Investigation of the rotational spectrum of CD₃OD and an astronomical search toward IRAS 16293-2422

Solar-type prestellar cores and protostars frequently display large amounts of deuterated organic molecules and, in particular, high relative abundances of doubly and triply deuterated isotopologs. Recent findings on CHD2OH and CD3OH toward IRAS 162932422 suggest that even fully deuterated methanol, CD3OD, may be detectable as well. However, searches for CD3OD are hampered in particular by the lack of intensity information from a spectroscopic model. The objective of the present investigation is to develop a spectroscopic model of CD3OD in low-lying torsional states that is sufficiently accurate to facilitate searches for this isotopolog in space. We carried out a new measurement campaign for CD3OD involving two spectroscopic laboratories that covers the 34 GHz-1.1 THz range. A torsion-rotation Hamiltonian model based on the rho-axis method was employed for our analysis. Our resulting model describes the ground and first excited torsional states of CD3OD well up to quantum numbers J <= 51 and Ka <= 23. We derived a line list for radio-astronomical observations from this model that is accurate up to at least 1.1 THz and should be sufficient for all types of radio-astronomical searches for this methanol isotopolog. This line list was used to search for CD3OD in data from the Protostellar Interferometric Line Survey of IRAS 16293-2422 obtained with the Atacama Large Millimeter/submillimeter Array. While we found several emission features that can be attributed largely to CD3OD, their number is still not sufficiently high enough to establish a clear detection. Nevertheless, the estimate of 2 x 10(15) cm(-2) derived for the CD3OD column density may be viewed as an upper limit that can be compared to column densities of CD3OH, CH3OD, and CH3OH. The comparison indicates that the CD3OD column density toward IRAS 16293-2422 is in line with the enhanced D/H ratios observed for multiply deuterated complex organic molecules.