Nice textbook illustration of microwave spectrum for a nearly free internal rotation?

Is it possible to find a nice textbookish example of experimental data for an internal rotation of a molecule, in which the rotation is nearly free? For example, 1-butene is supposed to have a nearly free rotation about its middle bond.


It would be fun, for example, to be able to show a microwave spectrum for this molecule that demonstrates the evenly spaced photon energies of a rotor.


