Reduction Wave

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

Figure 5.16. Plot of data for the external heavy-atom quenching of pyrene fluorescence in benzene at 20°C. Polaro-graphic half-wave reduction potentials Ein are used as a measure of the electron affinity of the quencher containing chlorine (O), bromine ( ), or iodine (3). From Thomaz and Stevens<148) with permission of W. A. Benjamin, New York.

From measurements of this type Thomaz and Stevens found a linear relationship for a graph of log(kJrPC,2) vs. where n is the number of halogen atoms in the molecule, is the spin-orbit coupling constant, and Em is the polarographic half-wave reduction potential of the heavy-atom quencher (Figure 5.16). This correlation suggests that an exciplex is formed by partial... 

It is interesting to note that cleavage takes place at the GeX rather than the GeR bond. In this case, as with the direct reduction, the ET step (k2) is rate-determining whereas k3 is fast and, despite the fact that reduction potentials of R3GeX are somewhat dependent on X, k2 could be calculated (Table 16). It is noted, however, that tabulated half-wave reduction potentials of the mediators appear to be higher than those of R3GeX (compare Table 16 with Table 3) though they appear very close on the CV trace. 

TABLE 16. Reduction of R3GeX in the presence of mediators A. Half-wave reduction potentials of A (vs SCE), and calculated rates k2, of regeneration of A187. 

We know that reduction potentials of arylsilanes are less negative than those of the corresponding aromatic compounds without silyl substituents (Sect. 2.2.1). The effect of silyl groups to facilitate the electron transfer to the neigbouring aromatic group is explained in terms of p interaction. For example, half wave reduction potentials of naphthylsilanes are less negative than that of... 

The same trends are expected to be found when the atom X is varied along another column of the Periodic Table, i.e. decreasing electronegativity of X along a column should lead to lower ctcx energy. Thus, for example, in the reductive C—X bond cleavage of PhCOCRR XPh, the half wave reduction potential is less for X=S than X=042). Relevant computational results are shown in Table 4. 

The above predictions are partially confirmed by the available electrochemical data. Specifically, the half wave reduction potentials for cis and trans 1,2-disubsti-tuted ethylenes have been measured and it is found that it is easier to reduce the trans isomer, i.e. E, > E°fa243 248). 

The half-wave reduction potentials (HWP) of dibenzothiophene and some of its derivatives have been measured for comparison with those of dibenzofuran and dibenzoselenophene. A shift to more negative HWP was observed for all of the methyl derivatives studied, the magnitude of which depended on the position of substitution. These shifts are in accord with LCAO molecular orbital theory predictions if the sulfur d orbitals are excluded from the calculations (Section III, A). ... 

The half-wave reduction potentials for a series of annelated 1,4 naphthoquinones (102-106) increase upon alkylation, and decrease as ring size decreases (Table 13). The more cathodic reduction potentials of 2,3-dimethylnaphtho-l,4-qui-none (106, 0.846 V) and l,2,3,4-tetrahydro-9,10-anthroquinone (105,0.854 V) as compared to 1,4-naphthoquinone (0.685 V) are expected from inductive electron donation of alkyl groups. A decrease in reduction potential from 105 to 2,3-cyclobutanaphtho-l,4-quinone (103) (0.695 V) as ring size decreases is observed such that the reduction potential of 103 is only slightly higher than the parent 1,4-naphthoquinone. 

Tab. 2 Half-wave reduction potentials (in V vs. Fc/Fc ) of C q using various solvents, supporting electrolytes, and temperatures...

Tab. 13 Half-wave reduction potentials at room temperature (in V vs. Fc/Fc+) of representative examples of cycloaddition derivatives of Cgo with oxygen-, carbon- and silicon-containing groups...

Plotting of the half wave reduction potentials (s1/2) of hexa- (— 1.49 V), hepta- (— 1.44), octa- (—1.43) and nonahelicene (—1.40), and benzo[i] (—1.44), benzo[l] (—1.43), benzo[c] (—1.48) and benzo[b]hexahelicene (—1.17) against the HMO electron affinities (—mm+1) gives a linear relation (e1/2 = 2.48mm+1 — 0.16)136). The slope of the line is equal to that given by Streitwieser 137) for a similar relation for planar aromatic molecules, but the intercept is different probably due to differences in entropy and solvation energy of molecules and anions which are incomparable for helicenes and planar aromatics. 

Figure 13.5 Correlation of the half-wave reduction potentials obtained for rotaxane l4...

Figure 13.34 Correlation between the half-wave reduction potentials of catenane 394+ and of its tetracationic ring component. Circles and squares correspond to the reduction of bipyridinium and /ra 1v-bis(pyridinium)ethylene units, respectively.

Half-Wave Reduction Potentials of Monoaza-Heterocycles... 

From the results already given, it can be seen that the ease of polarographic reduction depends on the ring position of the reducible group and also on the positions and electronic nature of other substituents. It was readily shown that the polar effects of the substituents on the half-wave reduction potentials could be expressed in the general form of the Hammett equation. 

Table 1. Half-wave reduction potentials of fluorinated alkenes (-E1/2, V vs SCE) [24]...

The solution electrochemistry is now well developed (Haufler et al. 1990 Allemand et al. 1991 Cox et al. 1991 Dubois et al. 1991 Miller et al. 1992 Wudl 1992) and the most recent experiments have shown the presence of six reversible reduction waves (Xie et al. 1992). There is a spread in the published half-wave reduction potentials as... 

Extensive mechanistic studies on the effect of substituents on hydride reductions of 2,4,6-triarylpyryliums with NaBELj and NaCNBH3 have been described. These studies determined first- and second-wave reduction potentials and also rates for each reaction. Comparisons of the data sets leads to the conclusion that charge neutralization in the hydride addition transition state precedes B-H bonding changes <2002JP0689>. 

TABLE 3.7 Half-Wave Reduction Potentials for Hydrocarbons in 75% Dioxane-HjO and Energies of Lowest Vacant Molecular Orbitals... 

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