Irreversible processes multistep mechanisms

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

However, later an alternative mechanism was established based on theoretical and experimental data that took into account the enhanced aromaticity of anion 7 (cf. Section 6.07.4.1). According to this mechanism, the interaction between tetrazolate anion and electrophile is a slow reaction stage of the multistep process. This bimolecular slow stage conceivably results in the formation of a labile intermediate 230, which fast and irreversibly transforms into isomeric tetrazoles 5 and 6 (Scheme 23) <2000H(53)1421>. 

Because this mechanism introduces a second electroactive species (B) into solution, its electron transfers are subject to the complexities of redox processes. Its reduction may be irreversible, it may itself reduce (or oxidize) by a multistep or ECE mechanism, etc. Only the simplest cases are treated above. 

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