As already mentioned he gross selection rule for a diatomic molecule to give a vibration spectrum is that the dipole moment of the molecule must change when the atoms are displaced due to vibration (so that it can set the electromagnetic field into oscillation). The condition required is only the change in dipole moment (possible from zero) and the molecule need not have a permanent dipole moment. For example, in case of photonuclear diatomic molecule like H₂, O₂, N₂ etc. which have only stretching motion/ vibrations and no bending motion/vibrations, the dipole moment does not change during vibration. Hence these molecules do not give vibration spectra i.e. they are said to be infrared-inactive. On the other hand, heteromuclear diatomic molecules like HCI, CO, NO etc. and polyatomic molecules like CO_2, H₂O, CH₄ etc. which show change in dipole moment in some mode of vibration spectra and are said to be infrared-active.

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