Some interesting aspects of N-oxides

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.