have a restriction.
Rotational transitions are only observed in the spectrum if the molecule has a permanent dipole moment. The reason for this is that the rotational motion must involve an oscillating dipole, which can interact with an electromagnetic field, there will be no absorption or emission if otherwise.
This means , as far as linear molecules are concerned, that molecules with a center of symmetry such as N2 and C2H2, or even pentane/hexane do not have a pure rotationnal spectrum whereas those without a center of symmetry, for example HF, do have a rotational spectrum.
So you understand that putting for example hexanes in the microwaves, even if you heat for 1 month, wont do a thing.
But think about it, if you change the frequency, so will the wavelenght and maybe you wont be in the microwave region anymore...
