Difference between ion-dipole and INDUCED ion-diople forces

[requal]

At first I thought an induced ion dipole only applied to non-polar molecules, but in the textbook it says it polar molecules are affected by induced ion-dipole forces as well ("for the polar molecule, it enhances the dipole moment already present", Silberberg) ? If that's so, does that mean that all molecules bonded with ion dipole forces have induced ion dipole forces as well.

 

[SpectraCat]

At first I thought an induced ion dipole only applied to non-polar molecules, but in the textbook it says it polar molecules are affected by induced ion-dipole forces as well ("for the polar molecule, it enhances the dipole moment already present", Silberberg) ? If that's so, does that mean that all molecules bonded with ion dipole forces have induced ion dipole forces as well.

Hmm .. ok ... let's clarify a bit. If you are talking about ion-dipole forces, presumably you are talking about an ion interacting with a neutral molecule or atom. In that case, the answer is yes, you always have induced ion-dipole forces. If the neutral molecule happens to be polar, you will also have normal ion-dipole forces as well ... these will generally be much stronger, especially at large distances. This is because the ion-dipole forces scale with distance as 1/r², while the induced ion-dipole forces scale with distance as 1/r⁴. The different scaling for the induced forces can be rationalized (although this is not strictly correct) as arising because the field from the ion interacts with the molecule twice .. first a polarization to induce the dipole, then the interaction with the induced dipole.

 

[DrDu]

If I remember correctly, e.g. in hydrogen bonding the induced dipole moment makes up for 10 to 20% of the binding energy.