Detection of deuterated methylcyanoacetylene, CH₂DC₃N, in TMC-1

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 K_a = 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 deuterated isotopologue. We derive a column density of (8.0 ± 0.4) × 10¹⁰ cm^-2. The abundance ratio between CH₃C₃N and 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 nor CH₃C₄D nor any ¹³C 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⁺.