Subject: CCL:concerted reaction/ kinetics

From: [email-masked] (Joe Durant)
Date: Wed, 4 Dec 1996 11:36:46 -0800 (PST)

Hi!

Your questions seem to boil down to two fundamental ones:

1) can one calculate the "free energy of activation" for the reaction
    AB + C -> [B-A-C]# -> B + AC  ?

and 

2) where is [B-A-C]# energetically located relative to the reactants?

If the reaction takes place in the gas phase then the answer (with
caveats) to 1 is yes.  To get the free energy of activation you need
to know relative energies of the species, rotational constants and
vibrational frequencies.  Throw in some statistical mechanics and you
will have an answer. (far easier said than actually done :-))

As to the second question I would direct you back to Johnston's BEBO
method.  He treats this sort of a reaction, and looks at the
thermodynamics as follows:  
in going from AB + C to [BAC] you have partially broken the AB
bond, and partially made a BC bond.  Additionally, you have a
anti-bonding B-C interaction (assuming A is an atom), which will be
repulsive.  The general atom-transfer reaction treated this way will have a
barrier, with the barrier height being maximum for a thermoneutral
reaction, and decreasing for more exothermic reactions.
If A is a polyatomic fragment the above analysis no longer holds, and
you are left with the task of deciding whether a (meta)stable BAC molecule
can be formed.

Hope this helps.

Joe

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