Interpretation of rate optima vs reaction parameters in steady state catalytic kinetics: Molecular aspects beyond concentration dependences

Abstract

Steady state catalytic reactions often exhibit rate maxima not only as a function of concentration, but also of other reaction parameters such as solvent polarity, Hammett substitution constants or metal cluster size. Analysis of several generic schemes operative in homogeneous and heterogeneous catalysis and comprise linear steps was made showing that kinetic expressions after incorporating dependence of rate constants on abovementioned reaction parameters can be used to model experimental data. Rate minima in steady state reactions depending on substituent constants cannot be easily explained by single route conventional mechanisms even after considering nonlinear steps. Such cases are conventionally explained by a switch in reaction mechanisms. An alternative developed in this study assumes presence of mechanistically similar parallel routes involving at least two different catalytically active species. The advanced model based on two step sequence is capable of explaining concave upward Hammett plots.