Subject: Dipole moment origin and electrostatics

From: Patrick Bultinck <[email-masked]>
Date: Tue, 26 Mar 1996 15:43:33 +0100 (NFT)

Dear,

I have a simple general question, which is troubling me. In an article by 
a well-known author several complexes are compared, and a 
Kitaura-Morokuma decomposition analysis carried out. From this analysis 
we see that the electrostatic interaction is the most important. Okay so 
far, but the author then calculates the dipole moment to explain this, 
and he calculates the dipole moment. This is important since the M-O 
distance is smaller than M-N, while for the N case the electrostatic 
interaction is stronger. He finds that the dipole moment for NH3 is 
closer to M than in case of H2O which should then explain a) the bond 
lengths and b) the stronger electrostatic interaction, and this despite 
mu (H2O) > mu (NH3). (Z*mu)/r^2 you know. 
All this explains everything very good, but... I have a problem with this.

All of the ion-ligand interaction is moved into the ion-dipole 
interaction, what about the rest of the multipoles ? This is especially 
interesting since when repeating some calculations by him, I found that 
he simply put the origin of the dipole in the center of mass. I would 
think that if you want to reduce the ion-ligand interaction to an 
ion-dipole model, you should choose your origin such that most of the 
higher order poles are minimal, so that you make the smallest mistake.

If you do not do so, it would seems to me that one could think of some 
ligand, which has a smaller interaction energy, but still has it's dipole 
closer to the ion because you chose things so that the center of mass was 
closer to the ion (say, throw a CH3 instead of H in it)...

If you want to compare some electrostatic interactions between different 
ligands with the same ion, and you want to make use of ion-dipole 
interactions to explain your observations, then how should you take the 
ORIGIN of the dipole vector ? Simply taking it in the center of mass 
seems like 'forget about the electrons, just have a look at nuclei'.

Is the fact that things are so well explained by the 
center-of-mass-origin coincidence, or am I missing the point (which is 
probably true).

Thanks,

Patrick Bultinck
Quantum chemistry
University of Ghent
Belgium