rotational spectrum

A rotational spectrum is a part of a molecular spectrum where bands originate from quantized changes in the energy of molecular rotation. In the gas phase, molecules can rotate freely. Quantum mechanics dictates that this rotational energy is quantized, meaning it can only exist at specific levels, described by the rotational quantum number J.12

Molecular rotational spectra occur when a molecule transitions between these rotational energy levels. For a transition to happen, the molecule must possess a permanent dipole moment, and the change in the quantum number must be \u0394J = \u00b11. The frequency of the absorbed or emitted radiation is directly related to the molecule's rotational constant, which in turn depends on its moment of inertia and internuclear distances. This allows scientists to determine molecular structures and properties.23