C_5H_6 formation in Dense Molecular Clouds

Cyclopentadiene (C_5H_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. The aim of this work is to reconcile the chemical network that predicts the formation of C_5H_6---a key intermediate in the growth of aromatic species---from gas-phase chemistry with its observed abundance in TMC-1. 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_5H_7^+. We then incorporated these results into the gas–grain chemical code NAUTILUS to model the formation of C_5H_6. We identify the reactions and as an important source of and, consequently, of in TMC-1. We experimentally determined the reaction rate to be 1 C2H4+ + CH3CCH C3H7+ + C2H2 C5H7+ C5H6 C2H4+ + CH3CCH 10^ -9 cm^ 3 s^ -1 . In addition, the radiative association reactions and under low-pressure conditions deserve further investigation, as they may constitute key intermediate steps in the formation of through neutral–neutral reaction pathways. C4H5 + H C5H5 + H C5H6 Our updated chemical model accounts for several previously missing formation pathways of . Although it reproduces only ∼ 20% of the observed abundance, this represents a significant improvement compared to existing models. We identify the reactions and as the two dominant sources of . However, the formation of through neutral–neutral chemistry remains poorly constrained. The main source is the radiative association of but 1,3-butadiene ( ) appears to be a key intermediate in the formation of ; 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 . C5H6 C2H4+ + CH3CCH C3H7+ + C2H2 C5H7+ C5H6 C5H5 + H C4H6 C5H6 C4H5 + H