Rotational spectroscopy of oxirane-$\mathit{\text{2,2}}$-$d_2$, $c$-CD${}_2$CH${}_2$O, and its tentative detection toward IRAS 16293$-$2422~B

We prepared a sample of oxirane doubly deuterated at one C atom and studied its rotational spectrum in the laboratory for the first time between 120~GHz and 1094~GHz. Accurate spectroscopic parameters up to eighth order were determined, and the calculated rest frequencies were used to identify $c$-CD${}_2$CH${}_2$O tentatively in the interstellar medium in the Atacama Large Millimeter/submillimeter Array Protostellar Interferometric Line Survey (PILS) of the Class 0 protostellar system IRAS 16293$-$2422. The $c$-CD${}_2$CH${}_2$O to $c$-C${}_2$H${}_4$O ratio was estimated to be $\sim$0.054 with $T_{\mathrm{r}\mathrm{o}\mathrm{t}}=125$ K. This value translates to a D-to-H ratio of $\sim$0.16 per H atom which is higher by a factor of 4.5 than the $\sim$0.036 per H atom obtained for $c$-C${}_2$H${}_3$DO. Such increase in the degree of deuteration referenced to one H atom in multiply deuterated isotopologs compared to their singly deuterated variants have been observed commonly in recent years.