Necessary Condition for Observing IR Spectrum:
The presence of quantised energy levels does not ensure transitions amongst them when the radiation of appropriate wavelength is made to fall on it. For the oscillating electrical field of the EM radiation to interact with the vibrating molecule, it is essential that the molecule has a fluctuating or oscillating dipole moment. In the absence of such a dipole moment the radiation cannot interact with the molecule and we say that the vibration in the molecule is IR inactive. For example, a homonuclear diatomic molecule like H2 would not give an IR spectrum because it does not have a dipole moment and also it does not generate a fluctuating dipole moment. On the other hand, HBr gas has a dipole moment and it does absorb in the IR region.
Present, if a molecule has a fluctuating dipole moment, those transitions would we observe? For this we need to know about the selection rule.