Chemistry Molecules with Microwave/Rotational spectrum

AI Thread Summary
Microwave spectroscopy is concerned with the rotational spectra of molecules, which depend on the presence of a dipole moment. Diatomic molecules like H2 and N2 lack a permanent dipole moment, thus they do not exhibit a microwave spectrum despite having rotational motion. The discussion highlights confusion about the definition of rotational spectra and the criteria for interaction with microwave radiation. It clarifies that while all molecules can rotate, only those with a dipole moment will interact with microwave photons. Therefore, molecules such as H2, N2, and O2 do not have a microwave spectrum due to their symmetrical charge distribution.
Tomtom
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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?
 
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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?
 
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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