Pyramidal inversion
In chemistry, pyramidal inversion is a fluxional process in compounds with a pyramidal molecule, such as ammonia "turns inside out". It is a rapid oscillation of the atom and substituents, the molecule or ion passing through a planar transition state. For a compound that would otherwise be chiral due to a stereocenter, pyramidal inversion allows its enantiomers to racemize.The identity of the inverting atom has a dominating influence on the barrier. Inversion of ammonia is rapid at room temperature. In contrast, phosphine inverts very slowly at room temperature. Consequently, amines of the type RR′R"N usually are not optically stable, but P-chiral phosphines are. Appropriately substituted sulfonium salts, sulfoxides, arsines, etc. are also optically stable near room temperature. Steric effects can also influence the barrier.
