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

Where;

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

  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.
 NON RIGID ROTATOR

  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.

NOTES:

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

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