Some interesting aspects of N-oxides

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.

Keywords:

• N-oxide bond lengths
• N-oxide dissociation energies
• N-oxide crystalline and gas phase structures
• local ionisation energies
• charge delocalisation

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.

Additional information

Funding

We greatly appreciate the support of this work by the Office of Naval Research [contract number N00014-12-1-0535], Program Officer Dr Clifford Bedford.