Detection of deuterated methylcyanoacetylene, CH 2 DC 3 N, in TMC-1

International audienceWe report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH 2 DC 3 N. A total of 15 rotational transitions, with J = 8–12 and K a = 0 and 1, were identified for this species in TMC-1 in the 31.0–50.4 GHz range using the Yebes 40 m radio telescope. The observed frequencies were used to derive for the first time the spectroscopic parameters of this deuterated isotopologue. We derive a column density of (8.0 ± 0.4)×10 10 cm −2 . The abundance ratio of CH 3 C 3 N to CH 2 DC 3 N is ∼22. We also theoretically computed the principal spectroscopic constants of 13 C isotopologues of CH 3 C 3 N and CH 3 C 4 H and those of the deuterated isotopologues of CH 3 C 4 H for which we could expect a similar degree of deuteration enhancement. However, we have not detected either CH 2 DC 4 H or CH 3 C 4 D, nor any 13 C isotopologues. The different observed deuterium ratios in TMC-1 are reasonably accounted for by a gas-phase chemical model where the low temperature conditions favour deuteron transfer through reactions with H 2 D +