Ion-molecule routes towards cycles in TMC-1

An automated study of the C2H4 + CH2CCH+ reaction

Departamento de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 123, 28006 Madrid, Spain

^★ Corresponding authors: maria.mallo@iff.csic.es; german.molpeceres@iff.csic.es

Received: 10 October 2025
Accepted: 31 October 2025

Abstract

Cyclopentadiene (c-C₅H₆) is considered a key molecule in the formation of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). The synthesis of PAHs from simpler precursors is known as the “bottom-up” theory, which, so far, has been dominated by reactions between organic radicals. However, this mechanism struggles to account for the origin of the smallest cycles themselves. However, it struggles to account for the origin of the smallest cycles themselves. Ion-molecule reactions emerge as promising alternative pathways to explain the formation of these molecules. We investigated the reaction network of the main ionic precursor of cyclopentadiene, c-C₅H₇⁺. To this end, we established an integrated protocol that combines astrochemical modelling to identify viable formation routes under cold ISM conditions, automated reaction path searches, and kinetic simulations to obtain accurate descriptions of the reaction pathways and reliable rate constants. In particular, we examined the reaction between ethylene (C₂H₄) and the linear propargyl cation (CH₂CCH⁺). Our results reveal that the formation of c-C₅H₇⁺ by radiative association is inefficient, contrary to our initial expectations. Instead, the system predominantly evolves through bimolecular channels yielding c-C₅H₅⁺ and CH₃CCH₂⁺, with the formation of c-C₅H₅⁺; this offers new insights into the reactivity that supports molecular growth in the ISM.