Quantitative Treatment of Structure Effects

Three types of the LFERs may be distinguished. The first group (A) includes expressions valid for a specific type of compound and certain processes. They are based on well-defined reference processes, which serve for the determination of the values of the constants These LFERs usually are named after the authors who have introduced them (Hammett, Taft, Brown, etc.), and special symbols for the parameters a and p are used (see Table I). The advantage of these established LFERs is the availability of 

CTp) Hammett Polar and resonance effects in conjugated systems (for meta and para substituents) 

In the second type of correlation (B) a search is made for relationships between two processes, one of them being under study and the other one being well known from the viewpoint of mechanism. The advantage of this approach lies in the clear kinship of the two processes when they show parallel structure effects on rate or equilibrium manifested by a good correlation. However, one must use the same derivatives in both series of compounds, and this may cause experimental problems. 

In the third type of correlation (C), experimental rate or equilibrium data are compared with reactivity indices calculated by some (usually) semiem-pirical method of theoretical chemistry. The main problem here is in the design of a suitable molecular model as the basis for calculation. 

The broad applicability of LFERs for heterogeneous catalytic reactions has been demonstrated independently by Kraus (23) and Yoneda (24-27). The first author concentrated mostly on the established relationships such as the Hammett and Taft equations, whereas Yoneda has concentrated particularly on correlations with reactivity indices and other quantities. Since then, LFERs have been widely applied to heterogeneous catalytic reactions, and experience has been gained as to the suitability of each different type. An important step has been made toward an interpretation of the slopes of linear correlations (parameter a in Eq. 3) as the quantities that are closely connected with reaction mechanisms. 

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