Persistent-Complex Formation

It was pointed out earlier that an important factor in persistent complex formation in reactive scattering systems is the depth of the potential well. In studying the elastic scattering of protons on molecules the Hahn-Meitner Institute group showed that even though one can only deduce average potentials in these molecular systems, such information can have significant applications in interpretation of ion-molecule reactions. 

Fig. 23. Cross section across a potential energy surface showing energy parameters relevant to persistent-complex formation. ...

C (02, 0)C0. All of them predominantly show forward scattering consistent with a direct mechanism. Kinematic study of the reaction Kr (D2, D)KrD showed that the product was rather isotropically scattered about the center of mass, though a persistent complex formation seemed unlikely in this exoergic reaction. 

In the cyclodextrins readily obtainable from starch, the six (a-CD), seven (P-CD) and eight (y-CD) a(l- 4)-linked glucose units are "locked up" in a strait-jacket type belt due to adoption of 4Cj chair conformations of the pyranoid rings and a net of 2-OH — OH-3 hydrogen bonds (73, 74). As this structural rigidity even persists on inclusion complex formation, as exemplified by the three represenatives in Fig. 1 (75 - 78) ... 

The above examples of results on reactions proceeding by direct and persistent complex mechanisms show that if one obtains a contour map with the product ion intensity asymmetrically disposed about the +90° line one can with some certainty conclude that the reaction is proceeding by a direct mechanism. On the other hand, if one has an isotropic or symmetric distribution of the product ions one must interpret such curves with some care since such distributions may not unambiguously indicate complex formation judging from the results obtained on such reactions as the Kr+(D2, D)KrD + and H2+(H2,H)HJ. 

This can be rationalized as follows. Consider an enzymatic reaction that displays ideal Michaelis-Menten kinetics, i.e. equilibrium formation of a Michaelis complex, followed by an irreversible chemical step to form products. Further assume that Michaelis complex formation involves one specific enzyme-substrate contact that causes a EIE. If that contact persists unchanged in the transition state, then there is no further isotope effect on the chemical step and the observable KIE will be equal to the EIE. In other words, if the specific enzyme-substrate contact is the same in the transition state as in the Michaelis complex, this will be reflected in the observable KIE for that label being equal to the EIE of Michaelis complex formation. If, on the other hand, the specific enzyme-substrate contact is removed in the transition state, then there will be an isotope effect on the chemical step that is opposite and equal in magnitude to the EIE on Michaelis complex formation. The observable KIE will be unity, accurately reflecting the lack of the specific enzyme-substrate contact at the transition state. 

Processes (2)-(4) represent the first reactions for which detailed information on collision dynamics is available over essentially the entire energy range. The fact that they are always predominantly direct indicates that even in slow collisions between strongly interacting reactants, the formation of a persistent complex cannot be automatically assumed. 

The existence of a deep well and a low excitation energy 8 is not in itself enough to assure formation of a persistent complex. As discussed in more detail elsewhere,the system must also be able to get into the well. This may not be possible if there is a barrier to achieving the required configuration. Thus, the intermediate in reaction (3),... 

On the basis of the simple criteria for complex formation discussed above, it thus appears possible to qualitatively account for all available data. Not only may the formation or nonformation of persistent complexes be explained, but also their energy dependence. With increasing energy, the lifetime of a complex will diminish until it is so short that there is a continuous transition to a corresponding direct mechanism. In addition, the second criterion discussed makes plausible the observation that at a given energy, long-lived complex modes may competitively coexist with quite distinct direct mechanisms. 

One of the best tests of purity of dioxane is the formation of the purple disodium benzophenone complex during reflux and its persistence on cooling. (Benzophenone is better than fluorenone for this purpose, and for the storing of the solvent.) [Carter, McClelland and Warhurst Trans Faraday Soc 56 343 I 960], TOXIC. 

The FT-IR spectroscopic measurements shown that in most cases the -COO or -0 groups formed a bridge between two Sn central atom, and polymerization occurred. The pqs approximations proved the formation of complexes with Oh, Tbp, and structures. H NMR measurements performed in DMSO solution have shown that the polymeric structure of the complexes does not persist in solution, and depolymerization occurs.. ... 

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