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.