Chemical Desorption versus Energy Dissipation: Insights from Ab Initio Molecular Dynamics of HCO• Formation
Chemical Desorption versus Energy Dissipation: Insights from Ab Initio Molecular Dynamics of HCO• Formation
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DOI：
10.3847/1538-4357/ab8a4b
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发表时间：
2020-07-01
影响因子：
4.9
通讯作者：
Rimola, Albert
中科院分区：
文献类型：
--
作者：
Pantaleone, Stefano;Enrique-Romero, Joan;Rimola, Albert
分子云是银河系中恒星形成的寒冷区域。它们被相当复杂的分子富集。据信，这些分子中的许多都是在遍布银河系的亚微米大小的星际尘埃颗粒的冰冷表面上合成的。在10 K的温度下，热解吸是低效的，因此，为什么这些分子会在寒冷的气体中被发现，这一直困扰着天文学家多年。当前模型的假设被称为化学解吸，即颗粒表面化学反应释放的分子形成能量部分被颗粒吸收，剩余的能量导致新形成的分子喷射到气体中。本文报道了精确的从头算分子动力学模拟，旨在研究发生在结晶冰表面模型上的H中心点与CO加成链H中心点+ CO -> HCO中心点第一次反应所释放的能量的命运。结果表明，在第一皮秒内，约90%的HCO中心点形成能量被注入到冰中，使得HCO中心点的能量含量（10-15 kJ mol(-1)）不到其结合能（30 kJ mol(-1)）的一半。因此，与实验室实验一致，我们得出的结论是，化学解吸对特定体系（即结晶冰上的H中心点+ CO）是低效的。我们怀疑这种行为在处理氢键时是相当普遍的，氢键负责冰地幔的内聚能和与吸附剂的相互作用，作为HCO中心点，即使需要特别的模拟来得出其他系统的具体结论。
Molecular clouds are the cold regions of the Milky Way where stars form. They are enriched by rather complex molecules. Many of these molecules are believed to be synthesized on the icy surfaces of the interstellar submicron-sized dust grains that permeate the Galaxy. At 10 K thermal desorption is inefficient and, therefore, why these molecules are found in the cold gas has tantalized astronomers for years. The assumption of the current models, called chemical desorption, is that the molecule formation energy released by the chemical reactions at the grain surface is partially absorbed by the grain and the remaining energy causes the ejection of the newly formed molecules into the gas. Here we report accurate ab initio molecular dynamics simulations aimed at studying the fate of the energy released by the first reaction of the H center dot addition chain to CO, H center dot + CO -> HCO center dot, occurring on a crystalline ice surface model. We show that about 90% of the HCO center dot formation energy is injected toward the ice in the first picosecond, leaving HCO center dot with an energy content (10-15 kJ mol(-1)) of less than half its binding energy (30 kJ mol(-1)). As a result, in agreement with laboratory experiments, we conclude that chemical desorption is inefficient for this specific system, namely H center dot + CO on crystalline ice. We suspect this behavior to be quite general when dealing with hydrogen bonds, which are responsible for both the cohesive energy of the ice mantle and the interaction with adsorbates, as HCO center dot, even though ad hoc simulations are needed to draw specific conclusions on other systems.