Subject: Summary: structure-kinetics correlation

From: YONG HUANG <[email-masked]>
Date: Fri, 22 Mar 1996 16:13:53 -0600 (CST)

On March 7, I posted a message about the correlation of structure - kinetics. I
received 3 responses. Many thanks to Drs. Michael Chabinyc, John Liebeschuetz 
and James Verth. The summary below is followed by a little additional
correspondence with Dr. Liebeschuetz.

Yong Huang ([email-masked])
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From:   SMTP%"[email-masked]"  7-MAR-1996 22:51:24.14
Subj:   CCL:reference on structure-kinetics relationship

Can anybody recommend some literature (books, papers) on the relationship
between kinetics and molecular structure? The reason is as follows.

It seems easier to make a guess of stability, acidity or basicity etc. by
looking at the structure of a molecule than make a guess of how fast it would
react with other molecules. In other words, the relationship between
thermodynamics and structure is more straightforward than that between kinetics
and structure. However, the Hammett equation, as well as other physico-organic
equations, shows correlation between sigma (substituent parameter, a structural
parameter) and K, where this K can be either equilibrium constant or rate
constant. This implies thermodynamics goes the same way as kinetics in those
series of compounds. In my mass spectrometry experiment, it seems compounds of
even similar structure show different thermodynamic trend (acidity in my case)
than kinetic trend.* I think the bridge between thermodynamics and kinetics is
the stability of reaction intermediate or activated complex. But I'm not
successful in making use of this knowledge to explain my experiments. Any
help will be appreciated.

Yong
________________________
*E.g., in the chart of degree of proton transfer vs. acidity, sinapic acid and
caffeic acid lie on the same line while 4-OH-cinnamic acid and ferulic acid on
the other line. The 2 lines may suggest 2 activation energies.
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From:	SMTP%"[email-masked]"  7-MAR-1996 22:38:44.18
Subj:	Re: CCL:reference on structure-kinetics relationship

  I saw your message and thought I'd send you a note.  One 
recent paper to look at is 
  "Intrinsic Structure-Reactivity Relationships in Gas-Phase
   SN2 reactions: Identity Exchange Reactions of Substituted
   Benzyl Chlorides with Chloride Ion" - B.D. Wladkowski,
   J.L. Wilbur, J. I. Brauman  JACS 116 2471-2480 1994

 There are lots of papers out on this topics many from our
group - just do a lit. search on John Brauman.  Terry MacMahon,
and Kebarble have also done some work in this area.  The paper
I mentioned has a good review of the literature also.
  Some issues to worry about are complexation energies of 
your reactants and competing reaction channels.  If you
want more info send me a specific area of interest.

         Michael
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From:	SMTP%"[email-masked]"  8-MAR-1996 06:53:15.14
Subj:	structure-kinetics

A correlation between kinetic rate and thermodynamic stability is not suprising
for a single step reaction. The height of the free energy `hill' between
reactant and product is not independant of the relative free energy between
these species. This will be especially true if the transition state is `late'
on the reaction coordinate i.e. product resembling.
	Where you get two classes of molecules, each of which show a different
linear relationship between kinetics and thermodynamics, then the liklehood is
that you are seeing two different reaction mechanisms. I don't know much about
the sort and the conditions of reaction you are looking at but is it possible
that in one of your classes some sort of intramolecular proton transfer is
occuring as part of your reaction and in your other class it is not ?

		Regards
				 John
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From:	SMTP%"[email-masked]"  9-MAR-1996 01:42:38.59
Subj:	thermodynamics vs. kinetics

See J. Org. Chem. 1995, 60, 3452-3458 for a related study.

> *E.g., in the chart of degree of proton transfer vs. acidity, sinapic acid and
> caffeic acid lie on the same line while 4-OH-cinnamic acid and ferulic acid
> on the other line. The 2 lines may suggest 2 activation energies.

James E. Van Verth              Department of Chemistry
[email-masked]           Canisius College, Buffalo, NY 14208
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From:	SMTP%"[email-masked]" 11-MAR-1996 04:18:48.96
Subj:	Re: structure-kinetics

On Mar 8, 10:13am, YONG HUANG wrote:
> Subject: RE: structure-kinetics

> John, thank you for your comment. The proton transfer IS inter-molecular in
> all cases. The experiment is MH(NH3)n cluster (MH is the acid) photoinduced
> intra-cluster proton transfer. Here I say intra-cluster because MH + nNH3
> forms the cluster.
>
> Although the molecules MH are very similar (cinnamic acid derivatives, OH
> and/or OCH3 attaching to benzene ring), we do suspect two mechanisms as you
> mentioned. I'm working on the effect of variable reaction time on the extent
> of proton transfer. Preliminary results indicate all show more proton transfer
> but only 4-OH-cinnamic acid increases A LOT.
> Why do you say the energy of the transition state is not independent of the
> reactant and product ESPECIALLY  when it's closer to the product? How about
> closer to the reactant? Thanks vey much.
>
> Yong
>-- End of excerpt from YONG HUANG

Yong,
	The reason that the transition state energy (and hence rate of reaction)
is often closely related to the Free Energy of the reaction when the transition
state is close in nature to the product is simply due to the fact that species
close to each other on the reaction coordinate are liable to have closely
linked energies, the energy function being continuous. Therefore the energy
change between the reactant and transition state closely parallels that between
reactant and product. This is embodied in what I believe is called the
Bell-Evans-Polanyi principle which describes why the product distribution of
some irreversible reactions is determined by the relative stability of the
products (I'll try to get a ref. for you).
	If the transition state is early on the reaction coordinate then the
transition state energy will be linked to that of the reactant rather than the
product and therefore is less likely to be related to the Free Energy change
for the reaction.
	I suppose another possibility you might have, other than two different
mechanisms, is different electron distributions in the transition state which
would imply different molecular orbitals are involved in stabilising the
developing anion in both cases. That would be interesting. I guess to explore
that possibility you'd have to do some fairly extensive high level MO
calculations. Looks as though you've got an exciting bit of research going on !

					John
John W. Liebeschuetz Ph.D.      |    [email-masked]

[Yong's words: Later I tried to find the transition state by CAChe MOPAC of
the reaction: MH...NH3 -> M-...NH4+, where MH is the simplest molecule in my
experiment, 4OH-cinnamic acid. The TS is extremely close to the reactant. I
tried the same calculation on the reverse reaction and the TS was very
different. I was advised to use a keyword LST. But I don't see it in the MOPAC
manual. This work is temporarily set aside. All the advice I've got so far is
very much appreciated.]