Rotational spectroscopy and astronomical search of propane-1-thiol (CH3CH2CH2SH) and propane-2-thiol ((CH3)2CHSH)

Abstract

Sulfur-bearing organic molecules provide essential insights into the chemical complexity and evolution of the interstellar medium (ISM). We report the laboratory rotational spectroscopic characterization of propanethiol (C3H7SH), a sulfur analogue of propanol and a key target for investigating interstellar sulfur chemistry, using free-jet absorption millimeter-wave (FJ-AMMW) spectroscopy in the 59.6–80 GHz range. For 1PT, three conformers, Aa, Ag, and Gg, were observed, with Gg reported for the first time. For 2PT, both the anti and gauche conformers were identified. We report the first assignments of several monosubstituted isotopologues in natural abundance, including 34S-anti-2PT, 13C1-anti-2PT, 13C2-anti-2PT, and 34S-gauche-2PT. Resolvable tunneling splitting arising from large-amplitude motion of the sulfhydryl (-SH) group was observed for Ag-1PT and gauche-2PT. Global fits incorporating previous measurements yielded accurate rotational constants, quartic centrifugal distortion constants, and vibrational coupling parameters. These spectroscopic data enabled an astronomical search toward the chemically rich Galactic Center molecular cloud G+0.693-0.027. While no evidence for propanethiol was found, we established rigorous 3σ upper limits for their column densities: N(Gg-1PT) <3 × 1012 cm−2 and N(g-2PT) <8 × 1012 cm−2. Thus, 1PT and 2PT are at least 13 and 5 times less abundant than ethanethiol in this source, respectively. The O/S ratio for the propane-1-ol/propane-1-thiol pair is ≥29, consistent with the solar value (≃ 37), providing a benchmark for sulfur depletion in the ISM. Ultimately, this work provides the precise spectral data necessary for future successful interstellar detections.