Rotational spectroscopic study of S-methyl thioformate

A global laboratory analysis of ground and excited torsional states up to 660 GHz^★

¹ Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Université Paris-Est-Créteil, Université de Paris, Institut Pierre Simon Laplace (IPSL), 61 av. du Général de Gaulle, 94010 Créteil, France
e-mail: kleiner@lisa.ipsl.fr
² Observatorio Astronómico Nacional (OAN-IGN). Calle Alfonso XII, 3, 28014 Madrid, Spain
³ Observatorio de Yebes (IGN). Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
⁴ Univ. Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, 59000 Lille, France
⁵ Radiospectrometry Department, Institute of Radio Astronomy of NASU, Kharkov, Ukraine
⁶ Instituto de Física Fundamental (IFF-CSIC). Calle Serrano 123, 28006 Madrid, Spain
⁷ Univ. Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR-UMR 6226, 35000 Rennes, France

Received: 19 April 2020
Accepted: 13 October 2020

Abstract

Context. S-methyl thioformate CH₃SC(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 CH₃SC(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 ν₁₈ = 1. Some 164 lines were also assigned to ν₁₈ = 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, V₃ = 127.4846(15) cm⁻¹.

Conclusions. Using our spectroscopy work, a deep search of S-methyl thioformate was carried out in the IRAM 30 m and ALMA data of different high-mass star-forming regions (Orion KL and Sgr B2). We derived an upper limit to the CH₃SC(O)H column density in these regions.