Free carbenes in the interstellar gas

Carbene molecules are a significant trace constituent of the gas in the interstellar medium (and in the expanding shell around at least one star), representing ca. 16% of the molecular species that have been identified in space. The identification of one of the most abundant and widespread interstellar carbenes, the cyclopropenylidene ring, C3H2, is described, and recent laboratory work on the excited vibrational states of this molecule and on the geometrical structure of one of its isomers, is presented. Several free carbenes which might be detected in space are considered. Two reasons why carbenes are comparatively conspicuous in astronomical sources relative to other reactive molecules are their high polarity and their production via ion–molecule reactions that make familiar stable species. Cumulene carbon chains somewhat longer than those so far detected in space are promising candidates for the carriers of the interstellar optical diffuse bands.