[论文解读] Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Joan Enrique-Romero, C. Ceccarelli+4 more|arXiv (Cornell University)|Aug 27, 2021

Astrophysics and Star Formation Studies参考文献 98被引用 38

一句话总结

本研究利用量子化学动力学与RRKM理论结合隧道效应，计算了在星际冰质颗粒上HCO与CH3自由基复合生成乙醛（CH3CHO）的效率。结果表明，该反应效率对扩散能与脱附能之比（f）极为敏感，当f从0.4降至0.3时，效率从1急剧下降至0.01，挑战了天体化学模型中普遍假设的单位效率，提示若f ≈ 0.3，则乙醛生成可能极不可能发生。

ABSTRACT

Interstellar grains are known to be important actors in the formation of interstellar molecules such as H$_2$, water, ammonia, and methanol. It has been suggested that the so-called interstellar complex organic molecules (iCOMs) are also formed on the interstellar grain icy surfaces by the combination of radicals via reactions assumed to have an efficiency equal to unity. In this work, we aim to investigate the robustness or weakness of this assumption by considering the case of acetaldehyde (CH$_3$CHO) as a starting study case. In the literature, it has been postulated that acetaldehyde is formed on the icy surfaces via the combination of HCO and CH$_3$. Here we report new theoretical computations on the efficiency of its formation. To this end, we coupled quantum chemical calculations of the energetics and kinetics of the reaction CH$_3$ + HCO, which can lead to the formation of CH$_3$CHO or CO + CH$_4$. Specifically, we combined reaction kinetics computed with the Rice-Ramsperger-Kassel-Marcus (RRKM) theory (tunneling included) method with diffusion and desorption competitive channels. We provide the results of our computations in the format used by astrochemical models to facilitate their exploitation. Our new computations indicate that the efficiency of acetaldehyde formation on the icy surfaces is a complex function of the temperature and, more importantly, of the assumed diffusion over binding energy ratio $f$ of the CH$_3$ radical. If the ratio $f$ is $\geq$0.4, the efficiency is equal to unity in the range where the reaction can occur, namely between 12 and 30 K. However, if $f$ is smaller, the efficiency dramatically crashes: with $f$=0.3, it is at most 0.01. In addition, the formation of acetaldehyde is always in competition with that of CO + CH$_4$.

研究动机与目标

• 评估在冰质颗粒上形成星际复杂有机分子（iCOMs）的自由基-自由基反应假设单位效率的有效性。
• 研究通过HCO + CH3复合在星际冰质上生成乙醛（CH3CHO）的效率。
• 评估温度以及扩散能与结合能之比（f）对反应效率的影响。
• 为整合进天体化学模型，提供可直接使用的速率常数和效率值。

提出的方法

• 在非晶态固态水（ASW）表面进行HCO + CH3 → CH3CHO与HCO + CH3 → CH4 + CO反应的反应热力学与能垒的量子化学计算。
• 应用带隧道效应校正的RRKM理论，计算温度依赖的速率常数。
• 引入竞争性的扩散与脱附过程，以确定有效反应效率（ε）。
• 计算扩散能与脱附能之比（f = Ediff / Edes）作为关键控制参数。
• 推导速率常数与效率的解析表达式，以实现直接用于天体化学模型。
• 采用真实的冰表面模型（平面位点与空穴位点），以考虑位点依赖的反应活性。

实验结果

研究问题

• RQ1 在星际冰质颗粒上，通过HCO + CH3复合生成乙醛的真实效率是多少？
• RQ2 扩散能与脱附能之比（f）如何影响反应效率与产物分支比？
• RQ3 量子隧穿效应对CH4 + CO生成的影响程度如何，相较于CH3CHO生成？
• RQ4 为何当前天体化学模型在IRAS16293B与NGC 7538 IRS 1等源中高估了乙醛的丰度？
• RQ5 若f ≈ 0.3，观测到的乙醛丰度能否与表面生成机制相容？

主要发现

• 由于CH3在较高温度下脱附，乙醛通过HCO + CH3复合生成仅在温度低于30 K时可行。
• 当f ≥ 0.4时，乙醛生成效率在12–30 K温度范围内保持为单位效率。
• 当f = 0.3时，乙醛生成效率在约30 K处峰值仅为0.01，表明效率出现急剧下降。
• 竞争反应CH4 + CO生成在低f条件下效率更高，因量子隧穿效应增强，且随温度降低而增加。
• 在f = 0.3且温度≤15 K时，CH4 + CO生成效率接近单位值，而乙醛效率仍可忽略不计。
• 本研究表明，若天体化学模型在未考虑f依赖效率的情况下假设ε = 1，则乙醛丰度会被高估数个数量级。