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 CHSC(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 CHSC(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 = 0 as well as 1391 transitions in the first excited state of torsion = 1. Some 164 lines were also assigned to = 2 for the A-species. A global fit was performed using the BELGI-C code taking into account the large splitting of A and E lines due to methyl internal rotation motion with a relatively low barrier, V = 127.4846(15) cm.

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 CHSC(O)H column density in these regions.