Cumulene carbenes in TMC-1: Astronomical discovery of l-H₂C₅ ^★

We report the first detection in space of the cumulene carbon chain l-H₂C₅. A total of eleven rotational transitions, with J_up = 7-10 and K_a = 0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. We derive a column density of (1.8±0.5)×10¹⁰ cm^-2. In addition, we report observations of other cumulene carbenes detected previously in TMC-1, to compare their abundances with the newly detected cumulene carbene chain. We find that l-H₂C₅ is ~4.0 times less abundant than the larger cumulene carbene l-H₂C₆, while it is ~300 and ~500 times less abundant than the shorter chains l-H₂C₃ and l-H₂C₄. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C _n H₂ isomers with n > 3.