The formation of dust in circumstellar environments is essentially a pure chemical problem. The conditions, however, under which chemical reactions occur in such environments are quite different from the conditions under which chemical reactions proceed in the laboratory.

First, particle densities in circumstellar shells generally are lower than in the laboratory by about 10 decades. This means that chemical reactions in circumstellar shells proceed much more slowly than in the laboratory, often so slowly, that reaction conditions change considerably before a reaction has run to completion. Especially the cooling of matter in an expanding stellar outflow may increase reaction time scales dramatically if activation-energy barriers are involved. This necessitates in the case of circumstellar environments that we consider in detail how reactions proceed in time in an environment where reaction conditions continuously change during the course of the reaction. The low densities additionally disable the stabilization of a complex of colliding particles by transferring the bond energy of two particles to a third particle while undergoing a transition into a bound state. Such ternary reactions are often essential in reactions under laboratory conditions but are completely negligible under circumstellar conditions. Important reaction routes under laboratory conditions thus are inaccessible in circumstellar shells.