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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]

Energy barrier

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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, inverting 30 billion times per second. Three factors contribute to the rapidity of the inversion: a low energy barrier (24.2 kJ/mol; 5.8 kcal/mol), a narrow barrier width (distance between geometries), and the low mass of hydrogen atoms, which combine to give a further 80-fold rate enhancement due to quantum tunnelling.[5] In contrast, phosphine (PH3) inverts very slowly at room temperature (energy barrier: 132 kJ/mol).[6] Consequently, amines of the type RR′R"N usually are not optically stable (enantiomers racemize rapidly at room temperature), but P-chiral phosphines are.[7] Appropriately substituted sulfonium salts, sulfoxides, arsines, etc. are also optically stable near room temperature. Steric effects can also influence the barrier.

Nitrogen inversion

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Nitrogen inversion in ammonia

Pyramidal inversion in nitrogen and amines is known as nitrogen inversion.[8] It is a rapid oscillation of the nitrogen atom and substituents, the nitrogen "moving" through the plane formed by the substituents (although the substituents also move - in the other direction);[9] the molecule passing through a planar transition state.[10] For a compound that would otherwise be chiral due to a nitrogen stereocenter, nitrogen inversion provides a low energy pathway for racemization, usually making chiral resolution impossible.[11]

Quantum effects

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Ammonia exhibits a quantum tunnelling due to a narrow tunneling barrier,[12] and not due to thermal excitation. Superposition of two states leads to energy level splitting, which is used in ammonia masers.

Examples

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The inversion of ammonia was first detected by microwave spectroscopy in 1934.[13]

In one study the inversion in an aziridine was slowed by a factor of 50 by placing the nitrogen atom in the vicinity of a phenolic alcohol group compared to the oxidized hydroquinone.[14]

Nitrogen inversion Davies 2006
Nitrogen inversion Davies 2006

The system interconverts by oxidation by oxygen and reduction by sodium dithionite.

Exceptions

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rigid Tröger's base scaffold prevents nitrogen inversion [15]

Conformational strain and structural rigidity can effectively prevent the inversion of amine groups. Tröger's base analogs[16] (including the Hünlich's base[17]) are examples of compounds whose nitrogen atoms are chirally stable stereocenters and therefore have significant optical activity.[15]

References

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  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, 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. ^ Arvi Rauk; Leland C. Allen; Kurt Mislow (1970). "Pyramidal Inversion". Angewandte Chemie International Edition. 9 (6): 400–414. doi:10.1002/anie.197004001.
  5. ^ Halpern, Arthur M.; Ramachandran, B. R.; Glendening, Eric D. (June 2007). "The Inversion Potential of Ammonia: An Intrinsic Reaction Coordinate Calculation for Student Investigation". Journal of Chemical Education. 84 (6): 1067. Bibcode:2007JChEd..84.1067H. doi:10.1021/ed084p1067. eISSN 1938-1328. ISSN 0021-9584.
  6. ^ 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. S2CID 98837101.
  7. ^ 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. PMID 25246969.
  8. ^ Ghosh, Dulal C.; Jana, Jibanananda; Biswas, Raka (2000). "Quantum chemical study of the umbrella inversion of the ammonia molecule". International Journal of Quantum Chemistry. 80 (1): 1–26. doi:10.1002/1097-461X(2000)80:1<1::AID-QUA1>3.0.CO;2-D. ISSN 1097-461X.
  9. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 423. ISBN 978-0-08-037941-8.
  10. ^ J. M. Lehn (1970). "Nitrogen Inversion: Experiment and Theory". Fortschr. Chem. Forsch. 15: 311–377. doi:10.1007/BFb0050820.
  11. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, pp. 142–145, ISBN 978-0-471-72091-1
  12. ^ Feynman, Richard P.; Robert Leighton; Matthew Sands (1965). "The Hamiltonian matrix". The Feynman Lectures on Physics. Vol. III. Massachusetts, USA: Addison-Wesley. ISBN 0-201-02118-8.
  13. ^ Cleeton, C.E.; Williams, N.H. (1934). "Electromagnetic waves of 1.1 cm wave-length and the absorption spectrum of ammonia". Physical Review. 45 (4): 234–237. Bibcode:1934PhRv...45..234C. doi:10.1103/PhysRev.45.234.
  14. ^ Control of Pyramidal Inversion Rates by Redox Switching Mark W. Davies, Michael Shipman, James H. R. Tucker, and Tiffany R. Walsh J. Am. Chem. Soc.; 2006; 128(44) pp. 14260–14261; (Communication) doi:10.1021/ja065325f
  15. ^ a b MRostami, MKazem (2019). "Optically active and photoswitchable Tröger's base analogs". New Journal of Chemistry. 43 (20): 7751–7755. doi:10.1039/C9NJ01372E. S2CID 164362391 – via The Royal Society of Chemistry.
  16. ^ MRostami; et al. (2017). "Design and synthesis of Ʌ-shaped photoswitchable compounds employing Tröger's base scaffold". Synthesis. 49 (6): 1214–1222. doi:10.1055/s-0036-1588913.
  17. ^ MKazem; et al. (2017). "Facile preparation of Λ-shaped building blocks: Hünlich base derivatization". Synlett. 28 (13): 1641–1645. doi:10.1055/s-0036-1588180. S2CID 99294625.
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