Pyramidal inversion

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Nitrogen inversion: The amine C3 axis is horizontal; the pair of dots represent the lone pair (on that axis). Note that the two amine molecules are symmetric across a mirror plane. If the three R groups attached are all unique, then the amine is chiral; isolability depends on the free energy required to invert the molecule.

In chemistry, pyramidal inversion (also umbrella 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. The general phenomenon of pyramidal inversion applies to many types of molecules, including carbanions, amines, phosphines, arsines, stibines, and sulfoxides.[4][2]

  1. ^ Arvi Rauk; Leland C. Allen; Kurt Mislow (1970). "Pyramidal Inversion". Angewandte Chemie International Edition. 9 (6): 400–414. doi:10.1002/anie.197004001.
  2. ^ a b IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Pyramidal inversion". doi:10.1351/goldbook.P04956
  3. ^ J. M. Lehn (1970). "Nitrogen Inversion: Experiment and Theory". Fortschr. Chem. Forsch. 15: 311–377. doi:10.1007/BFb0050820.
  4. ^ Arvi Rauk; Leland C. Allen; Kurt Mislow (1970). "Pyramidal Inversion". Angewandte Chemie International Edition. 9 (6): 400–414. doi:10.1002/anie.197004001.
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