Subject: 2:electrostatics and dipole moment

From: Patrick Bultinck <[email-masked]>
Date: Wed, 27 Mar 1996 13:15:24 +0100 (NFT)

Dear,

There seems to be some mis-understanding. I know that the VALUE for the 
dipole moment is indeed origin-independant for a neutral species, and not 
so for a charged species.
What I mean is : 'How to chose an origin for a neutral species IF you 
need not only the value ?'.
I will give an example :

Say C3v NH3 and C2v H2O. 
a) Complexes of the first with e.g. Na+ gives a larger interaction energy 
than with H2O.
b) mu H2O > mu NH3

In an article (I will not give the reference, since I would not like to 
let it look like I think something negative on some author, which no 
doubt knows more about this than I do) I found an explanation which 
should go as follows :

We know that mu H2O > mu NH3, so also that Z*mu is larger for the H2O 
complex than for the NH3 complex.
On the other hand, the interaction also depends on r, the distance 
between the dipole moment and the point charge or ion. The author found 
that the dipole vector in NH3 lies more to the right than in H2O (see 
figure).

	H		H
	 \		 \
        H-N		  O
	 /		 /
	H		H
           *->		*->	(PURELY QUALITATIVE AND EXAGERATED, please...)

For the interaction with an ion, r(NH3) would be substantially smaller 
than in case of H2O, which SHOULD explain the observed electrostatic 
interaction differences. BUT, I did some calculations myself, and the 
trouble is that I have great suspicions that the origin was simply put in 
the center of mass, which seems to me as an unjustifiable approach...I 
would put the origin where other multipoles are minimal. It seems to me 
that you could even play a little with the results, say you let all the 
chemical properties the same, but play a little with isotopes, and get 
another result, which should influence you perception of the 
metal-ligand bond.

So the question was : 'If I really need to have a method not only to have 
the value of the dipole moment, but also it's spacial position (origin, 
we have all components from our wavefunction, aka the value of the 
x-xomponent, y etc.), how  should I do this'.

Thanks,

*******************************************************************************
Patrick Bultinck			Macrocycles Quantum Chemical
Ph. D. Student				Calculations
Dept. Inorganic & Physical Chemistry 
University of Ghent			Tel. Int'l code/32/9/264.44.44
Krijgslaan 281 (S-3)			Fax. Int'l code/32/9/264.49.83
9000 Gent				E-mail : [email-masked]
Belgium					http://allserv.rug.ac.be/~pbultink/
*******************************************************************************