2016A&A...586A.110S

The partially deuterated linear isomer HDCCC of the ubiquitous cyclic carbene (c-C₃H₂) was observed in the starless cores TMC-1C and L1544 at 96.9GHz, and a confirming line was observed in TMC-1 at 19.38GHz. To aid the identification in these narrow line sources, four centimetre-wave rotational transitions (two in the previously reported K_a=0 ladder and two new ones in the K_a=1 ladder) and 23 transitions in the millimetre band between 96 and 272GHz were measured in high-resolution laboratory spectra. Ten spectroscopic constants in a standard asymmetric top Hamiltonian allow the main transitions of astronomical interest in the K_a≤3 rotational ladders to be calculated to within 0.1km/s in radial velocity up to 400GHz. Conclusive identification of the two astronomical lines of HDCCC was provided by the V_LSR, which is the same as for the normal isotopic species (H₂CCC) in the three narrow line sources. In these sources, deuterium fractionation in singly substituted H₂CCC (HDCCC/H₂CCC∼4-19%) is comparable to that in c-C₃H₂ (c-C₃H₂/c-C₃HD∼5-17%) and similarly in doubly deuterated c-C₃H₂ (c-C₃D₂/c-C₃HD∼3-17%), implying that the efficiency of the deuteration processes in the H₂CCC and c-C₃H₂ isomers are comparable in dark clouds.