Sulfur-bearing species in molecular clouds
  We study several molecules that could help in the solution of the missing
sulfur problem in dense clouds and circumstellar regions, as well as in the
clarification of the sulfur chemistry in comets. These sulfur molecules are:
the trimer (CH2S)3 and the tetramer (CH2S)4 of thioformaldehyde, pentathian
S5CH2, hexathiepan S6CH2, thiirane C2H4S, trisulfane HSSSH, and thioacetone
(CH3)2CS. Infrared spectra of these species are calculated using density
functional theory methods. The majority of calculated bands belong to the
mid-infrared, with some of them occurring in the near and far-infrared region.
We suggest that some of unidentified spectral features measured by Infrared
Space Observatory in several active galactic nuclei and starburst galaxies
could be caused by 1,3,5-trithiane ((CH2S)3), 1,3,5,7-tetrathiocane ((CH2S)4),
and thiirane (C2H4S). The objects whose unidentified infrared features we
compare with calculated bands are: NGC 253, M82, NGC 1068, Circinus, Arp 220,
30 Doradus, Orion KL, and Sgr B2.
