Subject: Dipole moments and charged species

From: <[email-masked]>
Date: Fri Dec 23 10:55:01 1994

Greetings from Firenze!

One of my students has asked me a question (really a group of 
related questions) which has arisen before in my research group, 
and to which I do not have truly satisfactory answers.  Now that
I have access to the CCL, I decided to ask it/them of all of you 
CCL subscribers:

1) Can a species already carrying a charge have a dipole moment
   as well?

   1a) If so, can such a moment (multipole, perhaps?) be calculated
       in any satisfactory way?

   1b) If not, why not?

2) When calculations are carried out on charged species, the result
   often includes a value for "dipole" moment, yet these values are
   unreliable.  If the "origin" of the ion is changed, with no other
   changes in bonds, angles, or torsions, the value for the "dipole"
   can also change.  If the origin for charged species was defined
   as always being at the center of mass, would the resulting value
   for dipole moment have any validity?

3) Can charge be treated as an "additive" property so that a dipole
   moment could be calculated for the uncharged species as the 
   structure determined for the charged species, and then adding
   the extra charge after that?  (In other words, making the dipole 
   moment of the charged species the same as the dipole moment for 
   the hypothetical uncharged species?)

I have only some general references from the 1930's concerning the
determination of quadrupole moments, but nothing more current than
that.

My apologies to all if these questions seem naive...my students often
stump me with seemingly simple ideas which are outside of my areas
of so-called "expertise".  Thanks in advance for your responses.  I
shall post a summary of replies.

Bob Zoellner

[email-masked]
[email-masked]

Robert W. Zoellner, Ph.D.; Associate Professor of Chemistry
[currently on sabbatical from Northern Arizona University]
ISSECC-CNR; Via Jacopo Nardi, 39; 50132 Firenze; Italy
Office telephone:  (39-55) 24.59.90  FAX:  (39-55) 24.78.366
Home address:  Via di Bellariva, 9; 50136 Firenze; Italy
Home telephone:  (39-55) 67.77.98