Subject: Re: CCL:AIM in g98 problems - NO GOOD SOLUTION --- SUMMARY

From: "Dr. N. SUKUMAR" <[email-masked]>
Date: Sat, 24 Nov 2001 5:05:00 EST

On Sat, 24 Nov 2001 01:07:29 -0500 (EST) "C.F. Matta" wrote:

> 
> Dear Dr. Sukumar,
> 
> 
> I do not agree, however, with the statement that AIM does not
> discriminate
> between multiple bonding: All what you have to do is to examine the
> magnitude of the density at the saddle point on the bond path,
> i.e. the density at the bond critical point [Rho(b)].A few examples:
> 
>	  Rho(b)[ethane    C-C]  =  0.253 au   Delta(C,C)=0.991 pair
>	  Rho(b)[benzene   C=C]  =  0.327 au   Delta(C,C)=1.388 pair
>	  Rho(b)[ethylene  C=C]  =  0.364 au   Delta(C,C)=1.890 pair
> 

I totally agree with you here. My statement "does not discriminate between
multiple bonding" was loosely worded. Of course the density carries all the
info.and, in particular, the density and curvatures at the critical point
provide an indication of bond order as well as bond character.

Dr. N. Sukumar
Rensselaer Department of Chemistry

---

From: "C.F. Matta" <[email-masked]>
Date: Sat, 24 Nov 2001 01:07:29 -0500 (EST)

Dear Dr. Sukumar,

I agree totally with you that the bond order is an artificial concept,
this is why I propose the physical measure of electron delocalization
(DELTA) provided by integrating the fermi hole density over the atomic
basins in question (see my previous CCL message distributed yesterday for
details and references). DELTA recover much of the chemistry and chemical
concepts by counting the number of electron pairs shared between two
specific atoms in a molecule.

You are correct in saying that AIM provides an all-or-none answer as to
the existance of bonding (which can be very helpful in assigning a bonded
structure in ambigiuous cases where distance alone can be missleading -
check our Inorg.Chem.(2001),40,5603 paper. The all-or-none nature of the
bond path also is important to locate points on the reaction
coordinate where a change in structure happen (i.e. when bonds form
or break).

I do not agree, however, with the statement that AIM does not discriminate
between multiple bonding: All what you have to do is to examine the
magnitude of the density at the saddle point on the bond path,
i.e. the density at the bond critical point [Rho(b)].A few examples:

Best wishes.

  Cherif
___________________________________________________________________________

  Cherif F. Matta		    	  tel. (905) 525-9140 ext. 22502
  Chemistry Department                    fax  (905) 522-2509
  McMaster University
  Hamilton, Ontario, CANADA L8S 4M1.
___________________________________________________________________________

On Fri, 23 Nov 2001, Dr. N. SUKUMAR wrote:

> Aren't bond orders a somewhat arbitrary concept?
>
> The simplest -- and also cheapest -- way of estimating bond orders I know
> of is simply to use a distance criterion and parametrize for different
> bonds. Some molecular modeling software do just this. But calculating bond
> orders ab initio? I'm afraid that really depends on how you define bond
> orders.
>
> More elaborate ways have been employed in the literature to calculate bond
> orders, but really it depends on whose definition you accept (see, e.g.
> J.S. Murray, N.Sukumar, S.Ranganathan and P.Politzer, “A Computational
> Analysis of the Electrostatic Potentials and Relative Bond Strengths of
> Hydrazine and some of its 1,1-Dimethyl Derivatives” Intl. J. Quantum Chem.
> 38, 611-629, 1990). I personally think that the bond order concept today
> provides no more useful information than bond length or bond dissociation
> energy, both of which can be unambiguously defined.
>
> AIM can be used to determine the existence or absence of a bond path
> between atoms in a molecule. It does not distinguish multiple bonds.
>
> Dr. N. Sukumar
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