Anodic spin trapping experiments

The electrochemistry of RH-Nu systems is well established (Eberson and Nyberg, 1976 Eberson et al., 1991 Childs et al., 1991). The radical cation mechanism has been shown to prevail for most situations where Nu = F , Cl-, RCOCT, OCN , CN , NO-r, Py and triethyl phosphite, all of them nucleophiles that are difficult to oxidize (Table 5). The initial formation of Nu is indicated for the redox-reactive SCN , NJ, I- and N02, with Br and (N02)3C occupying a somewhat indeterminate position. 

Tetrabutylboride ion, Bu4B , is oxidized anodically with pa = 0.35 V and thus combines low nucleophilicity with high redox reactivity. Electrolysis of its 

Nu ST Anode potential/V vs SCE Spin adduct formed Probable mechanism 

In principle, this reaction is a good model for the design of a proper spin trapping situation in an oxidative system (see Section 16). The radical to be trapped is formed from the initially reacting species in a secondary reaction, and the outcome of this reaction is not of a type that is likely to result from PBN + in a single step (reaction (35)) even if there were a chance that PBN + would be formed. The low anode potential additionally refutes the latter possibility. 

Somewhat surprisingly, no spin adduct was seen from the oxidation of Ph4B ( pa = 0.92 V) under similar conditions, the anode potential being varied between 0.5 and 2.2 V. Since Ph-PBN could be independently formed in a thermal reaction and was stable under the anodic conditions used, and Ph was judged to be electroinactive, it was concluded that Ph4B decomposed intramolecularly with direct formation of biphenyl. 

---

Spin trapping experiments have been performed recently in a fuel cell inserted in the ESR resonator ("in situ" cell), using DMPO and a-(4-pyridyl-l-oxide)-N-ferf-butylnitrone (POBN) as the spin traps [78,82,83], These experiments allowed the separate examination of spin adducts at the anode and cathode sides. 

Table 11 summarizes results of spin trapping experiments where PBN-Nu and other ST-Nu" systems have been oxidized anodically at platinum. Originally, all the reactions were suggested to proceed via Nu radicals (Janzen et al., 1980 Walter et al., 1982), but the fact that PBN is oxidized at a lower potential than Cl-, CNO and CN- (Tables 1 and 5) clearly shows that the faster electrochemical process must be PBN— PBN + at the potentials employed. On the other hand, azide ion is oxidized in a faster reaction than any of the spin traps used and thus azide radical is implicated as being trapped. The Cr 4MePyPBN [17] system is a case where possibly Cl is involved in view of the high pa of this spin trap. 

Chen et al. [16] achieved complete mineralization of trichloroethylene at an Ebonex anode, the chief products being CO2 (and about 10 % CO), chloride, and chlorate. The authors determined the fractions of the anodic current carried out to form O2, carbon oxides, and CIOs and observed the expected relationship that the current efficiency for TCE oxidation decreased at the expense of that for O2 with increasing anode potential. Spin trapping experiments were... 

Experiments were performed in an in situ FC inserted in the resonator of the ESR spectrometer that allowed separate monitoring, by spin trapping with DMPO, of radical formation at anode and cathode sides. The FC was operated at 300 K imder CCV and OCV conditions, respectively, with an MEA based on Nafion 10% neutralized by Ce(III) and Pt as catalyst, notation MEA/Ce. Results were compared to an in situ FC based on Nafion in the acid form, notation MEA/H, as baseline. The differences between the two types of experiments are significant. Figure 28 shows the presence of different radicals HO" for MEA/H and HOO for MEA/Ce. 

---