Photomodulation voltammetry

The conception and building of a photomodulation voltammetry (PV) apparatus was stimulated by the need to measure redox potentials of short-lived 

One of the methods used in PV to produce the free radicals is similar to the one illustrated in chapter 13, that is, when a substance like di-tert-butylperoxide is irradiated with energy of a suitable wavelength, the 0-0 bond is cleaved and two tert-butoxyl radicals are generated. These radicals can then abstract a hydrogen atom from another molecule RH, yielding the radical R. 

An alternative method used to produce the radicals is through the photodecomposition of the ketone RC(0)R  

A source of problems in PV experiments is that the signals of other species associated with the production of R, including that of the precursor, may interfere 

The electrochemical technique used in PV is known as linear sweep voltammetry with a slow sweep rate. It can be shown [332] that under the conditions just described (a constant S) and for a reversible process, the applied potential (E) is related to the measured current ( ) by 

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Several electrochemical techniques may yield the reduction or oxidation potentials displayed in figure 16.1 [332-334], In this chapter, we examine and illustrate the application of two of those techniques cyclic voltammetry and photomodulation voltammetry. Both (particularly the former) have provided significant contributions to the thermochemical database. But before we do that, let us recall some basic ideas that link electrochemistry with thermodynamics. More in-depth views of this relationship are presented in some general physical-chemistry and thermodynamics textbooks [180,316]. A detailed discussion of theory and applications of electrochemistry may be found in more specialized works [332-334],... 

Figure 16.11 Diagram of a photomodulation voltammetry apparatus. Adapted from [351],...

Reduction potentials of radicals may be determined by pulse radiolysis (Chap. 13.3) or photomodulated voltammetry (Wayner and Houman 1998 for a compilation, see Steenken 1985 Wardman 1989). 

Oxidation potentials for radicals have been determined by photomodulation voltammetry to be in the range of -1.03 V (vs see) for the A, A -dimethylamino-methylradical, —0.24 V for methoxymethyl, 0.09 V for fbutyl, 0.35 V for diphe-nylmethyl, 0.73 V for benzyl, <0.99 V for ethyl and <2.49 V for methyl [137]. In anodic addition of anions to double bonds it has been found that, depending on the oxidation potential of the adduct radical, either radical- or carbocation-derived products are obtained (Section 2.6.3.4). 

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