Nitrogen inversion
In chemistry, nitrogen inversion is a fluxional process in nitrogen and amines, whereby the molecule "turns inside out". It is a rapid oscillation of the nitrogen atom and substituents, the nitrogen "moving" through the plane formed by the substituents ; the molecule passing through a planar transition state. 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.
Nitrogen inversion is one case of the more general phenomenon of pyramidal inversion, which applies to carbanions, phosphines, arsines, stibines, and sulfoxides.Energy barrier
The ammonia interconversion is rapid at room temperature, inverting 30 billion times per second. Two factors contribute to the rapidity of the inversion: a low energy barrier and a narrow width of the barrier itself, which allows for frequent quantum tunnelling. In contrast, phosphine inverts very slowly at room temperature.Quantum effects
Ammonia exhibits a quantum tunnelling due to a narrow tunneling barrier, and not due to thermal excitation. Superposition of two states leads to energy level splitting, which is used in ammonia masers.Examples
The inversion of ammonia was first detected by microwave spectroscopy in 1934.
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.
The system interconverts by oxidation by oxygen and reduction by sodium dithionite.Exceptions
Conformational strain and structural rigidity can effectively prevent the inversion of amine groups. Tröger's base analogs are examples of compounds whose nitrogen atoms are chirally stable stereocenters and therefore have significant optical activity.
