Summary: structure-kinetics correlation

 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 (y0h8797 (+ at +)
 From:   SMTP%"Y0H8797 (+ at +) ACS.TAMU.EDU"  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.
 *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.
 From:	SMTP%"chabinml (+ at +) chem.Stanford.EDU"  7-MAR-1996
 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.
 From:	SMTP%"johnL (+ at +)"  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 ?
 From:	SMTP%"JVSQUARE%canisius.BITNET (+ at +)"
 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
 > 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
 jvsquare (+ at +)           Canisius College, Buffalo, NY 14208
 From:	SMTP%"johnL (+ at +)" 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
 > of proton transfer. Preliminary results indicate all show more proton
 > 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
 	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 W. Liebeschuetz Ph.D.      |    johnl (+ at +)
 [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.]