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