A quantum-mechanical study of charge transfer in redox catalysis: Decomposition of hydrogen peroxide on copper ammoniates and iron porphyrin

Abstract

Reactions of redox type at a catalyst are initiated by transfer of a charge δq, with accompanying change in the electronic structure of the reagent. It is shown that quantum chemistry provides estimates of δq sufficient for many qualitative and semiquantitative purposes, provided that the interaction is one of stationary coordination, or nearly so. The semiempirical approximation in the MO LCAO method is used with self-consistent charges in calculations for the decomposition of H₂O₂ on copper ammoniates. The results indicate indirectly that the reaction has a radical mechanism (δq ∼ 1 e in stationary coordination). An approximate analytic formula for δq is derived for a catalyst containing a large conjugated system (chelate or enzyme), in which a major part is played by the redox (catalytic) capacity of the system, which is defined as the derivative of the charge with respect to the Fermi level and which is easily calculated or determined empirically. This concept explains the high catalytic activity of such systems in redox processes. Detailed calculations are presented for the decomposition of H₂O₂ on iron porphyrin.