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There are some important features of rotational spectra which are as follows :
  • Pure rotational molecular spectra arises from transition between rotational energy states & are observed in far infrared (10^6 A° to 10^7 A°) or microwave region (10^7 A° to 10^8 A°)
  • Only those molecule which have permanent electric dipole moment can give rise to a pure rotational spectra. Thus homonuclear  diatomic molecules such as H2 , O2 ,N2 do not exhibits pure rotational spectra, while the hetronuclear diatomic molecules such as HCl, HBr, HI etc. exhibits pure rotational spectra. 

  • In practice ,rotational spectra are observed in absorption. For hetronuclear diatomic molecules the rotational spectra consists of simple series of absorption maxima, which are nearly equidistant. A measurement of frequencies help us to determine the moment of inertia and intermolecular distance of molecule.