thumb|300px|Isomerization of azobenzene, induced by light (hν) or heat (Δ) The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by isomerization or a dissociation into one or more other molecules. Such reactions require the addition of energy in the form of heat or light. A typical example of a unimolecular reaction is the cis–trans isomerization, in which the cis-form of a compound converts to the trans-form or vice versa.
thumb|300px|Isomerization of azobenzene, induced by light (hν) or heat (Δ) The most important elementary reactions are unimolecular and bimolecular reactions. Only one molecule is involved in a unimolecular reaction; it is transformed by isomerization or a dissociation into one or more other molecules. Such reactions require the addition of energy in the form of heat or light A typical example of a unimolecular reaction is the cis–trans isomerization in which the cis-form of a compound converts to the trans-form or vice versa.