Why is dinitrogen tetroxide a planar molecule?

[![enter image description here](https://i.stack.imgur.com/prOhom.png)](https://i.stack.imgur.com/prOhom.png)


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?


[![enter image description here](https://i.stack.imgur.com/AkWme.png)](https://i.stack.imgur.com/AkWme.png)




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](https://acdc.amherst.edu/view/octagon:153/CONTENT).


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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