Nitrogen dioxide (NO2) is a reddish brown molecule, while its dimer nitrogen tetroxide (N2O4) is colorless. Can you give a reasonably detailed explanation for this from electronic structure theory?

Previous queries were not answered because the question posed was deemed to be too broad.

  • $\begingroup$ I'm afraid that like the previous question, this will also be deemed as too broad very soon. EST is a very broad and complicated theory and may be hard to explain that in a single answer. $\endgroup$ – Nilay Ghosh Nov 20 '17 at 4:27
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    $\begingroup$ No, I think what made the other question too broad is wanting to include the other coloured gases, which would require an explanation for every single gas. $\endgroup$ – Jan Nov 20 '17 at 11:24
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    $\begingroup$ You are asking why NO2 has low lying energy levels compared to its dimer. (It just happens that these low levels lie in the visible part of the spectrum). To put the question the other way round, why would you expect the energy levels to be similar when the molecules are different? Perhaps more interesting is to ask why N2O4 is planar yet has a very long 0.175 nm N-N bond whereas the NO bond lengths and ONO angles are similar to NO2. $\endgroup$ – porphyrin Nov 21 '17 at 11:07
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    $\begingroup$ The color of NO2 is due to presence of single unpaired electeon, while in its dimer N2O4 that lone electron got paired which cause loss of its color. $\endgroup$ – Deadwing Mar 9 at 19:18

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