Reversibility quasi

Figure Bl.28.7. Schematic shape of steady-state voltaimnograms for reversible, quasi-reversible and irreversible electrode reactions.

Fig. 22. The capacity of the one volt plateau measured during the second cycle of several series of samples versus the H/C atomic ratio in the samples. The solid line suggests that each lithium atom binds quasi-reversibly to one hydrogen atom.

Tire deprotonation of thiazolium salts (see Section II) under argon at room temperature allowed the characterization of nonfused DTDAF of types 52 and 53 by cyclic voltammetry. Their very good donor properties were confirmed by two quasi-reversible peaks of equal intensity (93CC601). It is noteworthy that upon a second scan the first oxidation peak was shifted from -0.03 to -0.04 V. Upon further scans the voltam-mogram remains unchanged. Tliis interesting feature has been observed previously with TTF analogs. It was demonstrated that the neutral form... 

Irreversible and Quasi-Reversible Systems For irreversible processes (those with sluggish electron exchange), the individual peaks are reduced in size and widely separated (Figure 2-5, curve A). Totally irreversible systems are characterized by a shift of the peak potential with the scan rate ... 

FIGURE 2-5 Cyclic voltaimnograms for irreversible (curve A) and quasi-reversible (curve B) redox processes. 

For quasi-reversible systems (with 10 1 > k" > 10 5 cm s1) the current is controlled by both the charge transfer and mass transport. The shape of the cyclic voltammogram is a function of k°/ JnaD (where a = nFv/RT). As k"/s/naD increases, the process approaches the reversible case. For small values of k°/+JnaD (i.e., at very fast i>) the system exhibits an irreversible behavior. Overall, the voltaimnograms of a quasi-reversible system are more drawn-out and exhibit a larger separation in peak potentials compared to those of a reversible system (Figure 2-5, curve B). 

Equations have been derived for less ideal situations, involving quasi-reversible and irreversible adsorbing electroactive molecules and different strengths of adsorption... 

Suimnarize the different features of the cyclic voltammogramic response for reversible and quasi-reversible systems. 

For quasi-reversible systems the limiting current is controlled by both mass transport and charge transfer ... 

Quasi-reversible systems, 32 Quaternary ammonium salts, 153 Quinliydrone electrode, 151... 

The chalcogen-capped clusters M M2Co(/(3-S)(CO)s()/ -CsH5) (M M2 = MoEc, MoRu, WFe) and MoFeCo(/r ,-Se)(CO)H()) -CsH3) underw ent a one-electron, quasi-reversible reduction. Addition of an electron proceeded more readily for the clusters w ith the lighter metals and for the selenium capped cluster relative to its sulfur analogue. 

Gold(I) ylides are oxidized in 0.1 M [Bu4N]BF4/THFat low potentials of +0.11 and + 0.23 V vs. Ag/AgCl (quasi-reversible). The dinuclear amidinate oxidizes under the same conditions at + 1.24 V vs. Ag/AgCl (reversible). These large differences in chemical character of the dinuclear gold(I) complexes appear to explain the widely different behavior of these compounds and especially toward the reaction with mercury cyanide. 

The electron transfer Au(R2voltametric measurements 163). The half-wave potentials of the quasi-reversible process depends on the substituent R according to the Taft relation, as was described for Mo, W and Mn 37). The value of p decreases in the series Au > Mn > Mo = W, which indicates that in this sequence the mixing of ligand orbitals into the redox orbital decreases. The dominant ligand character of the unpaired electron MO in Au(R2dtc)2 relative to those in copper and silver compounds is found from Extended Hiickel MO calculations, as will be discussed later on. 

This means that Ei lies about midway on the SV wave between Ev and Epl2 at the high, i.e. positive, side, as we have already mentioned in connection with Fig. 3.64. However, in practice and on the basis of the above arguments (1)—(3), the position of E, will deviate from this more or less theoretical approximation where the sphericity term of eqn. 3.80 has been neglected. Further, with incompletely reversible redox couples, e.g. for quasi-reversible systems (cf., pp. 125-126), there will of course be more deviations, but at any rate the linear relationship between ip and C appears to remain. 

Figure 8.16 Reaction of a serine (3-lactamase with sulbactam. The central intermediate can go on to form products, can transiently inhibit the enzyme in a quasi-reversible fashion, or can irreversibly inactivate the enzyme.

The redox chemistry of several phosphaferrocenes,31,50 l,l -diarsaferro-cene (7),13 the complete series of 2,2, 5,5 -tetramethyl-l,r-diheteroferro-cenes (89, 26, 29, 32),22 and octamethyl-1,1 -diheteroferrocenes (90, 44, 48, 49)22 has been investigated by cyclic voltammetry. These compounds undergo quasi-reversible one-electron oxidations (0/+) to their radical cations and irreversible one-electron reductions (0/-) to their radical anions. The data are summarized in Table VI. 

Square planar Ni11 complexes (50a) and (50b) of the quinoxaline-2,3-dithiolate ligand are oxidizable in chemically reversible, electrochemically quasi-reversible processes to yield Ni111 species, also featuring the (dxy)1 configuration.198 Interestingly, the difference in protonation state makes for a 0.20V difference in oxidation potential ((50a) +0.12V (50b) +0.32V vs. SCE), consistent with the less basic S-donors in the thione form. 

Ni—Npyrazoie.572 Tridentate (167) forms five-coordinate [Ni(167)Cl2] with trigonal bipyramidal geometry, while (168) forms octahedral [Ni(168)(H20)2]Cl2. The presence of the basic phenantro-line allows the ligand to stabilize the Nirspecies as revealed by a quasi-reversible NiII/Nil wave at Eii2 = 0.46 V vs. AgCl/Ag+ (for [Ni(167)Cl2]).573... 

In thiosalen-type complexes the Ni11 ion usually has a four-coordinate square planar geometry.1360,1361 Complex [Ni(510)] shows a quasi-reversible reduction at —1.36 V and an irreversible... 

Reaction of the Ni11 thiolate species [Ni(L)] (L = /V,/V -diethyl-7V,7V -bis(2-mercaptoethyl)-l,3-propanediamine) with the tetraiodo cluster anion [Fe4S4I4]2 yields [Ni(L)(Fe4S4I2)(L)Ni] (793).1984 It incorporates a dithiolate bridge between Ni and Fe centers with a Ni—Fe distance of 2.827(1) A and exhibits a quasi-reversible oxidation wave at 1/2 = +0.15V (vs. SCE). The corresponding monosubstituted cluster anion [Ni(L)Fe4S4I3] (794) was also reported.1985... 

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

The present chapter will cover detailed studies of kinetic parameters of several reversible, quasi-reversible, and irreversible reactions accompanied by either single-electron charge transfer or multiple-electrons charge transfer. To evaluate the kinetic parameters for each step of electron charge transfer in any multistep reaction, the suitably developed and modified theory of faradaic rectification will be discussed. The results reported relate to the reactions at redox couple/metal, metal ion/metal, and metal ion/mercury interfaces in the audio and higher frequency ranges. The zero-point method has also been applied to some multiple-electron charge transfer reactions and, wheresoever possible, these results have been incorporated. Other related methods and applications will also be treated. 

The reduction of zinc ions at d.m.e. has widely been studied and the reaction has been reported to be quasi-reversible.94 Van Der Pol and co-workers54 studied this reaction by the faradaic rectification polarographic technique using high-frequency modulated signals. The kinetic parameters have been evaluated by the... 

In order to clarify redox properties of phosphaquinoid and phosphathienoquinoid compounds, electrochemical measurement and direct observation of the anion radical were carried out (Tables 3 and 4). Phosphaquinone 2 undergoes the first quasi-reversible reduction followed by the second irreversible reduction (Scheme 15) [9],... 

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