Nitrogen inversion
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File:Nitrogen-inversion-3D-balls.png
Nitrogen inversion in ammonia
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| Inversion of an amine. The pair of dots represents the lone electron pair on the nitrogen atom. | ||
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]
Contents
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[clarification needed], 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[clarification needed]. 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:
- the nitrogen atom must have one lone pair, and
- both isomers must not be under significant strain.
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
- ↑ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
- ↑ 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
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- ↑ 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
