I don't understand why nitrogen dioxide $\ce{NO2}$ is coloured brown when nitrate $\ce{NO2-}$ and dinitrogen tetroxide $\ce{N2O4}$ are not. I understand it is due to absorption of light when its frequency matches the energy of an electronic transition. But what makes $\ce{NO2}$ special, i.e. what electronic transition gives rise to the brown color in $\ce{NO2}$, and why do $\ce{NO2-}$ and $\ce{N2O4}$ lack these transitions? Is there are way to explain this difference using molecular orbital theory?
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