Nitrogen inversion
In chemistry, nitrogen inversion is a fluxional process in compounds with a nitrogen atom that has a pyramidal geometry, such as ammonia (NH3), whereby the molecule "turns inside out". It is a rapid oscillation of the nitrogen atom from one side of the plane formed by the substituents to the other side, passing through a planar transition state. For a compound that would otherwise be chiral due to a nitrogen stereocenter, nitrogen inversion allows its enantiomers to rapidly interconvert, making chiral resolution impossible unless the inversion process is prevented by steric or electronic effects. The concept of nitrogen inversion can be extended to other compounds that contain atoms of other elements with trigonal pyramidal geometry, such as carbanions, phosphines, arsines, stibines and sulfoxides, in which case it is called pyramidal inversion.
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).
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