Woon David E
Department of Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Avenue, Urbana, Illinois 61801, United States.
Acc Chem Res. 2021 Feb 2;54(3):490-497. doi: 10.1021/acs.accounts.0c00717. Epub 2021 Jan 14.
ConspectusInterstellar clouds and the outer reaches of protostellar and protoplanetary systems are very cold environments where chemistry is limited to processes that have little or no reaction barrier (in the absence of external energy input). This account reviews what is known about cation-ice reactions, which are not currently incorporated in astrochemical network models. Quantum chemical cluster calculations using density functional theory have shown that barrierless reactions can occur when gas phase cations such as HCO, OH, CH, and C are deposited on an icy grain mantle with energies commensurate with other gas phase species. When cations react with molecules on ice surfaces, the pathways and products often differ significantly from gas phase chemistry due to the involvement of water and other molecules in the ice. The reactions studied to date have found pathways to abundant and important astromolecules such as methanol, formic acid, and carbon dioxide that are very favorable and may be more efficient pathways than gas phase processes. Other products that can be produced include glycolonitrile, its precursors, and related isocyanide compounds. This account describes for the first time ice surface reactions between the carbon cation, C, and two common astromolecules, methanol (CHOH) and formic acid (HCOOH), which can yield precursors to glyoxal, hydroxyketene, vinyl alcohol, and acetaldehyde. The quantum chemical methodology used to explore reaction surfaces is also used to predict both vibrational and electronic spectra of reactant and product ices, which offers guidance for possible experimental studies of these reactions. While theoretical calculations indicate that cation-ice reactions are efficient and offer novel pathways to important astrochemical compounds, experimental confirmation would be very welcome. Cations and ice-covered grain mantles are certainly present in cold astrophysical environments. The account concludes with a discussion of how cation-ice reactions could be incorporated into reaction network models of the formation and destruction of molecules in interstellar clouds and protoplanetary systems. Further studies will involve characterizing additional rcactions and more extensive treatment of the most important cation-ice reactions to better ascertain reaction branching outcomes.
综述
星际云以及原恒星和原行星系统的外层区域是非常寒冷的环境，在那里化学过程仅限于那些几乎没有或完全没有反应势垒的过程（在没有外部能量输入的情况下）。本综述回顾了关于阳离子 - 冰反应的已知情况，目前这些反应尚未纳入天体化学网络模型。使用密度泛函理论的量子化学簇计算表明，当气相阳离子如HCO、OH、CH和C以与其他气相物种相当的能量沉积在冰粒幔上时，无势垒反应就会发生。当阳离子与冰表面的分子发生反应时，由于冰中存在水和其他分子，反应途径和产物通常与气相化学有显著差异。迄今为止所研究的反应已经找到了通往丰富且重要的天体分子（如甲醇、甲酸和二氧化碳）的途径，这些途径非常有利，并且可能比气相过程更有效。还可以产生的其他产物包括乙醇腈及其前体以及相关的异氰化物化合物。本综述首次描述了碳阳离子C与两种常见的天体分子甲醇（CH₃OH）和甲酸（HCOOH）之间的冰表面反应，这些反应可以产生乙二醛、羟基乙烯酮、乙烯醇和乙醛的前体。用于探索反应表面的量子化学方法也用于预测反应物和产物冰的振动光谱和电子光谱，这为这些反应的可能实验研究提供了指导。虽然理论计算表明阳离子 - 冰反应是有效的，并且为重要的天体化学化合物提供了新的途径，但实验证实将非常受欢迎。阳离子和覆盖冰的颗粒幔肯定存在于寒冷的天体物理环境中。综述最后讨论了如何将阳离子 - 冰反应纳入星际云和原行星系统中分子形成和破坏的反应网络模型。进一步的研究将涉及表征更多反应，并对最重要的阳离子 - 冰反应进行更广泛的处理，以更好地确定反应分支结果。
