Discovery of the cyclic C5H radical in TMC-1

Abstract

Cyclic C5H (c-C5H), the radical formed by substituting an ethynyl group CCH for the hydrogen atom in thec-C3H radical, has been detected for the first time in the space. Thec-C5H species is an isomer of the well-known linear radicall-C5H and is ∼6 kcal mol−1less stable. A total of 17 rotational transitions were detected for thec-C5H species in TMC-1 within the 31.0–50.3 GHz range using the Yebes 40m radio telescope. We derive a column density of (9.0 ± 0.9) × 1010cm−2forc-C5H. Additionally, we observed 12 lines forl-C5H and derive a column density for it of (1.3 ± 0.3) × 1012cm−2, which results in an abundance ratioc-C5H/l-C5H of 0.069. This is in sharp contrast with the value found for the analogue systemc-C3H/l-C3H, whose ratio is 5.5 in TMC-1. We discuss the formation ofc-C5H and conclude that this radical is probably formed in the reaction of atomic carbon with diacetylene.