When two atoms of a diatomic molecule are having different electronegativity they generate a dipole moment. Molecules with dipole moments absorb the radiation from Infra-red region and set the vibrational motion due to fluctuation in the bond. This increases the vibrational energy. Such molecules are IR active. Simple diatomic linear polar molecules have only one mode of vibration i.e. the stretching and contraction of bond. Therefore, hetero nuclear polar molecules like HCI, CO and NO are IR active.
While homo-nuclear non-polar molecules with symmetrical structure like H2, O2, Cl2 does not posses dipole moment and are IR inactive.
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