Rate coefficient cyclic oxides

At25 °C,k() = 0.24sec SfcH = 26l.mole sec , and oH = 2.6x 10 Lmole sec". The latter coefficient is about 1/40 of the rate coefficient for a diffusion controlled reaction in water. Kaiser and Weidman suggest that the intermediate is similar to the cyclic periodate ester postulated for the 1,2-diol oxidations, viz. 

For a cyclic voltammetric experiment at a potential scan rate v, involving oxidized and reduced species with identical diffusion coefficient, D, the peak current, i, is given by (Stromme et al., 1995) ... 

Figure 18. (A) Cyclic voltammetry of purified cytochrome c at doped indium oxide optically transparent electrodes. Solution contained 73 /uiM cytochrome c, 0.21 M Tris, 0.24 M cacodylic acid, pH 7.0, 0.20 M ionic strength. Electrode area = 0.71 cm. Potential scan rates in mV/s are (a) 100 (b) 50 (c) 20 (d) 10 (e) 5.0 (f) 2.0. (B) Derivative cyclic voltabsorptometry of purified cytochrome c at a tin-doped indium oxide optically transparent electrode. Same conditions as described above. Circles are calculated derivative cyclic voltabsorptometric responses for 73 /iM cytochrome c, formal potential = 0.260 V, n = 1.0, diffusion coefficient of oxidized and reduced cytochrome c = 1.2 x 10 cm /s, difference molar absorptivity at 416 nm = 57,000 cm" formal heterogeneous electron transfer rate constant = 1.0 x 10 cm/s, and electrochemical transfer coefficient = 0.5. Adapted from Reference (126) with permission.

Alwe HD, Walavalkar MP, Sharma A, Dhanya S, Naik PD. Tropospheric oxidation of cyclic unsaturated ethers in the day-time comparison of the reactions with Cl, OH and O3 based on the determination of their rate coefficients at 298 K. Atmos Environ. 2014 82 113-120. 

In this chapter, we discuss the rate coefficients and the mechanisms of oxidation of ketones. The classes covered include alkanones, hydroxyketones, diketones, unsaturated ketones, ketenes, cyclic ketones, ketones derived from biogenic compounds, and halogen-substituted ketones. Photolysis is a major atmospheric process for many ketones, and will be discussed in chapter IX. The major bimolecular reactions removing ketones from the atmosphere are with OH. Although less important than the OH... 

The electronic adsorption spectra for the complexes [Ir(OH)6]", where n = 0-2, have been resolved and peak maxima locations, molar extinction coefficients, oscillator strengths, and band half-widths calculated.44 Bands have been assigned in the main part to be one-electron MLCT transitions. Spectrophotometrically determined rate constants for the OH reduction of the IrVI and Irv complexes at 25 °C in 3M NaOH are (2.59 0.09) x 10—3 s—1 and (1.53 0.05) x 10 4 s 1 respectively. The activation energy for the reduction, Irv—>IrIV, is nAkcalmoC1. Cyclic voltammetry and potentiostatic coulometry of [Ir(OEI )r,]2 in 3M NaOH on a Pt electrode show that during the electro-oxidation compounds of Irv and IrVI are formed.45... 

As noted above, the duration of the retarding action of an inhibitor is directly proportional to the / value. In systems with a cyclic chain termination mechanism, the / coefficient depends on the ratio of the rate constants for two reactions, in which the inhibitor is regenerated and irreversibly consumed. In the oxidation of alcohols, aminyl radicals are consumed irreversibly via the reaction with nitroxyl radical formation (see earlier) and via the following reaction [11] ... 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

Using a computer, carry out simulations of cyclic voltammetry for a quasireversible system. Let = 50 and Dm = 0.45, Take a = 0.5 and let the diffusion coefficients of the oxidized and reduced forms be equal. Cast your dimensionless intrinsic rate parameter in terms of the function ij/ defined in (6.5.5), and carry out calculations for ip = 20, I, and 0.1. Compare the peak splittings in your simulated voltammograms with the values in Table 6.5.2. 

Laviron55 has recently noted that linear potential sweep or cyclic voltammetry does not appear to be the best method to determine the diffusion coefficient D of species migrating through a layer of finite thickness since measurements are based on the shape of the curves, which in turn depend on the rate of electron exchange with the electrode and on the uncompensated ohmic drop in the film. It has been established that chronopotentiometric transition times or current-time curves obtained when the potential is stepped well beyond the reduction or oxidation potential are not influenced by these factors.55 An expression for the chronopotentiometric transition has been derived for thin layer cells.66 Laviron55 has shown that for a space distributed redox electrode of thickness L, the transition time (r) is given implicitly by an expression of the form... 

To provide electronic coupling between the incorporated CoP(pyH)44+ counterions and the graphite electrode surface it was necessary to add to the coatings suitable redox mediators wiUi much larger diffusion coefficients. In Figive IB is shown the cyclic voltammetric response obtained from a Nafion coating which contained Os(bpy)32+ (bpy = 2,2-bipyridine) and Ru(NH3)e3+ as well as CoP(pyH)44+. The pair of current peaks near —0.25 V arises fix)m the Ru(NH3)63+/2+ couple and the pair near 0.6 V from the Os(bpy)33+/2+ coimle. This pair of mediators was chosen because they both have reasonable diflusional rates in Nafion and their formal potentials lie on either side of the formal potential of the CoP(pyH)4 couple (—O.IV) so that the cobalt center of the porphyrin could be repetitively cycled between its oxidized and reduced states by me mediator counterions which diffused to and from the electrode surface where they were oxidized or reduced. 

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