Hydroazoic acid
Hydroazoic acid (HN3) is an explosive and toxic compound that is composed of 97.7% nitrogen by weight. It is frequently used in stock solutions and as a reagent. The molecular structure of the molecule has been known for a long time; however, it wasn’t until recently that a crystal structure has been solved by using x-ray diffraction. The unique composition of hydroazoic acid makes it possible for the compound to form 3 resonance structures. In this write-up, the nitrogen’s in HN3 will be named a, b and g; with a being the nitrogen connected to the hydrogen, b the middle nitrogen and g the third and final nitrogen.

In the first resonance structure, the a nitrogen is sp2 hybridized, with one bond to hydrogen and a double bond to the b nitrogen. The b nitrogen has 2 double bonds (one with the a and one with the g nitrogen) and thus has a +1 positive charge. The g nitrogen is double bonded to the b nitrogen and has two lone pairs, making its formal charge -1. In the second resonance structure, the a nitrogen is sp3 hybridized, with 2 single bonds, 2 lone pairs of electrons and a formal charge of -1. The b nitrogen has a triple bond with the g nitrogen, which results in it having a +1 formal charge. In the third resonance form, the a nitrogen has 2 single bonds, a lone pair and a radical. The b nitrogen has a single bond, a lone pair and a double bond to the g nitrogen. There is a second radical present on the g nitrogen and there appears to have some sort of intermolecular force between the two radicals on the a and g nitrogens.

The second resonance structure appears to be predominant. The average bond angle determined by x-ray crystal investigation was found to be 172.8 degrees for the Na-Nb -Ng bond, while the average bond angle for the H-Na-Nb bond was found to be 109 degrees. It’s possible that the different resonance forms of hydroazoic acid may be due to the hydrogen bonds that exist between the polar molecules

References

Evers, J., Gobel, M., Krumm, B., Martin, F., Medvedyev, S., Oehlinger, G., Steemann,

F.X., Troyan, I., Klapotke, T.M. and Eremets, M.I. 2011. Molecular Structure of Hydroazoic Acid with Hydrogen-Bonded Tetramers in Nearly Planar Layers. Journal of the American Chemical Society. 133: 12100-12105

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Of the three resonance structures shown above, the second one is most common and most important. The average H-N-N bond angle determined by x-ray crystallography was found to be 109°.
http://pubs.acs.org/doi/abs/10.1021/ja2027053
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