2023MNRAS.518..185M

We prepared a sample of mono-deuterated oxirane and studied its rotational spectrum in the laboratory between 490 and 1060 GHz in order to improve its spectroscopic parameters and consequently the calculated rest frequencies of its rotational transitions. The updated rest frequencies were employed to detect c-C₂H₃DO for the first time in the interstellar medium in the Atacama Large Millimetre/submillimetre Array Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293-2422. Fits of the detected lines using the rotation diagrams yield a temperature of T_rot = 103 ± 19 K, which in turn agrees well with 125 K derived for the c-C₂H₄O main isotopologue previously. The c-C₂H₃DO to c-C₂H₄O ratio is found to be ∼0.15 corresponding to a D-to-H ratio of ∼0.036 per H atom, which is slightly higher than the D-to-H ratio of species such as methanol, formaldehyde, and ketene but lower than those of the larger complex organic species such as ethanol, methyl formate, and glycolaldehyde. This may reflect that oxirane is formed fairly early in the evolution of the prestellar cores. The identification of doubly deuterated oxirane isotopomers in the PILS data may be possibly judged by the amount of mono-deuterated oxirane and the observed trend that multiply deuterated isotopologues have higher deuteration rates than their mono-deuterated variants.