Millimeter- and submillimeter-wave spectroscopy of thioformamide and
  interstellar search toward Sgr B2(N)
  Thioformamide NH2CHS is a sulfur-bearing analog of formamide NH2CHO. The
latter was detected in the interstellar medium back in the 1970s. Most of the
sulfur-containing molecules detected in the interstellar medium are analogs of
corresponding oxygen-containing compounds. Therefore, thioformamide is an
interesting candidate for a search in the interstellar medium. The rotational
spectrum of thioformamide was measured and analyzed in the frequency range 150
to 660 GHz using the Lille spectrometer. We searched for thioformamide toward
the high-mass star-forming region Sagittarius (Sgr) B2(N) using the ReMoCA
spectral line survey carried out with the Atacama Large
Millimeter/submillimeter Array (ALMA). Accurate rotational constants were
obtained from the analysis of the ground state of parent, 34S, 13C, and 15N
singly substituted isotopic species of thioformamide. For the parent
isotopolog, the lowest two excited vibrational states, v12 = 1 and v9 = 1, were
analyzed using a model that takes Coriolis coupling into account. Thioformamide
was not detected toward the hot cores Sgr B2(N1S) and Sgr B2(N2). The sensitive
upper limits indicate that thioformamide is nearly three orders of magnitude at
least less abundant than formamide. This is markedly different from
methanethiol, which is only about two orders of magnitude less abundant than
methanol in both sources. The different behavior shown by methanethiol versus
thioformamide may be caused by the preferential formation of the latter (on
grains) at late times and low temperatures, when CS abundances are depressed.
This reduces the thioformamide-to-formamide ratio, because the HCS radical is
not as readily available under these conditions.
