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

TL;DR

This work tackles the formation of cyclopentadiene in cold interstellar environments by probing ion–molecule routes toward the cyclopentadienyl cation precursor c-C5H7+. Using an automated reaction-path discovery framework (AFIR/GRRM23) coupled with high-level quantum chemistry and RRKM/MESS kinetics, the authors map the C2H4 + CH2CCH+ PES and identify two dominant bimolecular channels forming c-C5H5+ and CH3CCH2+ with rate constants that dominate over radiative association under TMC-1 conditions. They find radiative association to c-C5H7+ is negligible and that c-C5H5+ appears as a reactive hub capable of driving ring-formation–type growth and possibly leading to benzonitrile and larger PAH precursors, though the observed c-C5H6 abundance remains underpredicted by the explored network. The study highlights the need for broader reaction networks and faster exploration methods (potentially ML-assisted PES) to fully capture interstellar hydrocarbon chemistry, and it provides concrete rate expressions for key channels suitable for astrochemical modeling. Overall, the work demonstrates a powerful, end-to-end pipeline for automated astrochemical reaction discovery that directly informs models of PAH formation in dense clouds like TMC-1.

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.

Figures (11)

• Figure 1: Schematic diagram of our automated protocol to explore the reaction network of ion-molecule systems.
• Figure 2: Prediction of the relative abundance of cyclopentadiene (c-C5H6) as a function of time in the chemical model of TMC-1. The black solid line is the abundance predicted by the model without ion-molecule reactions, dashed lines represent how it is modified including only each of the reactions separately and the green solid line is the total abundance when all the reactions from the dashed lines are included. The horizontal, dotted line represents the observed abundance of cyclopentadiene in TMC-1, calculated from its column density cernicharo2021pure.
• Figure 3: From left to right: structures of R1, R2 and R3 (all of them isomeric forms of C5H7+)
• Figure 4: Depiction of the reaction network of the C2H4 + CH2CCH+ reaction. The energies are calculated at the DLPNO-CCSD(T)/aug-cc-pvtz level with the $\omega$B97XD/def2-TZVPD harmonic zero point energy. In the diagram EQ are used to refer to the wells (local minima) on the PES, TS refer to transition states, and DC means "Dissociation channels". This nomenclature is chosen to be consistent with the one provided by the GRRM code grrm23.
• Figure 5: Rate constants (k) for the formation of the exothermic products of the C2H4 + CH2CCH+ reaction as a function of temperature in a range of 80-300 K and a pressure of $10^{-7}$ atm.