Subject: TS compared with PMO; Re: Prediction of reactive sites.

From: [email-masked] (A.V.Sokolov)
Date: Fri, 10 Feb 1995 19:37:24 +0300 (MSK)

Hullo, chemists!

Michael D. Bartberger wrote:
>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 ......
............ skipped
>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.

I have some comments on the topic.

To study validity of PMO calculations (pi-aproximation) I compared
its results with directly calculated activation energies,
NOT WITH EXPERIMENTAL DATA.
Using MNDO I got the following radical reactions TS (and so activation
energies):

CH2=CXY + HOO, CH3, CH3O, CCl3, CF3 and some substituted peroxy and
                                                         alkyl radicals,
where X is H, CH3, Cl, -COOCH3, -O-COCH3 or C6H5.

Having compared these act. energies with "charge transfer" energies of
PMO methods I dare state now that Frontier MO approximation is good
for nothing (in my case it fails completely).

So, FIRST: using PMO use full summing over set of MOs (or subset,
in pi-approximation).
Then, if you don't want to deal with a great deal of MOs, either:

1. project occupied shell on AOs of your reactive center and brood
   upon the coefficients and H{_i_i} of localized MO (when center
   is non-conjugated projection will give MO nearly 99% localized on it)
or
2. I can propose an additive scheme of PMO sums evaluation.
  (you need to calculate some sums beforehand to parametrizate your
  compounds. Each reagent gets 2 parameters, you can use them instead
  of FMO coefficients). I've tested it on my set of reactions.

BUT: all said doesn't mean that you will correctly predict results of
     direct calculations (in my case there were also strong influence
     of steric effect).

________________________________

Bye for now. Respectfully yours,
A. Sokolov,
Dept. of Phys.Chem., Yaroslavl Polytech. Inst.,  Russia
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[email-masked]