能足够有效地促进H-PAH的形成，尽管其他过程可能更重要。
The chemisorption of hydrogen atoms on polycyclic aromatic hydrocarbons (PAHs) is studied at low temperatures via quantum mechanical tunnelling through reaction barriers. PAHs are ubiquitous in the interstellar medium and may exist in various charge states as well as hydrogenation states. PAHs have been suggested to catalyze H2 formation in photon-dominated regions via chemisorbed hydrogen atoms. Hydrogenated PAHs are also implicated by the relative strengths of the infrared bands in protoplanetary nebulae, reflection nebulae and H ii regions. The activation barrier for the chemisorption of hydrogen atoms to graphite is prohibitively high (∼5000 K) at low to moderate temperatures for this reaction to occur classically. On PAHs, however, edge sites are more flexible and can accommodate the incoming hydrogen atom more easily, resulting in a lower barrier. Combined with a further rate enhancement via tunnelling, hydrogen chemisorption on PAH edges may become feasible in various regions in the interstellar medium. We present harmonic quantum transition state theory calculations, which incorporate tunnelling, on pyrene as a model PAH system. Indeed the relatively low (∼2000 K) classical activation barriers for hydrogen atom chemisorption on edge sites combined with strong tunnelling give rise to non-negligible rates of the order of 10−16–10−18 cm3 site−1 s−1 at temperatures as low as 40 K, with a large kinetic isotope effect kH/kD≈ 64, characteristic for tunnelling. At this temperature, chemisorption on the core of a PAH is orders of magnitude slower, ∼10−22.5 cm3 site−1 s−1 even for the lightest H isotope. The addition of H atoms to PAH edge sites via tunnelling could be efficient enough to contribute H-PAH formation, although other processes may be more important.