Subject: CCL: conformers and rate, MORE

From: Errol Lewars <elewars^at^trentu.cA>
Date: Fri, 16 Jul 2004 11:36:58 -0400

2004 July 16

Two more contributions to the question below. Thanks very much.
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QUESTION

2004 July 8

The rate of formation of products Pa and Pb from quickly interconverting
conformers Ca and Cb is treated with the the Curtin/Hammett and
Winstein-Holness equations (the Curtin-Hammett principle, in particular,
is well-known): J. I Seeman, Chem Rev, 1983, 83, 83; E. L. Eliel, "From
Cologne to Chapel Hill", ACS, 1990, pp. 31-33.

Question: has anyone a reference to a discussion of the case of rapidly
interconverting conformers Ca and Cb, where Ca, although the minor
conformer, is the only one that can react with X to form a product CaX,
through transition state Ta; intuition aside, has someone explicitly
addressed whether the _experimental_ activation E is measuring the Ta-Ca
energy difference, or the Ta-Cb difference?

Thanks.

EL
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1

A _very_ detailed kinetic treatment of various situations:

"It looks to me that some part of our paper
A computational study-of the hydroxy-group directivity in the peroxyformic
acid epoxidation of the chiral allylic alcohol (Z)-3-methyl-3-penten-2-ol:
Control of threo diastereoselectivity through allylic strain and hydrogen
bonding" JOURNAL OF ORGANIC CHEMISTRY 65 (20): 6715-6728 OCT 6 2000
could be of some use for you. We discuss there multi-transition state
reaction using an extended Curtin-Hammet principle."

Best regards,
Olga Dmitrenko

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2

An addendum:

Dear Per-Ola,
regarding the discussion in CCL on the rates, where both of us replied, 
I now fully agree with you. In my comment I forgot the fact that the 
population of Ca increases with exp(-(Ca-Cb)/kT) and hence this adds to 
the apparent activation energy. On the other hand in a rigorous 
definition of the activation energy we would  have extracted the 
population of the start species out of the  rate constant and then  the 
temperature dependence of the  rate constant obviously would  depend on 
exp(-(Ta-Ca)/kT). But I guess your andwer is more what is requested here.

Andreas

Dr Andreas Klamt
COSMOlogic GmbH&CoKG

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