ELECTROCHEMISTRY AT OMEGA-HYDROXY THIOL COATED ELECTRODES .2. MEASUREMENT OF THE DENSITY OF ELECTRONIC STATES DISTRIBUTIONS FOR SEVERAL OUTER-SPHERE REDOX COUPLES

The use of Au electrodes blocked with omega-hydroxy thiols for kinetics measurements at large electrode overpotentials is discussed. The adsorbed monolayer acts as a tunneling barrier slowing the rate of electron transfer allowing the measurement of redox kinetics at electrode overpotentials over 1 V without mass-transfer limitations. The cathodic heterogeneous electron-transfer kinetics for a series of six outer-sphere transition-metal complexes are measured at Au electrodes derivatized by a self-assembled organic monolayer of 16-hydroxy-1-hexadecylmercaptan from aqueous solutions. Plots of the logarithm of the apparent heterogeneous electron-transfer rate constants with electrode overpotential for all complexes studied display strong negative curvature at high electrode overpotentials. These data are analyzed by using the approach of Bennett to obtain the density of electronic states for the oxidized form of these complexes. The density of electronic states are well described by Gaussian distributions and are compared to previous determinations and the predictions of Marcus theory.