The University of Liverpool maintains a ChemTube 3D page, where it says


*> Highly symmetric polyatomic molecules, such as carbon dioxide, also

have no net dipole moment - the dipoles along the C-O bonds are always equal and opposite and cancel each other out. It is important to recognise also that if a molecule has a permanent dipole, but this dipole lies along the main rotation axis, then the molecule will not have a rotational spectrum - such as for a water molecule.

1) A chemistry student brought this question to me, and I am also surprised by their claim. Why can't water have a rotational spectrum? It defies the very purpose of microwave oven and the gas phase rotational spectrum of water is all over the spectroscopic literature. I would like to confirm the with the opinion of spectroscopists that this is incorrect or not.

2) Another question from the student electromagnetic waves have an sinusoidal electric and a magnetic field of microwaves. How does the magnetic field interact with the permanent electric dipole moment of a molecule say HCl? I have checked many physical chemistry texts and they are quiet about the interaction from varying magnetic component and the dipole moment of the molecule. Electric field interaction is discussed in detail.

Thank you.

  • $\begingroup$ It is interesting as to what you have written in that the animation on the web page shows possible rotations of the carbon dioxide molecule. $\endgroup$ – Farcher Mar 9 '19 at 6:33
  • $\begingroup$ Yes, the page is itself contradictory. CO2 can have pure rotations but since it does have a permanent dipole, its rotations can be probed by Raman spectroscopy. $\endgroup$ – M. Farooq Mar 9 '19 at 6:38
  • $\begingroup$ Related Why are magnetic fields so much weaker than electric? $\endgroup$ – Farcher Mar 9 '19 at 7:40
  • $\begingroup$ Thanks, it says the strength is different but the energies are same. Wouldn't a electrical dipole interact with magnetic field? $\endgroup$ – M. Farooq Mar 9 '19 at 14:00

For question 1. I think it basically said that the emission was due to the changing of electric dipole moment caused by rotation. In water molecular, the axis of rotation lies along with the axis of rotation, and thus the rotation does not change the geometry of electric dipole in space, and therefore not emission.

For question 2. from EMM we knew that $B$ filed was very weak and can be ignored during the calculation for those cases. (If you accept that C-O bound does not change length, then you should accept $B$ field just doesn't do anything as well.)

  • $\begingroup$ JC, rotational spectrum is studied as an absorption spectrum in the microwave region. However, if we check Google Scholar, there are so many papers on rotational spectrum of water. $\endgroup$ – M. Farooq Mar 9 '19 at 6:22
  • $\begingroup$ @M.Farooq If it's elector magnetic interaction, of course it doesn't cause rotation,(unless you count the angular momentum from photon), the answer is the same in the above statement. The different states were usually considered "unlocked" at different stages, when it can be treated as "locked"/"unlocked" based on different conditions. But what that web page said is one of those "locked" treatment. $\endgroup$ – J C Mar 9 '19 at 6:28

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