1. Molecules with permanent dipole moments can only be studied by microwave or rotational spectroscopy e.g. HCI, HCN, CH3Cl.
2. Homonuclear molecules which do not possess any dipole moment will not give rotation spectra and cannot be studied e.g. H2, N2, O2, etc.
3. Similarly, linear polyatomic molecules like CO2, C2H2 and non-linear polyatomic molecules like C6H6, and CH4, do not posses dipole moment thus don't show rotational spectra.
4. In case of polyatomic molecules single moment of inertia will not give complete molecular structure.
5. Rotational spectra are studied only in gaseous phase, as the molecules have free rotational motion. In solids and liquids the strong intra-molecular forces of attraction restrict the molecular free rotations. Hence, the substances in solid and liquid phase cannot be used.
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