Pyramidal inversion
In chemistry, pyramidal inversion is a fluxional process in compounds with a pyramidal molecule, such as ammonia (NH3) "turns inside out".[1][2] It is a rapid oscillation of the atom and substituents, the molecule or ion passing through a planar transition state.[3] For a compound that would otherwise be chiral due to a stereocenter, pyramidal inversion allows its enantiomers to racemize.
Energy barrier
The identity of the inverting atom has a dominating influence on the barrier. Inversion of ammonia is rapid at room temperature. In contrast, phosphine (PH3) inverts very slowly at room temperature (energy barrier: 132 kJ/mol).[4] Consequently, amines of the type RR′R"N are not optically stable. Unlike amines, P-chiral phosphines are optically stable (enantiomers racemize very slowly at room temperature). Appropriately substituted Sulfonium salts, sulfoxides, arsines, etc. are also optically stable.
steric can also influence the barrier.
References
- ^ "Pyramidal Inversion". Angew. Chem. Int. Ed. 9: 400–414. 1970. doi:10.1002/anie.197004001.
{{cite journal}}: Unknown parameter|authors=ignored (help) - ^ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Pyramidal inversion". doi:10.1351/goldbook.P04956
- ^ J. M. Lehn (1970). "Nitrogen Inversion: Experiment and Theory". Fortschr. Chem. Forsch. 15: 311–377. doi:10.1007/BFb0050820.
- ^ Kölmel, C.; Ochsenfeld, C.; Ahlrichs, R. (1991). "An ab initio investigation of structure and inversion barrier of triisopropylamine and related amines and phosphines". Theor. Chim. Acta. 82 (3–4): 271–284. doi:10.1007/BF01113258.