Gustavo A. Arteca1, Paul G. Mezey1
1Departments of Chemistry and Mathematics, University of Saskatchewan, Saskatoon, Canada, S7N OWO
Tóm tắt
AbstractThe Hammond postulate is a useful, qualitative tool that interrelates structural similarities between reactants, transition structures, and products with the exo‐ or endothermicity of reactions. It applies to most chemical reactions, although several exceptions are known. In this study the following problem is addressed: is it possible to formulate conditions for the validity of the quantitative Hammond postulate in terms of simple physical quantities characteristic to the molecules involved? A detailed analysis is given for the conditions of validity of the postulate, in terms of bounds on the internal forces and force constants of nuclear arrangements encountered along a reaction path. We have determined a broad class of constraints on barrier shapes that must be satisfied in order to obtain a critical situation that violates the Hammond postulate: a reactant‐like transition structure (“transition state”) for endothermic reactions, and a product‐like one for exothermic reactions. The general constraints are formulated in terms of physically meaningful quantities: (i) energy differences, (ii) restrictions on slopes (e.g., an upper bound on internal forces), and (iii) restrictions on curvatures (e.g., upper bounds on force constants) along potential curves.
