The basal plane of highly ordered pyrolytic graphite (HOPG) serves as an ordered model of more commonly used electrode surfaces such as glassy carbon (GC) and pyrolytic graphite. The defect density on the basal plane HOPG was reduced by careful cleaving and cell design and was verified to be low by requiring that AEp for Fe(CN)6*123"/4" (1 M KCI) be greater than 700 mV for a 0.2 V/s scan rate. Then a variety of redox systems were examined on "validated" HOPG surfaces, and variations In the electron transfer rate constant, Ab°, were observed. All 13 redox systems exhibited relatively fast kinetics on laser activated GC (A® > 0.03 cm/s for eight Inorganic systems), and In several cases A® exceeded the Instrumental limit. On HOPG, however, A£p varied greatly for the 13 systems, ranging from 66 to >1200 mV. The reasons for this variation fall into three general classifications. First, reactions Involving proton transfer (e.g. catechols) were all slow on HOPG, Implying some role of the surface In mediating multistep processes. Second, the observed rate correlated with the exchange rate for homogeneous electron transfer, but the heterogeneous rates on HOPG were 3-5 orders of magnitude slower than that predicted from simple Marcus theory. Third, the physical properties of HOPG, such as density of electronic states and hydrophoblclty, may depress A® relative to GC and metals.

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The electron‐transfer kinetics for three aquated metal ions with low self‐exchange rates were examined on well‐characterized carbon surfaces. All three ions exhibited slow kinetics on clean fractured glassy carbon (GC) and on the basal plane of highly ordered pyrolytic graphite (HOPG). The reactions appeared to be outer sphere and had rates approximately consistent with those predicted from the self‐exchange rates. Electrochemical oxidation of either GC or HOPG greatly increased the electron‐transfer rate constant to values significantly greater than predicted for outer‐sphere reactions. The increase was greatest for Eunormalaq+3/+2 and was eliminated if the surface was silanized. Surface oxides provide sites for inner‐sphere catalysis of the aquated ions at carbon, perhaps involving a surface oxide structure similar to the common ligand acetoacetonate.

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ChemInform Abstract As shown by cyclic voltammetry the title complex is reversibly oxidized in an one-electron process, but a stable cation could not be generated. Upon chemical oxidation in MeOH, the complex undergoes oxidatively induced CO insertion. The reaction probably involves reversible electron-transfer followed by competing chemical reactions. Either insertion of CO followed by reaction with MeOH and possibly further oxidation yields C6F5COOMe or reaction with MeOH before CO insertion and further oxidation produces C6F5H.