Difference Between Rigid And Non Rigid Rotator In Spectroscopy

Rigid VS Non Rigid Rotator In Spectroscopy

A non-rigid rotor is a molecule that can undergo internal rotation around one or more of its bonds, whereas a rigid rotor is a molecule whose shape is fixed in spectroscopy.

The following equation can be used to characterise a molecule’s rotational energy levels:

E(J) = J(J+1)h^2/2I


  • I is the molecule’s moment of inertia
  • h is Planck’s constant
  • and J is the rotational quantum number.

  1. ·         The moment of inertia and the spacing between the rotating energy levels are both constant in a stiff rotor.
  2. ·         The microwave spectra of these compounds show this discrepancy. Rigid  rotors are made up of lines that are evenly spaced.

  1. ·         In contrast, as the molecule undergoes internal rotation in a non-rigid rotor, the moment of inertia changes and the rotational energy levels are no longer evenly spaced.
  2. ·         While the spectra of non-rigid rotors are made up of lines that are clustered and whose spacing between the clusters is smaller than that of the lines within each cluster.


Because of their different moments of inertia, rigid and non-rigid rotors in spectroscopy differ primarily in the spacing of their rotating energy levels.