Ultrafast Electrochemical Techniques

In transient electrochemical measurements involving a solution phase redox couple, one seeks to create a competition between the reaction of interest, electron transfer at the electrode surface or coupled homogeneous steps, and diffusion of the species to and from the electrode surface (56). 

Despite the many elegant investigations that have been conducted on heterogeneous electron transfer dynamics of solution phase reactants, the magnitude of the diffusion-controlled current at short times ultimately places a lower limit on the accessible timescale. For diffusive species, the thickness of the diffusion layer, 8, is defined as = (nDty (equation (6.1.5.1)) and is, therefore, proportional to the square root of the polarization time, t. One can estimate that the diffusion layer thickness is approximately 50 A if the diffusion coefficient is 1 X 10 cm sec and the polarization time is 10 nsec. Given a typical bulk concentration of the electroactive species of 1 mM, this analysis reveals that only 10,000 molecules would be oxidized or reduced at a 1-pm radius microdisk under these conditions. The average current for this experiment is only 170 nA, which is too small to be detected with low nanosecond time resolution. 

The decrease in the lower accessible time limit possible using microelectrodes has important implications for probing the dynamics of rapid homogeneous chemical reactions. 

Step chronoamperometry (DPSC) conducted on a microsecond timescale reveal that l-methyl-3-carbamidopyridinyl radicals react via a dimerisation mechanism involving direct coupling of the electrogenerated neutral radicals at a rate of approximately 1.6 0.1 X 10 sec in DMF. The l-methyl-4-carbamidopyridinyl and l-methyl-3,4-dicarbamidopyridinyl radicals react via a pH-dependent ECE-DISPI mechanism, where E, C and DISP denote electron transfer, following chemical and disproportionation reactions, respectively. 

Steady-state measurements are important in studies of chemical reactivity. Steady-state measurements using electrodes of different radii provide insight into the kinetics of 

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Forster RJ (2000) Ultrafast electrochemical techniques. In Meyers R (ed.) Encyclopaedia of Analytical Chemistry, pp. 10142-10171. New York Wiley. 

Forster RJ (2006) Ultrafast electrochemical techniques. Encyclopedia of emalytical chemistry. Wiley, New York, NY. doi 10.1002/9780470027318.a5319... 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Most of the early investigations of interfacial ET rates were based on electrochemical current or voltage measurements (Lewis, 1991 Miller et al, 1995 Nozik and Memming, 1996 Sachs et al, 1997). The advantages and limitations of these methods are detailed in Chapter 12. It is often difficult to measure ET rates on the snb-nanosecond timescale with these techniqnes. Eor ET in this nltrafast regime, techniqnes based on direct nltrafast time-resolved spectroscopy are now extensively used. A comprehensive description of the method and application of ultrafast spectroscopy can be fonnd in text books (Fleming, 1986 Rnlliere, 2003). In this section, we describe some techniques that have been used for interfacial ET studies. 

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