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Pyramidal inversion

October 27, 2023

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 edit

 
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 edit

 
Nitrogen inversion in ammonia
   ⇌   
Inversion of an amine. The C3 axis of the amine is presented as horizontal, and the pair of dots represent the lone pair of the nitrogen atom collinear with that axis. A mirror plane can be imagined to relate the two amine molecules on either side of the arrows. If the three R groups attached to the nitrogen are all unique, then the amine is chiral; whether it can be isolated depends on the free energy required for the molecule's inversion.

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 edit

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 edit

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

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

Exceptions edit

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

 
rigid Tröger's base scaffold prevents nitrogen inversion [17]

References edit

  1. ^ Arvi Rauk; Leland C. Allen; Kurt Mislow (1970). "Pyramidal Inversion". Angew. Chem. Int. Ed. 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". Angew. Chem. Int. Ed. 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. 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. ^ 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.
  16. ^ 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.
  17. ^ 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.

October 27, 2023
pyramidal, inversion, chemistry, pyramidal, inversion, also, umbrella, inversion, fluxional, process, compounds, with, pyramidal, molecule, such, ammonia, turns, inside, rapid, oscillation, atom, substituents, molecule, passing, through, planar, transition, st. 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 Contents 1 Energy barrier 2 Nitrogen inversion 2 1 Quantum effects 2 2 Examples 2 3 Exceptions 3 ReferencesEnergy barrier edit nbsp 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 edit nbsp Nitrogen inversion in ammonia nbsp nbsp Inversion of an amine The C3 axis of the amine is presented as horizontal and the pair of dots represent the lone pair of the nitrogen atom collinear with that axis A mirror plane can be imagined to relate the two amine molecules on either side of the arrows If the three R groups attached to the nitrogen are all unique then the amine is chiral whether it can be isolated depends on the free energy required for the molecule s inversion 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 edit 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 edit 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 nbsp Nitrogen inversion Davies 2006The system interconverts by oxidation by oxygen and reduction by sodium dithionite Exceptions edit Conformational strain and structural rigidity can effectively prevent the inversion of amine groups Troger s base analogs 15 including the Hunlich s base 16 are examples of compounds whose nitrogen atoms are chirally stable stereocenters and therefore have significant optical activity 17 nbsp rigid Troger s base scaffold prevents nitrogen inversion 17 References edit Arvi Rauk Leland C Allen Kurt Mislow 1970 Pyramidal Inversion Angew Chem Int Ed 9 6 400 414 doi 10 1002 anie 197004001 a b IUPAC Compendium of Chemical Terminology 2nd ed the Gold Book 1997 Online corrected 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 Arvi Rauk Leland C Allen Kurt Mislow 1970 Pyramidal Inversion Angew Chem Int Ed 9 6 400 414 doi 10 1002 anie 197004001 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 doi 10 1021 ed084p1067 eISSN 1938 1328 ISSN 0021 9584 Kolmel 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 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 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 lt 1 AID QUA1 gt 3 0 CO 2 D ISSN 1097 461X Greenwood Norman N Earnshaw Alan 1997 Chemistry of the Elements 2nd ed Butterworth Heinemann p 423 ISBN 978 0 08 037941 8 J M Lehn 1970 Nitrogen Inversion Experiment and Theory Fortschr Chem Forsch 15 311 377 doi 10 1007 BFb0050820 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 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 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 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 MRostami et al 2017 Design and synthesis of Ʌ shaped photoswitchable compounds employing Troger s base scaffold Synthesis 49 6 1214 1222 doi 10 1055 s 0036 1588913 MKazem et al 2017 Facile preparation of L shaped building blocks Hunlich base derivatization Synlett 28 13 1641 1645 doi 10 1055 s 0036 1588180 S2CID 99294625 a b MRostami MKazem 2019 Optically active and photoswitchable Troger s base analogs New Journal of Chemistry 43 20 7751 7755 doi 10 1039 C9NJ01372E S2CID 164362391 via The Royal Society of Chemistry Retrieved from https en wikipedia org w index php title Pyramidal inversion amp oldid 1182020608, wikipedia, wiki, book, books, library,

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