Usually compounds' colours are explained by the presence of unpaired electrons which is free to excite and de-excite giving the compound a charectistic colour. NO has atleast one unpaired electron but even then it is colourless. Can anyone tell me what the reason is?
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I am not a physics major, this post will be rather free of equations and spherical chickens. This is a chemist's view of the situation ;)
You need more than an unpaired electron to create color in a substance. You need two places for it to be, with a given energy difference in the visible color range. Looking at the molecular orbital diagram of NO we see that no such organisation is possible (within reasons). The only choice is to go from one $\pi^*$ to the other, but those are at the same energy level. There is no way for this diatomic molecule to deform in any meaningful way so as to create a difference in energy levels between them. Metal complexes, especially iron- and cobalt-based ones, can do this, but not with the $\pi^*$ in any case. There is only $\sigma^*_2$ left, but the distance from $\pi^*$ to $\sigma^*_2$ is in huge, larger than visible spectrum (an event that knocks an electron up there is likely to unravel the entire molecule).
All in all, there is no possibility for the electron to move anywhere that requires the emission of a photon in visible range. It cannot produce the color you seek.
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$\begingroup$ What do you mean by HOMO, and when you said 'it needs to change it's spin direction to "there" '. What exactly did you mean? I understand that you mean other places it can go to but what are the other places and why does it need to change it's spin to go there? $\endgroup$– BeastOct 9, 2017 at 12:44
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1$\begingroup$ @Beast You would probably benefit from reading on the molecular orbital theory, you need that to understand fully. From MO theory: HOMO is the highest occupied molecular orbital. The uppermost arrow in the diagram. For 2 electrons to co-habitate in a single orbital, they need to have opposing spin. (Pauli exclusion principle) $\endgroup$– StianOct 9, 2017 at 14:24
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$\begingroup$ I know the basics of MO theory and Pauli's Exclusion Principle, I just wanted to know where else the electron could go where it had to change it's spin, did you mean into N or O's orbitals? $\endgroup$– BeastOct 9, 2017 at 14:48
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$\begingroup$ @Beast Well, at second glance, I see I was a bit fast writing that. There is no open singly occupied orbital in the MO. So nowhere to go even if the spin flips. Sorry for the confusion there. I'll update the answer. $\endgroup$– StianOct 9, 2017 at 19:28