Quinones cyclic voltammograms

In order to explain the formation of the product 9 during oxidation at two different potentials we have performed the experiments with cyclic voltammetry [45]. The cyclic voltammogram of catechol (QH2) exhibits the anodic wave at 0.25 V vs SCE (Fig. la) corresponding to the formation of o-quinone (Q) which is reduced in the cathodic sweep at 0.05 V vs SCE. The cathodic counterpart of the anodic peak disappears, when a sufficient amount of 4-hydroxycoumarin was added, and a second irreversible peak at 0.95 V vs SCE appeared (Fig. lb). 

Figure 18. Cyclic voltammograms of 1,4-benzoquinone (p-quinone) as permeability marker. Curve A in the absence of a cyclodextrin monolayer on a buffer solution containing no guest (p.1 M CH3C02Na-CH3C00H, pH 6.0). Curve B in the presence of the condensed monolayer of p-cyclodextrin derivative 41 on a buffer solution containing no guest. Curve C-E in the presence of the condensed monolayer of 41 on a buffer solution containing guest 59 at concentrations of 5.0 x 10", 1.0 x 10 , and 2.0 x 10 M, respectively (reprinted with permission from Anal. Chem. 1993, 65, 930. Copyright 1993 American Chemical Society).

Figure 12.2 illustrates the cyclic voltammograms for (a) 3,5-di-terf-butyl-o-quinone (3,5-DBTQ), (b) its anion radical (3,5-DTBSQT), and (c)... 

The azo function [e.g., azobenzene (PhN=NPh)] is reduced in a manner that is similar to that for quinones (discussed above). The electrochemistry for azo groups is a part of the discussion of the nitrogen compounds in Chapter 11 (Figure 11.10 illustrates the cyclic voltammogram for azobenzene). 

Table 12.4 summarizes the voltammetric oxidation potentials and peak currents for l,4-(MeO)2Ph and other alkoxy-substituted benzenes, phenols, and benzyl alcohols. Only the 1,4-(MeO)2PhX members of the series exhibit an initial irreversible anodic cyclic voltammogram via the sequence of Eq. (12.37). These plus the l,2-(MeO)2Ph isomer yield a metastable product from the second oxidation [species A, Eq. (12.37)] that undergoes a reversible reduction. Thus, the two-electron oxidation of dimethoxy benzenes yields the corresponding quinone. 

Fig. 18. Cyclic voltammograms for a polycrystalline diamond electrode exposed to ICC4 M solutions of (a) Fe(CN)63-, 4 (b) Ru(NH3)62+,3+ and (c) IrCb2- 3- on the background of 0.1 M KC1. (d) Quinone/hydroquinone on the background of 0.1 M HC104. Reprinted with permission from [92], Copyright (1997) American Chemical Society.

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

A report that hydroxytamoxifen can form quinone methides (QM) as a result of bio-oxidation [151] inspired an electrochemical study of some of the compounds discussed in the structure-activity relationship study ([128, 129], reviewed in [152]). In MeOH medium alone (these compounds are only sparingly soluble in water), the cyclic voltammograms (CV) of most of the compounds exhibited the expected reversible Fc/Fc+ redox couple, often followed by that of the phenolic moiety (where appropriate). However, when an organic base such as pyridine was added, two distinct types of electrochemical behaviour were observed. In the cases of the compounds which showed low or no cytotoxic effects in vitro, very little change in the CV was observed upon the addition of base. However, for the most... 

B) shows cyclic voltammograms of the la/lb-monolayer in the electro-chemically active traws-quinone (la)-state after irradiation, X > 430 nm (curve b), and the electrochemically inactive itmt-quinone (Ib)-state produced upon irradiation, 305 nm < A, < 320 nm (curve c). In the presence of the lb-monolayer, only the background current of the electrolyte is observed, implying that this photoisomer monolayer is redox-inactive within this potential range. By the cyclic photoisomerization of the monoiayer between the la and lb states, the transduced current is switched reversibly between ON and OFF -states [Figure 7.3 (B, inset)]. 

The absence of silver oxidation and/or reduction peaks is evidence for the electrochemical inactivity of the silver deposited on this carbon (in the form of metallic crystallites). The cyclic voltammogram recorded for the D—Ox carbon (Fig. 50, curve 2) exhibits two anodic peaks (fp., = +0.27 V, p,a = +0.77 V) due to the oxidation of adsorbed silver and surface hydroquinone-like groups, respectively. A single cathodic peak (Ep,) = +0.16 V) is due to the reduction of quinone-like surface groups according to Scheme 19. The large cathodic reduction wave confirms the presence of adsorbed silver cations and their reduction... 

The cyclic voltammograms of all the carbons carrying preadsorbed silver, recorded in dilute nitric acid solution (Fig. 51), exhibit a Ag"/Ag" couple (cathodic wave < +0.4 V and an anodic response in the +0.40-0.60 V potential range), as well as the electroactive quinone/hydroquinone-like surface system ( p, s +0.50 V p.a2 = +0.90 V). The presence of distinctly shaped anodic silver oxidation peaks indicates the partial solution of sorbed (deposited) metal. An almost sixfold higher anodic peak for D—Ox carbon confirms the partially ionic form of the adsorbed silver. 

Several publications on electrochemical mechanistic studies of the oxidative transformations of catecholamines followed the contribution by R. N. Adam s group (256) and involved a-methyldopamine, a-methylnor-adrenaline, dopamine (257), a-methyldopa, 5,6-dihydroxy-2-methylin-dole (255), and dopa (259). These studies (257) (Scheme 5), which confirmed the validity of the melanization scheme by Mason and Raper (Ref. 7, p. 50), explored the pH effect on the sequence of events that characterize the electrooxidation of catecholamines. Thus, the cyclic voltammogram in I M HCIO4 (pH 0.6) shows only peaks corresponding to the catechol-quinone redox couple as the protonation of the amino group prevents the cyclization step. 

Fig. 19. A Assembly of a phenoxynaphthacenequinone 1-tetradecanethiol mixed monolayer on a Au electrode and its photoisomerization. B Cyclic voltammograms of the trans-quinone monolayer (42a) a before rigidification with tetradecanethiol and b after rigidification with tetradecanethiol. c Cyclic voltammogram of the mixed monolayer after photoisomerization of the frans-quinone to the ana-quinone state. Cyclic voltammograms were recorded in 0.01 M phosphate buffer (pH 7.0) with a potential scan rate of 50 mV s-1. Inset Switching behavior of the cathodic peak current in the cyclic voltammogram of the mixed monolayer upon reversible photoisomerization...

Fig. 20. A Use of dibenzylviologen (43) to amplify the electrochemical signal of the photoswitchable phenoxynaphthacenequinone/tetradecanethiol mixed monolayer. B Cyclic voltammograms of the electrode in the presence of benzyl viologen (1 mM) a in the trans-quinone state and b in the ana-quinone state. Recorded at pH 7.5, scan rate 5 mV s-1. Inset Photoswitching behavior of the electrocatalytic current...

While we characterized this conformational transition using nuclear magnetic resonance investigations, the cyclic voltammograms of MQC exhibits two clear reversible redox reactions (Figure 23). In aprotic media, quinones... 

The cyclic voltammograms of MQC exhibits two clear reversible redox reactions (Fig. 34.14). In aprotic media, quinones exhibit two reduction peaks separated by 0.7 V, which corresponds to the formation of a radical anion species and a dianion species of quinones, respectively. This is in agreement with the reduction characteristics of MQC. Two well-separated reduced states of MQC are formed in the aprotic solvent of acetonitrile upon reduction. Therefore, the electronic states of MQC and MHQC can be easily transformed into each other by simple electrochemical control of the redox reaction, which results in large conformational flapping motions due to a preference for the stable conformation caused by the change in the electronic state of the quinone moiety. 

As already outlined in the section which covers colorimetric assays based on redox reactions, orffto-diphenols are more easily oxidized to the respective quinones than meta-diphenols. This has been confirmed by recording cyclic voltammograms of (-)-epicatechin. Two separate oxidation... 

The formation of carbon surface oxides, phenols, quinones, lactones, and car-boxyhc acids upon the electrooxidation of carbon has been detected by physical methods such as infrared spectroscopy [262], ellipsometry [263], x-ray photoelectron spectroscopy [262,264,265], thermal desorption, and electrochemistry (see refs. [8, 96, 248, and 261] and references therein). Cyclic voltammograms of oxidized carbons exhibit increased charge in the potential interval from 0.4 to... 

A cyclic voltammogram of 2,3,5-TMHQ in acetonitrile is shown in Figure 18. Peak Ic is a reversible e reduction of a-tocopherylquinone or its model quinone (Q) to an anion radical (Q ) [Eq. 11a)] and peak lie the further reversible e reduction of the anion radical to a dianion [Q", Eq. (11b)]. 

Figure 19. Cyclic voltammogram at a platinum electrode of 1.4 mM 2,3,5-trimethyl-6-(3 -methyl-3 -hydroxybutyl)quinone and 2.8 mM ethyl malonate in acetonitrile containing 0.1 M tetraethyl-ammonium perchlorate as supporting electrolyte. Sweep rate 83.5 mV s (Reprinted with permission of Elsevier Publishers, Amsterdam).

FIGURE 11.3. (a) Structure of poly(ether amine quinone). (b) Cyclic voltammogram for the cross-linked poly(ether amine quinone)/glucose oxidase/carbon paste electrode in phosphate buffer of pH 7.0 scan rate 5 mV/s, added electrolyte 0.1 M KCl (I) glucose absent and (II) 0.1-M glucose present. (From Ref. 53). 

---