Rotational spectroscopic study of S-methyl thioformate: A global laboratory analysis of ground and excited torsional states up to 660 GHz*

CONTEXT: S-methyl thioformate CH(3)SC(O)H is a monosulfur derivative of methyl formate, a relatively abundant component of the interstellar medium (ISM). S-methyl thioformate being, thermodynamically, the most stable isomer, it can be reasonably proposed for detection in the ISM. AIMS: This work aims to experimentally study and theoretically analyze the ground and first torsional excited states for CH(3)SC(O)H in a large spectral range for astrophysical use. METHODS: S-methyl thioformate was synthesized as a result of a reaction of methyl mercaptan with acetic-formic anhydride. The millimeter-wave spectrum was then recorded for the first time from 150 to 660 GHz with the solid-state spectrometer located at Lille. RESULTS: A set of 3545 lines is determined and combined with 54 previously measured lines in the microwave region, belonging to ground state ν (t) = 0 as well as 1391 transitions in the first excited state of torsion ν (18) = 1. Some 164 lines were also assigned to ν (18) = 2 for the A-species. A global fit was performed using the BELGI-C(s) code taking into account the large splitting of A and E lines due to methyl internal rotation motion with a relatively low barrier, V(3) = 127.4846(15) cm(–1). CONCLUSIONS: Using our spectroscopy work, a deep search of S-methyl thioformate was carried out in the IRAM 30m and ALMA data of different high-mass star-forming regions (Orion KL and Sgr B2). We derived an upper limit to the CH(3)SC(O)H column density in these regions.