Woon David E
Department of Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Avenue, Urbana, Illinois 61801, United States.
J Phys Chem A. 2022 Aug 11;126(31):5186-5194. doi: 10.1021/acs.jpca.2c01739. Epub 2022 Jul 27.
One of a small number of known magnesium-containing astromolecules, magnesium isocyanide (MgNC) was first detected in 1986. MgNC is an intriguing reactant to consider: it is an open-shell radical in which its metal atom forms a bond with CN that is a mixture of ionic and covalent character. While its gas phase astrochemistry has received prior attention, the grain surface chemistry of MgNC has never been studied. Because of its ionic character, MgNC is found to interact far more strongly with an ice surface than molecules with a greater degree of covalency. As a radical, it may react with closed-shell molecules deposited from the gas phase. In this work, cluster calculations treated with density functional theory and correlation consistent basis sets were used to model the deposition of MgNC on clusters containing 17 and 24 water molecules, which were then allowed to react with acetylene (HCCH) and hydrogen cyanide (HCN) as well as with H atoms. The addition of H to MgNC-HO yields hydromagnesium isocyanide (HMgNC), a known astromolecule that may be ejected into the gas phase. HCCH and HCN bind to MgNC-HO to form intermediate radical compounds that may then also react with H atoms. There is enough reaction energy from H addition to eject fragments of the intermediates into the gas phase: the vinyl radical (CH) for HCCH and the methaniminyl radical (HCN) for HCN. That leaves MgNC-HO to perform further catalytic activity. Alternatively, various hydrogenated divalent Mg compounds may also be stabilized and frozen into the ice or potentially ejected into the gas phase. Benchmark coupled cluster theory calculations in limited systems were used to characterize the submerged reaction barriers present when HCCH or HCN add to MgNC in the gas phase.
异氰化镁（MgNC）是少数已知的含镁天体分子之一，于1986年首次被探测到。MgNC是一种值得关注的有趣反应物：它是一种开壳自由基，其中金属原子与具有离子和共价混合特征的CN形成键。虽然其气相天体化学此前已受到关注，但MgNC的颗粒表面化学从未被研究过。由于其离子特性，发现MgNC与冰表面的相互作用比具有更高共价程度的分子要强得多。作为一种自由基，它可能与从气相沉积的闭壳分子发生反应。在这项工作中，使用密度泛函理论和相关一致基组进行的团簇计算来模拟MgNC在含有17个和24个水分子的团簇上的沉积，然后使其与乙炔（HCCH）、氰化氢（HCN）以及H原子发生反应。向MgNC - HO添加H会生成异氰化氢镁（HMgNC），这是一种已知的天体分子，可能会被喷射到气相中。HCCH和HCN与MgNC - HO结合形成中间自由基化合物，然后这些化合物也可能与H原子发生反应。H添加产生的反应能量足以将中间体的碎片喷射到气相中：对于HCCH是乙烯基自由基（CH），对于HCN是甲亚胺基自由基（HCN）。这使得MgNC - HO能够进行进一步的催化活性。或者，各种氢化二价Mg化合物也可能被稳定并冻结在冰中或潜在地喷射到气相中。在有限系统中使用基准耦合簇理论计算来表征HCCH或HCN在气相中添加到MgNC时存在的浸没反应势垒。
