Page "Rotational spectroscopy" Paragraph 43

Instead of defining these two rotational quantum numbers for two independent axes, we associate one quantum number () with the total angular momentum of the molecule and the other quantum number () with the angular momentum of the axis that has different moment of inertia ( i. e., axis for oblate symmetric top and axis for prolate symmetric tops ).

The rotational energy of such a molecule, based on rigid rotor assumptions can be expressed in terms of the two previously defined rotational quantum numbers as follows