C₅H₆ formation in Dense Molecular Clouds

¹ Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91405 Orsay, France
² Université Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, 33400 Talence, France
³ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁴ Université Paris-Saclay, CNRS, Institut de Chimie Physique, UMR8000, 91405 Orsay, France
⁵ Synchrotron SOLEIL, L’Orme des Merisiers, 91192 Saint Aubin, Gif-sur-Yvette, France
⁶ Department of Physics, University of Trento, Via Sommarive 14, 38123 Trento, Italy

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 January 2026
Accepted: 16 February 2026

Abstract

Context. Cyclopentadiene (C₅H₆) has been recognized as a crucial precursor in the formation of nonplanar polycyclic aromatic hydrocarbons (PAHs) and carbon-rich nanostructures in space. Despite its significance and detection in the Taurus Molecular Cloud (TMC-1), the elementary gas-phase reaction pathways leading to cyclopentadiene from acyclic hydrocarbon precursors remain poorly constrained. This gap is emphasized by persistent discrepancies between astrochemical model predictions and the abundances inferred from its radioastronomical detection.

Aims. The aim of this work is to reconcile the chemical network that predicts the formation of C₅H₆ – a key intermediate in the growth of aromatic species – from gas-phase chemistry with its observed abundance in TMC-1.

Methods. We used combined experimental work conducted with the CERISES tandem mass spectrometer at the SOLEIL synchrotron, together with quantum chemical calculations, to determine and refine reaction rate coefficients and branching ratios for reaction pathways that lead to the formation of C₅H₇⁺. We then incorporated these results into the gas–grain chemical code NAUTILUS to model the formation of C₅H₆.

Results. We identify the reactions C₂H₄⁺ + CH₃CCH and C₃H₇⁺ + C₂H₂ as an important source of C₅H₇⁺ and, consequently, of C₅H₆ in TMC-1. We experimentally determined the C₂H₄⁺ + CH₃CCH reaction rate to be 1 × 10⁻⁹ cm³ s⁻¹. In addition, the radiative association reactions C₄H₅ + H and C₅H₅ + H under low-pressure conditions deserve further investigation, as they may constitute key intermediate steps in the formation of C₅H₆ through neutral–neutral reaction pathways.

Conclusions. Our updated chemical model accounts for several previously missing formation pathways of C₅H₆. Although it reproduces only ~20% of the observed abundance, this represents a significant improvement compared to existing models. We identify the reactions C₂H₄⁺ + CH₃CCH and C₃H₇⁺ + C₂H₂ as the two dominant sources of C₅H₇⁺. However, the formation of C₅H₆ through neu-tral-neutral chemistry remains poorly constrained. The main source is the radiative association of C₅H₅ + H but 1,3-butadiene (C₄H₆) appears to be a key intermediate in the formation of C₅H₆; however, its abundance is uncertain due to the disputed detection of its cyano-derivative proxy. Further work is therefore required to better constrain the abundance of 1,3-butadiene, which may be efficiently formed through the radiative association C₄H₅ + H.