Subject: Replies to Bader analysis of CO

From: [email-masked] (Miquel Sol�)
Date: Fri, 17 Feb 1995 15:45:35 +0100

Dear CCLers,

Many thanks to all who replied to my question.
For all who may be interested in the conditions of the calculations, the r(C-O)
bond length in CO molecule was taken as 1.128 A. The results yield a
charge on the oxygen atom of -0.0145 au, while the dipole moment is 0.1024 D.
For the LiF molecule the r(Li-F) was 1.564 A, the charge on fluorine atom 
-0.6478 au and the dipole moment -6.4895 D. Further, the value of the density 
in the bond critical point (bcp) is 0.4829 au in CO and 0.0722 au in LiF.
Probably, the main conclusion is that the mere value of the laplacian 
in the bond critical point is not enough to define ionicity or covalency 
and that one should look also the shape of the laplacian in larger 
regions around the bcp, as well as the value of the electron density
in the bcp. 

My post was:

>An electronic analysis of the CO and LiF systems using the generalized 
>density from a QCISD/6-311++G** wavefunction yields a value for the laplacian
>of the density of 0.8328 au for CO and 0.7389 au for LiF. Could this mean,
>from a Bader analysis point of view, that CO is more ionic than LiF !!??
>Shoud the positive value of the laplacian (charge depletion) mean that the C-O 
>bond in CO molecule is ionic?.
>
>           If there is enough interest, I shall summarize the responses
>that I get on the net.
>
>           Thanks in advance,
>
>                       Miquel Sola
>                      	Institut de Quimica Computacional
>			Universitat de Girona
>			Voice: +34.72.41.83.62
>			FAX:   +34.72.41.83.61
>			World Wide Web: http://stark.udg.es
>			e-mail: [email-masked]
>				[email-masked]

Answers:

From: [email-masked]

Miquel,
  A positive value of the Laplacian at a 'bond crt. pt'
does not neccessarily mean that the bond is an ionic
bond.  It simply means that the density is locally
depleted.  Other types of interactions can give you
a positive value of the Laplacian as well.. eg H-bond.
The positive values have been correlated with 'closed-
shell' interactions which include ionic bonds.  Out of
curiousity,  how does your dipole moment stack up in both
cases?  Just to get an idea of the general charge
distribution.
  Take care... hope this helps.
     Ken Edgecombe
     Dept. of Chem.
     Queen's Univ.
     Kingston, ON

From: [email-masked] (Steven Bachrach)

>the laplacian
>of the density of 0.8328 au for CO and 0.7389 au for LiF. Could this mean,
>from a Bader analysis point of view, that CO is more ionic than LiF !!??

Well, suprising as it may seem, this is probably a correct conclusion!
If you calculate the charge on C and O in CO, my recollection is that the
values are soemthing like +1.2 and -1.2 respectively. The charge on Li and
F in LiF is probably something like +0.95 and -0.95, respectively. So which
is more ionic?

Finally, I don't think that Bader ever suggested that the values of the laplacian are interpretable in this way. All he indicated was that if the laplacian
is positive that this indicates a preponderance of ionic character in the bond
and a negative value indicates a mainly covalent character. I actually had a
long and heated argument with Bader many years ago about this and just what
the laplacian really means. I personally have my doubts about interpreting the
laplacian in terms of ionic/covalent character.

Steve

Steven Bachrach				
Department of Chemistry
Northern Illinois University
DeKalb, Il 60115			Phone: (815)753-6863
[email-masked]			Fax:   (815)753-4802