C 5 H 6 formation in Dense Molecular Clouds

Context . Cyclopentadiene (C 5 H 6 ) 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 C 5 H 6 – 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 C 5 H 7 + . We then incorporated these results into the gas–grain chemical code NAUTILUS to model the formation of C 5 H 6 . Results . We identify the reactions C 2 H 4 + + CH 3 CCH and C 3 H 7 + + C 2 H 2 as an important source of C 5 H 7 + and, consequently, of C 5 H 6 in TMC-1. We experimentally determined the C 2 H 4 + + CH 3 CCH reaction rate to be 1 × 10 −9 cm 3 s −1 . In addition, the radiative association reactions C 4 H 5 + H and C 5 H 5 + H under low-pressure conditions deserve further investigation, as they may constitute key intermediate steps in the formation of C 5 H 6 through neutral–neutral reaction pathways. Conclusions . Our updated chemical model accounts for several previously missing formation pathways of C 5 H 6 . Although it reproduces only ~20% of the observed abundance, this represents a significant improvement compared to existing models. We identify the reactions C 2 H 4 + + CH 3 CCH and C 3 H 7 + + C 2 H 2 as the two dominant sources of C 5 H 7 + . However, the formation of C 5 H 6 through neu-tral-neutral chemistry remains poorly constrained. The main source is the radiative association of C 5 H 5 + H but 1,3-butadiene (C 4 H 6 ) appears to be a key intermediate in the formation of C 5 H 6 ; 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 C 4 H 5 + H.