Paving the way to the synthesis of PAHs in dark molecular clouds: The formation of cyclopentadienyl radical (c-C₅H₅)

¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, Torrejón de Ardoz, 28850 Madrid, Spain
² Department of Organic and Inorganic Chemistry, Faculty of Sciences, and IACYS-Green Chemistry and Sustainable Development Unit, University of Extremadura, 06006 Badajoz, Spain
e-mail: jugarco@unex.es

Received: 5 January 2023
Accepted: 8 March 2023

Abstract

Context. The interest of astrophysicists and astrochemists in studying polycyclic aromatic hydrocarbons (PAHs) has grown since their assignment to previously unidentified IR emission bands of dust grains. Although observations show that PAHs are present in the interstellar medium (ISM), there is still no consensus about their formation. PAH formation has been studied following two approaches: bottom-up (i.e. through the association of smaller hydrocarbons) and top-down (via the photo-dissociation or hydrogenation of larger-structures). The recent detection of simple aromatic rings in the TMC-1 molecular cloud seems to suggest the bottom-up scenario is favoured.

Aims. We study the reaction between the propargyl (C₃H₃) radical and the vinyl radical (C₂H₃) under interstellar conditions, providing new kinetic parameters for the formation of PAH precursors.

Methods. We used high-level quantum-chemical calculations to describe the reaction mechanism between these two radicals. We calculated the rate constant of the individual product channels in the temperature range 10–400 K and at 1 × 10⁻⁷ atm by solving the one-dimensional master equation to quantitatively understand whether this reaction is viable in the ISM.

Results. Our results show that this reaction is likely to occur even in the low-density (~10⁴ cm⁻³) and low-temperature (~10 K) conditions of molecular dark clouds. The main product is the cyclopentadienyl radical (C₅H₅). The unsaturated open-chain hydrocarbon C₅H₅ in the trans (trans-C₅H₅) and cis conformations (cis-C₅H₅) are also formed, but to a lesser extent.

Conclusions. Our results show that the reaction of vinyl radical with propargyl radical is efficient under interstellar conditions and yields the cyclopentadienyl radical, which could be an important precursor for the formation of more complex polycyclic hydrocarbons (e.g. indene) and simple PAHs (e.g. naphthalene) in molecular dark clouds.