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I was wondering why $\ce{N2O4}$ is a planar species in spite of having a σ-bond in between the two $\ce{N}$ atoms for free rotation. Does it not form a conformer?

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    $\begingroup$ Because pi conjugation. $\endgroup$ – Ivan Neretin May 15 at 16:29
  • $\begingroup$ Why there would be delocalisation of pi electron cloud over N-N bond as the RS formed would be highly unstable as it would have + charge on oxygen atom with high electron deficiency $\endgroup$ – user93741 May 15 at 16:52
  • $\begingroup$ No, it wouldn't have + on oxygen. $\endgroup$ – Ivan Neretin May 15 at 16:54
  • $\begingroup$ The oxygen with the -charge is in conjugation only with the oxygen present in the same N atom .It can't be in conjugation with the =O present in the other N atom . How there can be partial bond between 2 N atoms $\endgroup$ – user93741 May 15 at 16:56
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    $\begingroup$ I think it is a very good question (why the down vote?) - by comparison the geometry that the isoelectronic oxalate ion adopts in crystals depends upon the counter cation (en.wikipedia.org/wiki/Oxalate) indicating that in that case at least the energy well keeping the ion planar is not that deep - so why is N2O4 planar? pi conjugation would suggest double bond character in the N-N bond, but as is well known that bond is weak and long, so I don't find that a very satisfying explanation. $\endgroup$ – Ian Bush May 15 at 18:45
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I think that one would expect that a stable form of the molecule might be planar and have D2h symmetry.

Some support for the claim:

  1. Early crystal structures suggested that the ONO bonds on either side of the N-N bond are easily modeled by low level DFT (doi.org/10.1039/C19660000082).
  2. More recently, simulations of the trans and cis N2O4 structure show planarity when optimized at B3LYP/ aug-cc-pVTZ (aka, pretty good not amazing) See here.

The simulations also suggest that the barrier to isomerization is low, meaning that while the stable structures are planar, there are low barrier (low temperature) out of plane rotations possible.

Summary:

  1. Just because something has sigma bonds doesn't imply that it will immediately be non-planar
  2. yes the molecule can form a conformer
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