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The lewis structure of trinitramide has four major resonance structures, whereby the lone pairs of the central nitrogen can resonate with the nitro groups.

This seems to indicate that each of the nitrogen atoms would be sp2 hybridized, and the entire molecule would be planar. In fact, the Wikipedia ball-and-stick image for trinitramide seems to indicate it so.

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However, based on quantum mechanical calculations, trinitramide is in fact non-planar. Why is this so? What drives its non-planarity?

Mithoron
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Cyclopropane
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    Steric hindrance, maybe? – Oscar Lanzi Jun 18 '19 at 23:01
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    I would assume it’s a combination of steric and charge repulsion between the oxygen atoms. The experimentally determined structure in https://doi.org/10.1002/anie.201007047 shows the nitro groups essentially at right angles to each other. – Andrew Jun 18 '19 at 23:12
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    I have extensively written about the isoelectronic trinitromethanide anion: https://chemistry.stackexchange.com/q/32354/4945 – Martin - マーチン Jun 18 '19 at 23:14
  • Doesn't the paper you have linked tell more than what we can presumably say? Interestingly, they not only say isn't planar, but pyramidal. There should be a comment atop the calculations. Otherwise take it at the level of their work. Perhaps one could calculate a different situati – Alchimista Jun 19 '19 at 12:42
  • Are there 4 electron domains around the central nitrogen (3 bonds to NO2 groups and one lone pair)? That would suggest trigonal pyramidal geometry. – Guest Oct 26 '20 at 21:14
  • The presence of nitro groups attached to the central nitrogen would lead to resonance between each nitro group and the central notrogen, which would have in fact made us expect trigonal planar geometry infact. The reason then, for the actual non-planarity, is best explained by the reasoning put forward by Andrew's comment above. Essentially, in the 3D confirmer as shown here the nitro groups are at right angles at each other to minimise steric as well as charge repulsion between the oxygens – Yusuf Hasan Oct 27 '20 at 00:43
  • A similar rationale regarding steric repulsion follows for the propeller-like arrangement of the three phenyl groups in triphenylphosphine – Yusuf Hasan Oct 27 '20 at 00:45
  • Also a similar repulsion between ortho hydrogens is the usual explanation for non planarity of biphenyl systems – Yusuf Hasan Oct 27 '20 at 00:53

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