Astronomy & Astrophysics manuscript no. AA202141371_Cabezas ©ESO 2021 June 8, 2021 Detection of deuterated methylcyanoacetylene, CH2DC3N, in TMC-1 ? C. Cabezas1, E. Roueff2, B. Tercero3; 4, M. Agúndez1, N. Marcelino1, P. de Vicente3 and J. Cernicharo1 1 Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain. e-mail:
[email protected];
[email protected] 2 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France 3 Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014, Madrid, Spain. 4 Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain. Received; accepted ABSTRACT We report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH2DC3N. A total of fifteen rotational transitions, with J = 8-12 and Ka = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies were used to derive for the first time the spectroscopic parameters of this 10 −2 deuterated isotopologue. We derive a column density of (8:0±0:4)×10 cm . The abundance ratio between CH3C3N and CH2DC3N 13 is ∼22. We also theoretically computed the principal spectroscopic constants of C isotopologues of CH3C3N and CH3C4H and those of the deuterated isotopologues of CH3C4H for which we could expect a similar degree of deuteration enhancement. However, we 13 have not detected either CH2DC4H nor CH3C4D nor any C isotopologue. The different observed deuterium ratios in TMC-1 are reasonably accounted for by a gas phase chemical model where the low temperature conditions favor deuteron transfer through + reactions with H2D .