# Why is the bond angle of N-N-N in hydrogen azide not 180 degrees?

Chen and Wang (2009) have reported an $\ce {N-N-N}$ bond angle in the hydrazoic acid molecule of $171.5$ degrees using B3LYP/6-311G** model. The wikipedia page for the molecule also reports a similar bond angle of $171$ degrees. However, this seems to go against predictions by VSEPR theory for the case of two regions of electron density (which gives a bond angle prediction of $180$ degrees). It is known that VSEPR theory is limited in its theoretical predictions. However, I would like to understand what accounts for the discrepancy in the two.

Reference

Chen, F., & Wang, F. (2009). Electronic Structure of the Azide Group in 3′-Azido-3′- deoxythymidine (AZT) Compared to Small Azide Compounds. Molecules, 14, 2656-2668. doi:10.3390/molecules14072656

• Also relevant paper on the recent SCXRD investigation (DOI: 10.1021/ja2027053) suggests somewhat higher average $\ce{N-N-N}$ angle 172.8°. Besides, all 3 N atoms and both bonds in $\ce{N3}$ are quite inequivalent, so there is no wonder the angle is not 180°. – andselisk Dec 16 '17 at 5:24
• VSEPR doesn't predict anything. – Mithoron Dec 16 '17 at 16:54
• – Mithoron Dec 20 '17 at 22:22
• acs.figshare.com/articles/dataset/… Molecular Structure of Hydrazoic Acid with Hydrogen-Bonded Tetramers in Nearly Planar Layers – Hal Nov 19 '20 at 20:17