Rotational spectroscopy measures the absorption or emission of light by molecules in order to understand changes in their rotational energy. Microwave frequencies are most often used by experiments, but some rotational transitions are observed in the far infrared. The earliest experiments in microwave spectroscopy involved studies of a vibrational mode of ammonia. Rotational spectroscopy is sometimes referred to as pure rotational spectroscopy and is distinct from ro-vibrational and ro-vibronic spectroscopies where changes in vibrational and/or electronic energies accompany those in rotations.
Rotational spectroscopy is practical only in the gas phase where it is possible to distinguish transitions between individual quantum states known as rotational energy levels. Molecular rotational motions are rapidly quenched and converted into other forms of energy in solids and liquids. Rotational spectra can be observed for molecules that have a permanent electric dipole moment. The electric field of the radiation exerts a torque on the molecule through its dipole moment causing the molecule to rotate more quickly (in excitation) or slowly (in relaxation). Homonuclear diatomic molecules such as dioxygen (O2), dihydrogen (H2), etc. do not have a dipole moment and, hence, no pure rotational spectrum. On rare occasions, the effects of centrifugal distortion allow spectra to be observed of molecules that do not have a permanent electric dipole moment. Likewise, electronic excitations can occasionally lead to asymmetric charge distributions and a net dipole moment.
Among the diatomic molecules, carbon monoxide (CO) has one of the simplest rotational spectra. As for tri-atomic molecules, hydrogen cyanide (HC≡N) has a simple rotational spectrum for a linear molecule and hydrogen isocyanide (HN=C:) for a non-linear molecule. The difficulty involved in the interpretation of rotational spectra increases with the size and conformational flexibility of a molecule.
Rotational transitions can also be observed by Raman spectroscopy. This method is complementary to microwave spectroscopy since the selection rules for many molecules are different for the two methods. Rotational Raman spectra therefore allow the observation of different transitions and even different molecules, including some molecules without a permanent dipole moment.
Read more about Rotational Spectroscopy: Applications, Historical Achievements, Overview, Quadrupole Splitting, Stark Effect, Experimental Determination of The Spectrum, Rotational Raman Spectroscopy