Bronsted correlation curvature

The geometries of reactant and product ion-molecule complexes, together with transition structures, were completely optimized, and for each reaction the enthalpies of activation and of reaction were computed for the elementary step of PT between the reactant and product complexes. Each of the non-identity reactions (n = 1, 2, 3) was considered in both the forward and backward direction together with the identity reaction n = 0), this yielded seven pairs of values for a plot of computed AHi vs. AH which showed noticeable curvature over a range -100 Marcus equation was obtained with AHj ,l = 67 kJ mol . The slope of the Bronsted correlation, as determined by Eq. (19.10), varied between a = 0.32 for the most exothermic PT to 0.68 for the most endothermic. ABK did not comment upon the position of the proton in the transition structures for the non-identity reactions. 

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). 

---