Are rotational energy levels of a molecule in general equally spaced or does the spacing increase with energy? How about a diatomic molecule; I have seen a derivation showing that the rotational levels in a diatomic molecule are equally spaced, but when drawn in an energy level diagram they clearly aren't? What is right? And another thing; in my book the expression for the rotational and vibrational energies of a diatomic molecule is derived in terms of a classical "dumb-bell"-picture? How is this justified taking quantum mechanics into account?