QUANTIFYING THE HAMMOND POSTULATE - INTRAMOLECULAR PROTON-TRANSFER IN SUBSTITUTED HYDROGEN CATECHOLATE ANIONS

A rigorous formulation of the Hammond postulate is presented. The concepts of the reaction exothermicity (gamma) and the structural proximity between the transition state and the reactants (beta) are quantified. Another quantity, called the isosynchronicity parameter (alpha), is introduced. The structural proximity and the isosynchronicity parameter are defined by using structural distances calculated with the help of the recently introduced NOEL similarity indices. The new indices-alpha, beta, and gamma, which can serve as theoretical tools for systematizing mechanisms of chemical reactions, are calculated at the HF/6-31++G** level for a series of 10 intramolecular proton transfers between both unsubstituted and (poly)fluorosubstituted hydrogen catecholate anions. A positive correlation, as implied by the Hammond postulate, is found between the calculated values of beta and gamma.