These reactions are more likely to involve a series of bimolecular reactions. Rapid post-reaction cooling and the dilution inherent in gases both suppress bimolecular degradation pathways. By suppressing bimolecular decomposition pathways, cyclobutadiene is well-behaved. The most important elementary reactions are unimolecular and bimolecular reactions. E1 stands for unimolecular elimination, and E2 stands for bimolecular elimination. The Williamson ether reaction follows an S2 bimolecular nucleophilic substitution mechanism. For bimolecular reactions, two molecules collide and react with each other. Almost all elementary steps are either unimolecular or bimolecular. When fluorescent proteins are reconstituted the PCA is called Bimolecular fluorescence complementation assay. Bimolecular reactions are highly dependent on the concentration of substrates. Fast internal conversion reduces the excited state lifetime, and thereby prevents bimolecular reactions. Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or trimolecular. Bimolecular electron transfer always produces a reactive chemical species, free radicals. These structures, formed from two separate G-rich strands, are termed bimolecular quadruplexes. In a bimolecular reaction, two molecules collide and exchange energy, atoms or groups of atoms.