Icy Grain Mantle Surface Astrochemistry of MgNC: The Emergence of Metal Ion Catalysis Studied via Model Ice Cluster Calculations

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-nH2O yields hydromagnesium isocyanide (HMgNC), a known astromolecule that may be ejected into the gas phase. HCCH and HCN bind to MgNC-nH2O 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 (C2H3) for HCCH and the methaniminyl radical (H2CN) for HCN. That leaves MgNC-nH2O 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.