Effect of a magnetic field on the physical size of a paramagnetic molecule

[amraud]

I know that placing a paramagnetic molecule inside a magnetic field will induce a magnetic force on the molecule from the unpaired electrons, but what will the effect be on the physical properties on the molecule (of any?)
Specifically will the size of the molecule be changed? My understanding is that the outer electrons define the outer border (the edges) of the molecule, so if the magnetic field influences the unpaired electrons, will that also change the size of molecule (or electron cloud) relative to a molecule not in a magnetic field?

Ex: Will a diatomic oxygen molecule (O2) INSIDE a magnetic field be any bigger or smaller than one NOT inside a magnetic field?

 

[kanato]

If the molecule has a spontaneous magnetic moment (as I believe oxygen does) then the answer is most likely no. Placing a molecule with a magnetic moment in a field doesn't change the moment, it just causes it to align with the field. In order to change the size of the molecule with a magnetic field, you'd have to change the size of the magnetic moment. I think this is very unlikely to happen in a molecule.

Now if you have a situation where the molecule is in a singlet ground state without a field, and by turning on a strong enough field you can force it into a triplet state (or doublet to quadruplet, etc.), then yes things would change. This would probably require a very large field, and might result in the dissociation of the molecule, if electrons have to move from bonding states to anti-bonding states to make their spins parallel.