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Molecules with Microwave/Rotational spectrum

  1. Oct 21, 2009 #1
    1. The problem statement, all variables and given/known data
    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.

    3. 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?
  2. jcsd
  3. Oct 21, 2009 #2
    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?
  4. Oct 21, 2009 #3


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    Staff: Mentor

    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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