Interaction of Small Nitriles Occurring in the Atmosphere of Titan with Metal Ions of Meteoric Origin.

Abstract

Meteoric material injected into the atmosphere of Titan, Saturn's moon, can react with nitriles and other organic compounds that constitute Titan's atmosphere. However, specific chemical outcomes have not been fully explored. To understand the fates of meteoric metal ions in the Titan environment, reactions of Mg⁺ and Al⁺ with CH₃CN (acetonitrile) and C₂H₅CN (propionitrile) were carried out using a drift cell ion reactor at room temperatures (300 K) and reduced temperatures (∼193 K) and modeled using density functional theory and coupled-cluster theory. Analysis of reactant ion electronic state distributions via electronic state chromatography revealed that Mg⁺ was produced in our instrument exclusively in its ground (²S) state, whereas Al⁺ was produced in both its ¹S ground state and ³P first excited state. Mg⁺(²S) and Al⁺(¹S) produce association products exclusively with both CH₃CN and C₂H₅CN. Primary association reactions with C₂H₅CN occurred with higher reaction efficiencies than those with CH₃CN. Mg⁺(²S) sequentially associates up to four nitrile ligands, and Al⁺(¹S) associates up to three, each via the nitrile nitrogen. Computed binding energies are strongest for the first ligand and diminish with subsequent nitriles. Mg⁺(²S) exhibits a stronger preference for binding nitriles than Al⁺(¹S) because its unpaired electron delocalizes to the nitrile ligands through back-bonding, whereas the lone pair on Al⁺(¹S) remains localized on the metal center. Al⁺(³P) exhibited evidence of bimolecular product formation with both nitriles. Computational modeling of Al⁺(³P) with CH₃CN suggests that the major product, AlCH₃⁺, is kinetically favored over the more energetically stable product, Al⁺(HCN).