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 40 m radio telescope. We derived a column density of (1.8 +/- 0.5)x10(10) cm(-2). In addition, we report observations of other cumulene carbenes detected previously in TMC-1 in order to compare their abundances with the newly detected cumulene carbene chain. We find that l-H2C5 is similar to 4.0 times less abundant than the larger cumulene carbene l-H2C6, while it is similar to 300 and similar to 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 CnH2 isomers with n>3.