Abstract
N-oxides feature a coordinate covalent N+→O− bond, in which both electrons formally come from the nitrogen. Such compounds are important in organic and biological chemistry, and increasingly in the area of energetic materials. The N+→O− linkage has the interesting property that it can be either a donor or an acceptor of electronic charge through resonance delocalisation. It will be shown that the relative reactivities toward N-oxide formation of nonequivalent nitrogens in a given polyazine molecule can be predicted by computing the respective local ionisation energies. Charge delocalisation plays a reinforcing role. However, it can be used to rationalise the remarkable range of N+→O− bond lengths in polyazine N-oxides, as well as the opposite effects of NO2 and NH2 substituents. It is pointed out that, at least for some N-oxides, there are surprisingly large differences between the N+→O− bond lengths in the crystalline and the gas phases. While the approaches being used to explain and/or rationalise N-oxide properties are able to achieve considerable success, it is important to recognise and keep in mind their limitations – examples of which are given.
Acknowledgements
P. Politzer was introduced to the area of energetic materials by Rod Bartlett during the second half of a hopelessly lost New Orleans Saints football game in 1979. P. Politzer remains grateful for Rod's help and for the one-sidedness of the game.