Cage effect

In chemistry, the cage effect, also called geminate recombination, describes how the immediate solvent surroundings of a molecule shape its chemical behavior, treating dissolved molecules or radicals as if temporarily encapsulated by neighboring solvent molecules rather than acting as fully free particles. The effect was introduced by James Franck and Eugene Rabinowitch in 1934, and the trapped species are described as forming a cage pair, or geminate pair, that typically persists for around ten to the minus eleven seconds before a particle can escape the solvent cage to react with other molecules. In free radical polymerization, radicals formed by the breakdown of an initiator become similarly enclosed within a cage of solvent or monomer molecules, where repeated collisions within the cage often lead to recombination or mutual deactivation of the radical pair; the chance that a radical pair escapes recombination has been measured at roughly 0.1 to 0.01 in polymers and 0.3 to 0.8 in liquids, and the effect has been studied using iodine molecules, heme proteins in solution, and small molecules trapped in solid noble gas matrices and triiodide crystals.