Letter to the Editor

Detection of deuterated methylcyanoacetylene, CH₂DC₃N, in TMC-1^⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: carlos.cabezas@csic.es; jose.cernicharo@csic.es
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: 22 May 2021
Accepted: 6 June 2021

Abstract

We report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH₂DC₃N. 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)×10¹⁰ cm⁻². The abundance ratio of CH₃C₃N to CH₂DC₃N is ∼22. We also theoretically computed the principal spectroscopic constants of ¹³C 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 have not detected either CH₂DC₄H or CH₃C₄D, nor any ¹³C 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 H₂D⁺.