Reduction polarographic half-waves

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). 

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... 

Quantitative structure-chemical reactivity relationships (QSRR). Chemical reactivities involve the formation and/or cleavage of chemical bonds. Examples of chemical reactivity data are equilibrium constants, rate constants, polarographic half wave potentials and oxidation-reduction potentials. 

For an irreversible reduction the half-wave potential is determined not only by the standard electrode potential but also by the polarographic overvoltage. For a simple electrode process the metal ion-solvent interaction is mainly responsible for the polarographic overvoltage and hence E[ j of such irreversible reductions may also be considered as a function of the solvation 119f... 

Thermodynamic reduction potentials of numerous aromatics were first measured by Hoijtink and van Schooten in 96% aqueous dioxane, using polarography [15, 16]. These fundamental works were decisive tests of the HMO theory, showing that the polarographic half-wave potentials vary linearly with the HMO energies of the lowest unoccupied molecular orbitals (LUMO) of the hydrocarbons [1]. Hoijtink etal. had already noticed that most aromatics can be further reduced to their respective dianions [17]. They proposed a... 

Conjugation with an electron-withdrawing group substantially lowers the energy of the lowest unoccupied molecular n-orbital, which results in less negative reduction potentials for the alkene system. The class of compounds is referred to as activated alkenes, Polarographic half-wave potentials for some activated alkenes in aprotic solvents are listed in Table 3.3... 

Polarographic half-wave potentials for the reduction of vjc-dibromides at mercury in dimethylformamide with a tetraalkylammonium salt electrolyte. 

Polarographic half-wave potentials Table 4.8) for the reduction of 1,2-dibromides are sensitive to the relative stereochemistry of the halide substituents... 

Tetrasubstituted onium salts are cleaved by reduction and lose the substituent which gives the most stable radical. Polarographic half-wave potentials for some of... 

Reduction of sulphonium salts polarographic half-wave potentials, Ey. ref. [54], in water cyclic voltammetry peak potentials, Ep ref. [55], in acetonitrile at glassy carbon, scan rate 50 mV s. ... 

Polarographic half-wave potentials (Ev , / V vs. see) and number of electrons involved (n) for reduction of one molecule of sulphoxides in methanol and in dimethylformamide. 

Fig. 10. Plot of data for external heavy-atom quenching of pyrene molecular fluorescence in benzene at 20°C according to Eq. (35) polarographic half-wave reduction potential E/2 is taken as measure of electron affinity of quenching species containing chlorine (O), bromine ( ), or iodine (O) atoms (Thomaz and Stevens52).

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

As noted earlier, the polarographic half-wave potential Estandard redox potential of the couple because EVl is governed by kinetic as well as thermodynamic parameters, so that attempts to correlate EVl for irreversible couples with structural features of the complex have been very difficult to obtain. One moderately successful correlation, however, was obtained by Vlcek13 and by Crow14 for a fairly extensive series of complexes of Co111, Crm and Rhm. It was suggested that before reduction of the metal complex... 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

Standard emf Values for the Cell H2/HCl/AgCl, Ag in Various Aqueous Solutions of Organic Solvents at Various Temperatures Temperature Dependence of the Standard Potential of the Silver Chloride Electrode Standard Electrode Potentials of Electrodes of the First Kind Standard Electrode Potentials of Electrodes of the Second Kind Polarographic Half-Wave Potentials (E1/2) of Inorganic Cations Polarographic E1/2 Ranges (in V vs. SCE) for the Reduction of Benzene Derivatives Vapor Pressure of Mercury... 

Polarographic half-wave potentials for the reduction of various monomeric aquachlororuthen-ium(III) complexes [RuCl (OH2)6 ](3, )+ (0 < 3) have been reported. The various Ru 1... 

Figure 3. Polarographic half-waves for the reduction of 03 wiM Cu(II) in a mineral oil/1-pentanol/SCS microemulsion (60% water) containing various concentrations of quinoline (Q). Curve 1, (Q) = 0 Curve 2, (Q) = 32 mM and Curve 3,...

For reduction of dicarboxylic acids in acetone solution, polarographic half-wave potentials and limiting currents are dependent on concentration in a way that suggests that at low concentration the acids are sufficiently dissociated for reduction of solvated protons to predominate, whereas at higher concentrations the more difficult reduction of undissociated acid is observed [10]. 

We have already seen (see Section 18C-6) that the potential for the oxidation or reduction of a metallic ion is greatly affected by the presence of species that form complexes with that ion. It is not surprising to find that similar effects are observed with polarographic half-wave potentials. The data in Table 23-1. show clearly that the half-wave potential for the reduction of a metal complex is generally more negative than that for reduction of the corresponding simple metal ion. In fact, this negative shift in potential permits us to determine the composition of the complex ion and its formation constant provided that the electrode reaction is reversible. Thus, for the reactions... 

Polarographic half-wave reduction potential measured in aprotic solvents. 

Where reduction is reversible, E can be considered equal to the polarographic half-wave potential Ei/g The exact relationship between E1/2... 

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