Some time ago, optical astronomy was successful in detecting the first molecular species in interstellar space. However, these have been limited to the three diatomic molecules CH, CH+, and CN. Consideration of this list made it rather natural, after the development of microwave astronomy, for some attention to be given to a search for OH, which has a rather strong microwave transition. This molecule was first detected, after a considerable search, by Weinreb et al.. The analogous Λ-doublet transitions in SH and CH have since been searched for assiduously; failure to detect them does not necessarily mean these radicals are excessively rare in interstellar space, since various unfavorable factors make the search difficult.

Any consideration of polyatomic molecules in interstellar space immediately raises the question as to whether and where such molecules are likely to exist. Of course, they can occur in stellar atmospheres, where they have already been found to a limited extent. But in free interstellar space they are less likely to exist because of slow rates of formation and a rather short lifetime, the latter being typically limited to a few hundred years because of dissociation by ultraviolet radiation from stars. In dust clouds, molecules are more protected from ultraviolet radiation and their formation may be aided by catalysis on the dust grain surfaces.