Abstract
In modern transition state theory, the rate constant for an electron transfer reaction is expressed as the product of four factors: an exponential factor, a pre-exponential factor, an electronic transmission coefficient, and a nuclear transmission coefficient. The activation energy of the reaction manifests inside the exponential factor, and on the conventional view, catalysis occurs by decreasing this activation energy below its catalyst-free value. In the present work we report the discovery of an unusual counter-example in which catalysis occurs by increasing the electron transmission coefficient far above its catalyst-free value. The mechanism involves the formation of a superexchange bridge between an electron donor (a graphite cathode) and an electron acceptor (a pentasulfide ion). The bridge consists of a dz2 orbital inside a cobalt phthalocyanine molecule. The dramatic result is the acceleration of the reduction of pentasulfide ions by more than 5 orders of magnitude compared with the catalyst-fre...
Highlights
Chemistry is the study of change at the molecular level
This shows the cyclic voltammetry of a 1.29 mol L−1 Na2S4.1 solution recorded on a series of lowsurface-area graphite composite electrodes containing different percentages of cobalt phthalocyanine
It is further evident that the EC reaction can be powerfully catalyzed by cobalt phthalocyanine
Summary
Chemistry is the study of change at the molecular level. Of special interest to chemists are the mechanisms of chemical reactions and the speeds at which they occur. Complicating matters still further, each individual molecular trajectory involves random changes of atomic and electronic configurations that occur on vastly different time scales. In the absence of a definitive method of solving the equations of chemical kinetics, theoreticians have developed a vast array of approximate methods. The physical configuration of the system inside the bottleneck is referred to as the transition state, and this gives the theory its name. An attractive feature of transition state theory is that it eliminates the need to compute a vast number of reaction trajectories that end in failure. Instead, it estimates the reaction rate constant from the molar Gibbs energy of activation via the heuristic equation: k
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