Submillimeter wave spectroscopy of propanoic acid (CH3CH2COOH) and its ISM search

Three compounds with a C3H6O2 formula have been detected in the Interstellar Medium (ISM): ethyl formate (CH3CH2OC(O)H), methyl acetate (CH3OC(O)CH3) and very recently hydroxyacetone (CH3C(O)CH2OH). The higher thermodynamic stability of another isomer, the propanoic acid (CH3CH2C(O)OH), clearly suggests that this molecule should be considered as a possible candidate for ISM detection. To provide reliable predictions for astronomical use in the millimeter and submillimeter wave ranges we performed a new study of propanoic acid rotational spectrum up to 545 GHz. The analysis of large amplitude methyl top torsional motion in this molecule was carried out using the rho axis method and the RAM36 program code. More than 3200 lines corresponding to the rotational transitions in the ground and first excited methyl top torsional states were newly assigned and fit within experimental error. This enabled us to produce new predictions which were used to search for propanoic acid in two high-mass star-forming regions: SgrB2 (public IRAM 30 m) and Orion KL (ALMA Science Verification). We report a non-detection of propanoic acid in these clouds and derive upper limits to the column density.