[shelanachium]

Coloured gases came up recently, and several blue ones were mentioned. They are all nitroso-compounds such as NCNO and CF3NO. In fact all NO compounds in which the nitroso-group pi-electrons do not interact with the rest of the molecule are blue. These include (CH3)3CNO and O2NNO which causes the blue colour of 'nitrous acid' solutions.

But why? Simple compounds are not normally coloured unless they contain unpaired electrons e.g. NO2, ClO2. Nitroso-compounds don't.

Has it anything to do with the paramagnetism of O2, with a triplet ground state? Nitroso-compounds are almost isoelectronic with this, so do they have a low-lying triplet state, production of which absorbs yellow light and causes the blue colour? And if so, is this triplet state reactive, causing nitroso-compounds to have many light-driven reactions?

[Demotivator]

I think liquid O2 is also blueish.

[shelanachium]

I have read that the blue colour of liquid O2 involves a photon being absorbed simultaneously by two O2 molecules, creating a transition that infra-red would be able to produce in a single molecule were it not forbidden. Because of the necessary involvement of two molecules the absorption occurs only when molecules are in close proximity, i.e. in the solid, liquid or highly-compressed gas. It is not therefore related to the blue colour of nitroso compounds or the yellow of the electronically similar azo-compounds RN=NR both of which involve isolated molecules.