Doubly deuterated acetaldehyde

Submillimetre wave transitions were measured for the non-rigid doubly deuterated acetaldehyde CHD_2_CHO displaying hindered internal rotation of its asymmetrical CHD_2_ methyl group. An analysis of a dataset consisting of previously measured microwave transitions and of the newly measured ones was carried out with an effective Hamiltonian which accounts for the tunneling of the asymmetrical methyl group. A line position analysis is carried out allowing us to reproduce 853 transition frequencies with a weighted root mean square standard deviation of 1.7, varying 40 spectroscopic constants.

A spectroscopic catalogue for astrophysical purposes is built from the analysis results. Using this catalogue we were able to detect for the first time CHD_2_CHO towards the low-mass protostellar system IRAS 16293-2422 utilizing data from the ALMA Protostellar Interferometric Line Survey. The first detection of the CHD_2_CHO species allows for the derivation of its column density with a value of 1.3x10^15^cm^-2^ and an uncertainty of 10-20%.

The resulting D_2_/D ratio of ~20% is found to be coincident with D_2_/D ratios derived for other complex organic molecules towards IRAS 16293-2422, pointing at a common formation environment with enhanced deuterium fractionation.