Rotational spectroscopy

Rotational spectroscopy determines molecular rotational energy levels from radiation absorbed or emitted during transitions between them. Its spectra reveal molecular moments of inertia and related structural information.

Rigid rotor: A model of a rotating molecule whose internal geometry remains fixed. Its quantized energy levels provide the basic predictions for rotational spectral lines.
Angular momentum: A physical quantity describing rotational motion and its associated conserved properties. Molecular rotational states are quantized according to angular momentum.
Microwave spectroscopy: Spectroscopy that studies matter through its interaction with microwave-frequency radiation. Many pure rotational transitions of small molecules fall in the microwave region.
Vibrational spectroscopy: The study of molecular vibrational energy levels through radiation-induced transitions. It probes bond vibrations rather than primarily the molecule’s rotational levels.
Rotational selection rule: A rule specifying which changes in rotational quantum number can produce allowed transitions. It determines which energy-level gaps appear as spectral lines.
Quantum number: A discrete value that specifies a permitted state or property in a quantum system. Rotational quantum numbers label the states joined by spectral transitions.
Molecular structure determination: The inference of molecular geometry and dimensions from experimental measurements. Rotational constants can constrain bond lengths and angles.
Electronic spectroscopy: The study of transitions between molecular electronic energy states. Its transitions involve electronic states, typically at higher energies than pure rotation.
Moment of inertia: A measure of how mass is distributed around an axis of rotation. Rotational transition frequencies depend directly on molecular moments of inertia.
Electric dipole moment: A measure of the separation of positive and negative electric charge in a system. A permanent dipole enables electric-dipole rotational transitions in molecules.

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