Site-Reactivity Relationships

Being able to measure the amounts of the saturation sites on the surface of a dispersed metal catalyst provides a means of further defining the specific activity 

As discussed previously, this reaction was also run under these same conditions over the series of specifically cleaved platinum single erystals shown in Fig. 3.2. 3 The results of these experiments show that it was the corner atoms on these crystals that promoted C-H bond breaking. Thus, the saturation sites on the dispersed metal catalysts are also comer atoms. Since this saturation site description agrees with that proposed on the basis of the butene deuteration described previously,5 -62 it is likely that the isomerization sites, M, are edge atoms and the hydrogenation inactive sites, M, are face atoms. A similar approach can be used to determine the nature of the active sites responsible for promoting almost any type of reaction. 5.70 

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The catalytic performances of the supported catalysts clearly demonstrated the improvement in terms of activity or selectivity by such optimized catalytic systems. This improvement is related to a nanometer control of the critical characteristics of the active sites. Enhancement of the catalytic performances and the understanding of structure-reactivity relationships can only be achieved by advancing the understanding of different preparation methods, eventually leading to better control over the characteristics of the active sites at a nanometer scale. Moreover, new properties of these solids may be found, which could have a great impact on catalytic reactions. 

Shin J., Kim B., Exploring the Active Site of Amine Pyruvate Aminotransferase on the Basis of the Substrate Structure-Reactivity Relationship How the Enzyme Controls Substrate Specificity and Stereoselectivity,/. Org. Chem. 2002, 67, 2848-2853. 

The possibility of introducing single-site mutations in azurins enabled a detailed analysis of structure-reactivity relationships where, for example, the impact of specific amino acid substitutions on the rate of intramolecular FT could be investigated. In order to understand better the role of the polypeptide matrix separating electron donor and acceptor on FT reactivity, the structure-dependent theoretical model developed by Beratan et al. (6, 7) was employed to identify relevant ET pathways (cf. Section I.B). In this model, the total electronic coupling of a pathway is calculated as a repeated product of the couplings of the individual links. The optimal pathway connecting the two redox sites, is thus identified (cf. Eq. 5). 

Provided the substituents are not ortho to the reactive site this relationship holds quite well. 

As non-toxic chiral Fe complexes have recently been used as catalysts [118-120], increased knowledge of their structure-reactivity relationships becomes pertinent. X-band CW-EPR spectra of [Fe °Cl(l)], reported by Bryliakov et al. [121], were found to be typical of high-spin S = 5/2 Fe complexes with EID K, 0.15. Using this complex, the conversion and selectivity of the asymmetric sulphide oxidation reaction was investigated in a variety of solvents. In previous studies [122], the active site was proposed to be the [Fe =0(l)] species. However an alternative active species was proposed [121]. Oxo-ferryl 7i-cation radicals are expected to have typical S = 3/2 spectra with resonances at geff 4... 

The structure-reactivity relationship for polyamine derivatives in activated ester hydrolysis was previously established [46]. Polyvinylamine (PVA), linear (LPEI) and branched (41% branching) polyethylene imine (BPEI) as well as their dodecyl- and imidazole-substituted derivatives with an approximate and equal degree of substitution (16-20%) were applied as catalysts. The compoundsp-NPA and 4-acetoxy-3-ni-trobenzoic acid (ANBA) as well as some of their homologues were used as substrates. At an excessive catalyst concentration relative to the substrate concentration, reactions proceeded at pseudo first order. In each series of polymers, the reaction rate constant was increased considerably by substitution of dodecyl (hydrophobic site) by imidazolyl (catalytic center) and when a charged substrate (electrostatic effect) was employed. At an equal degree of substitution, the catalytic activity increased in the following order LPEK PVA < BPEI. 

Based on an analysis of the overall rate coefficients for the reactions of OH radicals with alkenes, Peeters has developed a structure-reactivity relationship for prediction of the relative yields of OH addition to specific sites in the parent alkenes. For mono-alkenes and non-conjugated poly-alkenes the site-specific rate coefficients depend on the stability of the ensuing hydroxyalkyl radical, which can be either a primary, secondary or tertiary radical. For conjugated dienes, the structure-reactivity analysis requires additional consideration of the resonance stabilisation energies of the adduct radicals. The quality of the predictions have been tested for a large number of alkenes and found in most cases to be in agreement with experiment within 10 %. 

The aim of many surface science experiments is to provide the fundamental detail of a reaction over a well-characterized single crystal surface in order to establish structure-reactivity relationships. Supported catalytic particles, on the other hand, may have various exposed surface facets along with defect sites. A choice then has to be made as to which single crystal surface will provide the most accurate representation of the active surface facets of the support particles. In order to address the similarities or differences in the rate over ideal surfaces and those over supported particles, Kelly and Goodmanl l compared the rate of methane formation from CO and H2 catalyzed by an Ni(lOO) single crystal with that over a supported catalyst taken under the same conditions (see Fig. 2.12). 

The study of structure-reactivity relationships by the organic chemist Hammett showed that there is often a quantitative relationship between the two-dimensional structure of organic molecules and their chemical reactivity. Specifically, he correlated the changes in chemical properties of a molecule that result from a small change in its chemical structure that is, the quantitative linear relationship between electron density at a certain part of a molecule and its tendency to undergo reactions of various types at that site. For example, there is a linear relationship between the effea of remote substituents on the equilibrium constant for the ionization of an acid with the effect of these substituents on the rate or equilibrium constant for many other types of chemical reaction. The relative value of Hammett substituent constants describes the similarity of molecules in terms of electronic properties. Taft expanded the method to include the steric hindrance of access of reagents to the reaction site by nearby substituents, a quantitation of three-dimensional similarity. In addition, Charton, Verloop, Austel, and others extended and refined these ideas. Finally, Hansch and Fujita showed that biological activity frequently is also quantitatively correlated with the hydrophobic character of the substituents. They coined the term QSAR, Quantitative Structure-Activity Relationships, for this type of analysis. 

J.-S. Shin, B.-G. Kim, Exploring the active site of amineipyruvate aminotransferase on the basis of the substrate structure-reactivity relationship how the enzyme controls substrate specificity and stereoselectivity, J. Oq . Chem. 67 (2002) 2848-2853. 

The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

With the addition of increasing amounts of electrolyte this variance decreases and an approximate linear relationship between internal and external pH exists in a 1 Af electrolyte solution. The cell-0 concentration is dependent on the internal pH, and the rate of reaction of a fiber-reactive dye is a function of cell-0 (6,16). Thus the higher the concentration of cell-0 the more rapid the reaction and the greater the number of potential dye fixation sites. 

In these acids the geometric relationship of the substituent X to the reactive site COOH approximates that in 4-substituted benzoic acids, but in 3 the resonance... 

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