# Why is nitrogen dioxide (NO2) brown? [closed]

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?