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
⇌
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
planartransition 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
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.
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]
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]
^Arvi Rauk; Leland C. Allen; Kurt Mislow (1970). "Pyramidal Inversion". Angewandte Chemie International Edition. 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.
eISSN1938-1328.
ISSN0021-9584.
^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.
S2CID98837101.
^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 Tröger's base scaffold". Synthesis. 49 (6): 1214–1222.
doi:
10.1055/s-0036-1588913.
^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.
S2CID99294625.
^
abMRostami, MKazem (2019). "Optically active and photoswitchable Tröger's base analogs". New Journal of Chemistry. 43 (20): 7751–7755.
doi:
10.1039/C9NJ01372E.
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