The spectroscopic study of the rotational motion of molecules. Rotational spectroscopy gives information about interatomic distances. The transitions between different rotational energy levels in molecules correspond to the microwave and far-infrared regions of the electromagnetic spectrum. It is only possible for there to be transitions between rotational energy levels in pure rotational spectra if the molecule has a permanent dipole moment. In the near-infrared region rotational transitions are superimposed on vibrational transitions, resulting in a vibrational–rotational spectrum. This type of spectrum is considerably more complicated than a purely rotational spectrum. See vibrational spectroscopy.