Formation of Complex Organics in the Gas Phase by Sequential Reactions of Acetylene with the Phenylium Ion

In this paper, we report a study on the reactivity of the phenylium ion with acetylene, by measuring product yield as a function of pressure and temperature using mass-selected ion mobility mass spectrometry. The reactivity is dominated by a rapid sequential addition of acetylene to form covalently bonded C8H7 + and C10H9 + ions with an overall rate coefficient of 7–5 × 10–10 cm3 s–1, indicating a reaction efficiency of nearly 50% at room temperature. The covalent bonding nature of the product ions is confirmed by high temperature studies where enhanced production of these ions is observed at temperatures as high as 660 K. DFT calculations at the UPBEPBE/6-31++G** level identify the C8H7 + adduct as 2-phenyl-ethenylium ion, the most stable C8H7 + isomer that maintains the phenylium ion structure. A small barrier of 1.6 kcal/mol is measured and attributed to the formation of the second adduct C10H9 + containing a four-membered ring connected to the phenylium ion. Evidence for rearrangement of the C10H9 + adduct to the protonated naphthalene structure at temperatures higher than 600 K is provided and suggests further reactions with acetylene with the elimination of an H atom and an H2 molecule to generate 1-naphthylacetylene or acenaphthylene cations. The high reactivity of the phenylium ion toward acetylene is in sharp contrast to the low reactivity of the benzene radical cation with a reaction efficiency of 10–4–10–5, confirming that the first step in the cation ring growth mechanism is the loss of an aromatic H atom. The observed reactions can explain the formation of complex organics by gas phase ion–molecule reactions involving the phenylium ion and acetylene under a wide range of temperatures and pressures in astrochemical environments.