ECHET article 107
Wed, 31 Jul 1996 09:54:13 +1000 (EST)

We have not attempted to obtain activation energies and delta S of
activation by the lineshape method in the fast exchange region. In
principle there is no difficulty in doing this in most cases. The
lineshape analysis could be carried out with both the benzylic protons and
the oxymethylenic protons. It may even be possible to determine separate
activation parameters for the ring flipping of E and Z amide isomers in the
acetyl and isopropyl cases.

In cases where we had two coalescences at different temperatures, we did
calculate approximate EA and S of activation using a two point Arrhenius!!
While activation energies were perhaps OK, we regarded this method as
hopelessly inadequate for delta S of activation since coalescence
temperatures in most cases only differed by between 10 and 20 K. The
lineshape method should give us a much wider range of temperatures and rate
constants. All entropies of activation were negative (-7 to
-25cal/deg.mol) and activation energies were typically 7-8kcal/mol for
benzoylbenzoxazepines through to 11kcal/mol for the isobutanoyl and
benzenesulfonyl derivatives but interpretations should be vey guarded for
the above reasons.

Thanks for the tip about gNMR. Concidentally a flyer from Softshell
advertising the package crossed my desk just two days ago.

Steve Glover

Dr Steve Glover, Department of Chemistry, University of New England,

Armidale, Best in the Country
NSW 2351, Australia.
Phone: 067-73 2361 Int. 61-67-732361
Fax: 067-73 3268 (new) Int. 61-67-733268

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