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
Qualitative reaction coordinate for inversion of an amine and a phosphine. The y-axis is energy.

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 usually are not optically stable (enantiomers racemize rapidly at room temperature), but P-chiral phosphines are.[5] Appropriately substituted sulfonium salts, sulfoxides, arsines, etc. are also optically stable near room temperature. Steric effects can also influence the barrier.

References
  1. Arvi Rauk, Leland C. Allen, Kurt Mislow (1970). "Pyramidal Inversion". Angew. Chem. Int. Ed. 9: 400–414. doi:10.1002/anie.197004001.CS1 maint: uses authors parameter (link)
  2. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected 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. 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.
  5. Xiao, Y.; Sun, Z.; Guo, H.; Kwon, O. (2014). "Chiral Phosphines in Nucleophilic Organocatalysis". Beilstein Journal of Organic Chemistry. 10: 2089–2121. doi:10.3762/bjoc.10.218. PMC 4168899.CS1 maint: uses authors parameter (link)
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