Bronsted correlation interpretation

These equations factor all the acid-base types in a series to that where p = = 1. For example, if triethylamine and diaminoethane monocation are to be interpreted in the same correlation they need corrections because the former has p = 1 and q = 1 whereas the latter has p = 2 and q = 1. The assignment of p and q is not satisfactory for acid-base pairs with uncertain structures (such as H3O and HO ) or species with multiple centres of differing reactivity hydronium and hydroxide ions almost always show anomalous reactivities in corrected Bronsted correlations, but these are partly due to solvation effects. The protons attached to a single atom such as ammonium (R-NHj or oxonium ion (R-OH2 ) are regarded as having p = 1 rather than the number of identical protons. A similar convention selects the q value for hydroxide ion as unity even though there are three lone pairs free to accept a proton. The statistical... 

Interpretation of the slightly curved Bronsted correlations in terms of Marcus type theory is clouded by the curvature not being accurately defined. This is quite apart from any reservations about the validity of the energy-profile equations. The data can often be force fitted to a linear plot with little difference in correlation coefficient from that to the quadratic (Fig. 7). 

Fig. 38. Attack of semicarbazide on 4-chlorobenzaldehyde catalysed by adds (knA) tbe data can be interpreted as a poor linear Bronsted correlation or as a classical Eigen-type corrdation with pK 9 [122].

Applications of the technique to heterogeneous catalysts have been few, but they have demonstrated that the method is useful for catalyst characterization. For example, the lifetime of the orthopositronium species is inversely proportional to the number of Bronsted acid sites present in alumina-silica cracking catalysts. This interpretation was derived from a correlation between the activity for the alkylation of cumene and the lifetime of the orthopositronium species. 

The Bronsted relation has proved to be a useful equation for correlating rate and equilibrium results for proton transfer reactions. However, following the analysis by Leffler and Grunwald [73] in 1963 considerable effort has been made to go further than this and understand why the relation should hold, and also to attach some significance to the values of a and 3 in terms of the structure of the transition state for proton transfer. An alternative approach from that to be discussed here interprets the Bronsted relation from molecular potential energy diagrams [74]. 

No definite assignment of these two bands was made. Lavalley and coworkers, however, report a correlation between the frequency of CO interacting with a Bronsted center and the shift suffered by the related O - H stretch, which helps in proposing an interpretation for the hidden species [70]. From this correlation, it results that the corresponding CO frequency of 2173 cm would imply a shift of ca. 200cm . This means that the 0-H stretch of the acidic hydroxyls before interaction with CO would fall, roughly speaking, around 3680 and 3620 cm respectively. 

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