Subject: CCL:Examples of DG maxima where no PE maximum exists?

From: "Daniel A. Singleton" <singleton(at)mail.chem.tamu.edu>
Date: Mon, 03 May 2004 22:58:20 -0500

There are a great many such reactions - we have published 
some, but it is really an old phenomenon.  Within physical 
organic, I would consider Houk's discussion of the 
addition of carbenes to alkenes in the mid 1980's as a 
"classic" example.  I would guess that a good way to find 
examples would be to search for "barrierless" reactions, 
as the scenario you describe will apply to perhaps a 
majority of reactions described as barrierless.  But 
beware calculationally, entropy at medium intermolecular 
separations is a four letter word.  It is used for some 
problems, for lack of choice, but buyer beware.  

On Mon, 3 May 2004 09:19:19 +0100
  "Rzepa, Henry" <h.rzepa(at)imperial.ac.uk> wrote:
>In the temporary absence of a working search of the  CCL 
>archives,
>does anyone know of any examples of the following 
>scenario?
>
>The potential energy surface of a dissociative reaction 
>may often
>rise to the asymptotic maximum of infinite separation of 
>the two
>components. These two components will show a large 
>increase in
>the entropy,  and hence  any free energy increase for 
>this process will be
>substantially less than the potential energy increase. 
>Depending 
>on how the entropy evolves along the reaction path, its 
>possible to imagine that 
>unlike the potential energy, the free energy reaction 
>path may
>show a clear maximum (transition state) unmatched by one 
>in the 
>potential energy.
>
>Does anyone know of any "classic" examples of such
>entropy induced transition states? 
>(its the sort of concept that eg a Scifinder search is 
>difficult
>to formulate for)
>-- 
>
>Henry Rzepa. Imperial College, Chemistry Dept.
>+44 0778 626 8220 +44 020 7594 5804 (Fax)