Molecules with Microwave/Rotational spectrum

[Tomtom]

Homework Statement

This question is about microwave (rotational) spectroscopy.
Which of the following molecules have a microwave spectrum?
H2, N2, NO, N2O, CH4, CO2, OCS, H2O, Ethene, Benzene.

The Attempt at a Solution

According to my textbook, it appears that all molecules have a rotational spectrum. However, Wikipedia, "Diatomic molecules such as dioxygen (O2), dihydrogen (H2), etc. do not have a dipole moment and hence no purely rotational spectrum."

I'm not really sure what that's supposed to mean. Do all molecules have a rotational spectrum (I don't see why not, everything with a bond should be able to rotate)?

Is the rotation considered relative to one of the atoms in the molecule, or to the centre of mass?

 

[Tomtom]

Have I understood it correctly if I suggest that H2, N2 and O2 all rotate, but (when not considering temporary dipoles) they should not interact with photons in the microwave (nor infrared for that matter) spectrum?

 

[Borek]

That would be my understanding. Both ends of the molecule interact identically, so there is no net force moment that can change rotation.

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