Subject: Prediction of reactive sites

From: "Michael D. Bartberger" <[email-masked]>
Date: Tue, 7 Feb 1995 11:06:57 -0500 (EST)

Dear Netters:

I would very much appreciate references concerning any methods used to 
correlate a given molecule's regioselectivity in a given reaction with a 
property calculable with ab initio methods.

For example, using a simple FMO approach, we are taught that "attack" by 
a nucleophile on a given substrate are predicted / rationalized by looking
at the AO coefficients at the LUMO of the substrate and envisioning 
maximum overlap in the transition state, etc etc.  The same goes for 
electrophilic reactions using the HOMO of the substrate; that (in this 
case) the electron density and overlap in a TS is greatest at the site 
with the greatest coefficient.

Now, AO coefficients are easily generated in semiempirical programs, 
where they are generated from a minimal basis, and seem to mimic what one 
would generate from a valence-only, Huckel-type approximation (for example, 
look at the AM1 coefficients for ethylene, allyl radical, etc.)  With ab 
initio methods and extended basis sets, "reading" the coefficients in 
such a manner doesn't seem as plausible, as one obtains coefficients for 
each of the sets of functions used in the basis.

I had asked a question similar to this some time ago; asking whether it is
possible to obtain a "total" coefficient of a given AO at a certain MO, or
whether one of the coefficients generated in this way should be "plucked 
out" and used as "the" coefficient.  The question was met with mixed 
sentiments without really ever getting any concrete answer. 
  
(By the way, I'm still very much open for suggestions here.)    :-) 

This brings me to the main question.  What is the "best" (if any) method for 
correlating the types of reactivities I've discussed above, whether 
in polar (like discussed above) or in radical reactions, with either 
calculated coefficients, or some other property caclulable by ab initio 
methods.  Of course, total TSs can be calculated and the energetics 
compared, but it would be nice to have some sort of qualitative or 
semi-quantitative rationale for what governs these reactivities.

I will certainly summarize if there is sufficient interest.  I hope what 
I have written makes some sense.    :-)

Thanks very much!

Regards,

Michael
  
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 Michael D. Bartberger         [email-masked]        TEL: (904) 392-3580
 Department of Chemistry       [email-masked]         FAX: (904) 846-0296
 University of Florida
 Gainesville, FL  32611
 USA
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