A-Level How atoms bond and how that governs a substance's behaviour: worked solution

Question

Predict how many IR absorptions nitrogen dioxide should show and state which vibrational modes are responsible for them.

NO2 is a bent (non-linear) triatomic molecule. A vibrational mode is IR active if it causes a change in dipole moment. For a bent AB2 molecule there are 3N-6 = 3(3)-6 = 3 vibrational modes: the symmetric stretch, the asymmetric stretch, and the bending mode. Because the molecule is bent (already polar) and none of these modes cancels the dipole change, all three modes change the dipole moment and are IR active. Hence NO2 shows 3 IR absorptions.