From Y0H8797*- at -*ACS.TAMU.EDU Fri Mar 22 17:49:45 1996 Received: from VMS1.TAMU.EDU for Y0H8797:~at~:ACS.TAMU.EDU by www.ccl.net (8.7.1/950822.1) id RAA05895; Fri, 22 Mar 1996 17:14:32 -0500 (EST) Date: Fri, 22 Mar 1996 16:13:53 -0600 (CST) From: YONG HUANG To: CHEMISTRY- at -www.ccl.net Message-Id: <960322161353.22a125b9 \\at// ACS.TAMU.EDU> Subject: 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 %-% acs.tamu.edu) -------------------------------------------------------------------------------- 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. 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. -------------------------------------------------------------------------------- From: SMTP%"chabinml.,at,.chem.Stanford.EDU" 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 -------------------------------------------------------------------------------- From: SMTP%"johnL.,at,.proteus.co.uk" 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 -------------------------------------------------------------------------------- From: SMTP%"JVSQUARE%canisius.BITNET # - at - # UBVM.cc.buffalo.edu" 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 jvsquare ^at^ canisius.edu Canisius College, Buffalo, NY 14208 -------------------------------------------------------------------------------- From: SMTP%"johnL # - at - # proteus.co.uk" 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. | johnl $#at#$ proteus.co.uk [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.]