Ion-molecule routes towards cycles in TMC-1. An automated study of the C2H4 + CH2CCH+ reaction

TL;DR

This study explores ion-molecule reactions in space, revealing that certain bimolecular channels, rather than radiative association, predominantly lead to cyclic molecule formation relevant to interstellar chemistry.

Contribution

It introduces an integrated protocol combining astrochemical modeling, automated reaction search, and kinetics to study ion-molecule pathways for cyclic molecule formation in space.

Findings

Radiative association is inefficient for c-C5H7+ formation.

Bimolecular channels produce c-C5H5+ and CH3CCH2+.

Supports molecular growth pathways in the interstellar medium.

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. Ion-molecule reactions emerge as promising alternative pathways to explain the formation of these molecules. In the present work, we investigate the reaction network of the main ionic precursor of cyclopentadiene c-C5H7+ . To this end, we establish an integrated protocol that combines astrochemical modelling to identify viable formation routes under cold interstellar medium conditions, automated reaction path search and kinetic simulations to obtain accurate descriptions of the reaction pathways and reliable rate constants. In particular, we examine the reaction between ethylene (C2H4) and the linear propargyl cation (CH2CCH+). Our results reveal that the formation of c-C5H7+ by radiative association turns out to be 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+ offering new insights into reactivity that supports molecular growth in the ISM.