Three-membered rings

Oxirene is a heterocyclic chemical compound which contains an unsaturated three-membered ring containing two carbon atoms and one oxygen atom. The molecule was synthesized in low temperature ices and detected upon sublimation by isomer selective photoionization reflectron time-of-flight mass spectrometry.

A diaziridine is a heterocyclic compound containing two nitrogen atoms in a three-membered ring. Diaziridines can be considered as strained hydrazines. Unlike most amine types of structures, the nitrogen atoms of diaziridines are configurationally stable because the ring strain prevents Walden inversion. As a result, there can be various stereoisomeric forms of this structure.

thumb|right|class=skin-invert-image|A generic oxaziridine derivative.

Thiirene is an organosulfur compound with the formula C2H2S. It can be viewed as a derivative of cyclopropene, but with the methylene group replaced by sulfur. It is antiaromatic and very labile.

allotrope of oxygen; hypothetical isomer of ozone

Borirane is a heterocyclic organic compound with the formula C2H4BH. This colourless, flammable gas is the simplest borirane, a three-membered ring consisting of two carbon and one boron atom. It can be viewed as a structural analog of aziridine, with boron replacing the nitrogen atom of aziridine. Borirane is isomeric with ethylideneborane.

Cyclopropatriene is a hypothetical compound () which is an allotrope of carbon. It was once proposed as a candidate for a spectroscopically observed tricarbon species. It is a cyclic cumulene.