As already explained he gross selection rules for the observation of a pure rotational spectrum is tat the molecule must have a permanent dipole moment so that it can interact with the electromagnetic radiation and absorb or emit a photon. Thus pure rotational spectrum is genuinely by polar molecules. Consequently, homonuclear diatomic molecules (like H₂ , N₂ etc.) and symmetrical linear molecules lie CO₂ (or symmetrical molecules like C₆ H₆) do not give rotational specters. Thus polar molecules like H₂O, NO, N₂O etc. give pure rotational spectral.

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