SE839: Can aromatic cations be involved in the growth of polycyclic aromatic hydrocarbons (PAH) by acetylene additions under astrochemical conditions?

The role of aromatic cations, such as phenylium (C6H5+), in aromatic growth in the interstellar medium and planetary atmospheres is debated in the literature. Some experimental works indicated that C6H5+ is nonreactive toward key interstellar molecules, including H2 and C2H2, challenging its proposed role in PAH formation. On the contrary, other experiments combined with DFT calculations at the M06-2X/AVTZ level showed that phenilium can barrierlessly react with acetylene producing, depending on conditions, either collisionally stabilized C6H5=CCH2+ cation or phenylacetylene radical cation C6H5CCH+ or other C8H6+ isomers after an atomic hydrogen loss. In the present work, we employ chemically accurate CCSD(T)-F12/cc-pVTZ-F12//ωB97X-D/6-311G** and CCSD(T)/CBS//ωB97X-D/6-311G** methods to explore potential energy surfaces (PES) of the C6H5+ + C2H2, C8H7+ + C2H2, and C8H6+ + C2H2 reactions and to unravel possible pathways for the formation of naphthalenylium (C10H8+) and their feasibility under low-temperature conditions, such as those in cold molecular clouds or in the stratosphere of Titan. The PES calculations are followed by RRKM-Master Equation (RRKM-ME) calculations of reaction rate constants and product branching ratios depending on the temperature and pressure. The RRKM-ME results indicate that the phenilium + C2H2 reaction predominantly forms pentalene radical cation + H under single-collision conditions (zero pressure limit) but produces C6H5=CCH2+ at the pressure of 1 mbar typical for Titan’s stratosphere. Pentalene cation C8H6+ appears to be nonreactive with acetylene and only can form a weakly bound C8H6+…C2H2 complex with large separation between the fragments. Alternatively, C6H5=CCH2+ may be reactive with acetylene to ultimately form naphthalenylium + H, but the reaction is hindered by transition states residing few kJ mol-1 above the initial reactants occurring on the pathway following the formation of the initial C6H5=CCH2+…C2H2 complex. Implications of the results of the ab initio and RRKM-ME calculations will be discussed to answer the title question.