  Methyl mercaptan (CH3SH) is a known interstellar molecule with abundances
high enough that the detection of some of its minor isotopologues is promising.
The present study aims at providing accurate spectroscopic parameters for the
(13)CH3SH isotopologue to facilitate its identification in the interstellar
medium at millimetre and submillimetre wavelengths. Through careful analysis of
recent CH3SH spectra from 49-510 GHz and 1.1-1.5 THz recorded at natural
isotopic composition, extensive assignments were possible not only for the
ground torsional state of (13)CH3SH, but also in the first and second excited
states. The torsion-rotation spectrum displays complex structure due to the
large-amplitude internal rotation of the (13)CH3 group, similar to the main and
other minor isotopic species of methyl mercaptan. The assigned transition
frequencies have been fitted to within experimental error with a 52-parameter
model employing the RAM36 programme. With predictions based on this fit,
(13)CH3SH was searched for in spectra from the Atacama Large
Millimetre/submillimetre Array (ALMA) towards the Galactic centre source Sgr
B2(N2). Several transitions were expected to be observable, but all of them
turned out to be severely blended with emission from other species, which
prevents us from identifying (13)CH3SH in this source.

Solar and Stellar Astrophysics;Astrophysics of Galaxies;Instrumentation and Methods for Astrophysics;Atomic and Molecular Clusters;Chemical Physics