IR Spectra: N-O Nitro group: Why two peaks?

nbd

My IR spectra correlation chart for organic chemistry says that the stretch for a nitro N-O bond occurs at "1550 and 1400cm^-1" and that it will look like "teeth". Why does N-O have two peaks? The rest of the functional groups on my chart list a range in which a single peak should appear, but why is N-O expected to generate two peaks?

Thanks!

DrDu

I suppose symmetric and antisymmetric stretching modes. Note the analogous feature for CH3 groups at around 1430 cm^{-1}.

nbd

That makes sense, thank you! In the same vein, I notice that my correlation chart also has the carbon dioxide stretch listed as a "doublet" appearing around 2350 cm^-1. Is that for a similar reason as N-O/CH³? In my mind it shouldn't be, because CO₂ is not bent and does not have resonance, so the anti-symmetrical stretching would not be IR-active. If that's true, what could be causing the two bands for the carbon dioxide stretch?

Edit: Since this is kind of a new question, I posted it in its own thread here.