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I don't understand why nitrogen dioxide $\ce{NO2}$ is coloured brown when all of the other nitrogen oxide species and even nitrate are otherwise colorless. I understand it is due to absorption of light when it frequency matches a vibration frequency but what makes $\ce{NO2}$ special?

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closed as too broad by NotEvans., Jannis Andreska, Mithoron, Jon Custer, Tyberius Sep 16 '17 at 23:13

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