Fioroni Marco, DeYonker Nathan J
Department of Chemistry, The University of Memphis, Memphis, TN, United States.
Front Chem. 2021 Feb 1;8:621898. doi: 10.3389/fchem.2020.621898. eCollection 2020.
Heterogeneous phase astrochemistry plays an important role in the synthesis of complex organic matter (COM) as found on comets and rocky body surfaces like asteroids, planetoids, moons and planets. The proposed catalytic model is based on two assumptions: siliceous rocks in both crystalline or amorphous states show surface-exposed defective centers such as siloxyl (Si-O•) radicals; the second phase is represented by gas phase CO molecules, an abundant C building block found in space. By means of quantum chemistry; (DFT, PW6B95/def2-TZVPP); the surface of a siliceous rock in presence of CO is modeled by a simple POSS (polyhedral silsesquioxane) where a siloxyl (Si-O•) radical is present. Four CO molecules have been consecutively added to the Si-O• radical and to the nascent polymeric CO (pCO) chain. The first CO insertion shows no activation free energy with ΔG = -21.7 kcal/mol forming the SiO-CO• radical. The second and third CO insertions show Δ ≤ 10.5 kcal/mol. Ring closure of the SiO-CO-CO• (oxalic anhydride) moiety as well as of the SiO-CO-CO-CO• system (di-cheto form of oxetane) are thermodynamically disfavored. The last CO insertion shows no free energy of activation resulting in the stable five member pCO ring, precursor to 1,4-epoxy-1,2,3-butanone. Hydrogenation reactions of the pCO have been considered on the SiO oxygen or on the carbons and oxygens of the pCO chains. The formation of the reactive aldehyde SiO-CHO on the siliceous surface is possible. In principle, the complete hydrogenation of the (CO) series results in the formation of methanol and polyols. Furthermore, all the SiO-pCO intermediates and the lactone 1,4-epoxy-1,2,3-butanone product in its radical form can be important building blocks in further polymerization reactions and/or open ring reactions with H (aldehydes, polyols) or CN (chetonitriles), resulting in highly reactive multi-functional compounds contributing to COM synthesis.
非均相天体化学在彗星以及小行星、类行星、卫星和行星等岩石天体表面发现的复杂有机物质（COM）的合成过程中发挥着重要作用。所提出的催化模型基于两个假设：结晶态或非晶态的硅质岩石都显示出表面暴露的缺陷中心，如硅氧基（Si - O•）自由基；第二相由气相CO分子代表，这是一种在太空中大量存在的碳构建单元。借助量子化学（DFT，PW6B95/def2 - TZVPP），在存在CO的情况下，硅质岩石的表面由一个简单的多面体倍半硅氧烷（POSS）模拟，其中存在一个硅氧基（Si - O•）自由基。四个CO分子已连续添加到Si - O•自由基和新生的聚合CO（pCO）链上。第一次CO插入显示无活化自由能，ΔG = -21.7 kcal/mol，形成SiO - CO•自由基。第二次和第三次CO插入显示Δ≤10.5 kcal/mol。SiO - CO - CO•（草酸酐）部分以及SiO - CO - CO - CO•系统（氧杂环丁烷的二酮形式）的闭环在热力学上是不利的。最后一次CO插入显示无活化自由能，形成稳定的五元pCO环，即1,4 - 环氧 - 1,2,3 - 丁酮的前体。已考虑在SiO氧上或pCO链的碳和氧上进行pCO的氢化反应。在硅质表面形成反应性醛SiO - CHO是可能的。原则上，（CO）系列的完全氢化会导致甲醇和多元醇的形成。此外，所有的SiO - pCO中间体以及自由基形式的内酯1,4 - 环氧 - 1,2,3 - 丁酮产物可能是进一步聚合反应和/或与H（醛、多元醇）或CN（酮腈）的开环反应中的重要构建单元，从而产生有助于COM合成的高反应性多功能化合物。
