Sulphur-bearing species in molecular clouds
Using density functional theory methods we studied several molecules that could help in the solution of the missing sulphur problem in dense clouds and circumstellar regions, as well as in the clarification of the sulphur chemistry in comets. We calculated infrared spectra of the following molecules: the trimer (CH$_2$S)$_3$ and the tetramer (CH$_2$S)$_4$ of thioformaldehyde, pentathian S$_5$CH$_2$, hexathiepan S$_6$CH$_2$, thiirane C$_2$H$_4$S, trisulfane HSSSH, and thioacetone (CH$_3$)$_2$CS. 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 (CH$_2$S)$_3$, (CH$_2$S)$_4$, and thiirane C$_2$H$_4$S. The objects whose unidentified infrared features we compared with calculated bands are NGC 253, M82, NGC 1068, Circinus, Arp 220, 30 Doradus, Orion KL, and Sgr B2.