Nitrogen inversion

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.

In chemistry, nitrogen inversion is the interconversion of the chirality of a nitrogen compound with a trigonal pyramidal geometry, such as ammonia, whereby the molecule "turns inside out".[1]

Energy barrier considerations

The ammonia interconversion is rapid at room temperature. Two factors contribute to the rapidity of the inversion: a low energy barrier (24.2 kJ/mol) and a narrow width of the barrier itself, which allows for frequent quantum tunnelling (see below). In contrast, phosphine (PH3) inverts very slowly at room temperature (energy barrier: 132 kJ/mol).[2]

Consequences for optical isomerism

Amines of the type RR′R"N and RR′NH are chiral, but they typically cannot be obtained as individual enantiomers because of the rapidity of the nitrogen inversion. The situation is very different for ammonium salts, RR′R″HN+ and RR′R″R‴N+, and amine oxides, RR′HNO and RR′R″NO, which are optically stable. The corresponding chiral phosphines (RR′R″P and RR′PH), sulfonium salts (RR′R″S+), and sulfoxides (RR′SO) are also optically stable.

Quantum effects

Ammonia exhibits a quantum tunnelling due to a narrow tunneling barrier,[3] and not due to thermal excitation. Superposition of two states leads to energy level splitting, which is used in ammonia masers.

Conditions

For nitrogen inversion to occur:

Examples

The inversion of ammonia was first detected by microwave spectroscopy in 1934.[4]

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 [5]

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

References

  1. ↑ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  2. ↑ Kölmel, C.; Ochsenfeld, C.; Ahlrichs, R. An ab initio investigation of structure and inversion barrier of triisopropylamine and related amines and phosphines. Theor. Chim. Acta. 1991, 82, 271–284. doi:10.1007/BF01113258
  3. ↑ Feynman, Richard P.; Robert Leighton; Matthew Sands (1965). "The Hamiltonian matrix". The Feynman Lectures on Physics. Volume III. Massachusetts, USA: Addison-Wesley. ISBN 0-201-02118-8.
  4. ↑ Cleeton, C.E.; Williams, N.H. (1934). "Electromagnetic waves of 1.1 cm wave-length and the absorption spectrum of ammonia". Physical Reviews 45 (4): 234–237. Bibcode:1934PhRv...45..234C. doi:10.1103/PhysRev.45.234.
  5. ↑ 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
This article is issued from Wikipedia - version of the Monday, February 08, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.