For this case (
) the rotational energy is given by:

We corresponding quantum mechanical expression for the rotational energy is given by:

Usually in spectroscopy eq. (34) is presented in the form

The energy of a rotational state is normally expressed in **rotational terms**
measured in wavenumbers, :

Then the separation between the neighbor energy levels is

As seen from eq. (36) the rotational constant decreases as the moment of inertia increases. Therefore, large molecules have closely spaced rotating energy levels.

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