Disproportionation equilibria

These methods can give us useful information on radicals in a manner similar to that for closed-shell systems, provided the exploitation is correct. Of course, in expressions for total energy, bond orders, etc., a singly occupied orbital must be taken into account. One should be aware of areas where the simple methods give qualitatively incorrect pictures. The HMO method, for example, cannot estimate negative spin densities or disproportionation equilibria. On the other hand, esr spectra of thousands of radicals and radical ions have been interpreted successfully with HMO. On the basis of HMO orbital energies and MO symmetry... 

Hence, the HMO method is inherently incapable of giving any picture for disproportionation equilibria, the reason being the neglect of electronic repulsion. In the SCF treatment, it is convenient to assume that all three systems in the redox equilibrium are built up from the same MO s, the only difference being the number of n electrons. Adopting this convention and denoting the oxidized form, radical, and reduced form as Ox, Sem (semiquinone), and Red, we can write... 

In spite of the complicated nature of disproportionation equilibria, we think that the J m values alone can give useful information on log K values through eqs. (120) and (128), because it appears that the enthalpy change, AH, accompanying disproportionation can be treated within many reaction series as... 

S dianions are, at least, partly dispro-portionated. These disproportionation equilibria are almost temperature independent. 

The study of Li28 + DMF solutions [60] also allowed to characterize the electrochemical properties of polysulfides only redox couples of the type 8 /8 are involved. The chemical reactions coupled to charge transfers are classical dissociation and disproportionation equilibria no complex rearrangement reaction or transient species has been necessary. Redox potentials and charge-transfer coefficients of the redox couples involved in sulfur and polysulfide solutions are summarized in Table 2. 

For elements of several possible oxidation states (e.g., sulfur, the transition metals) all relevant disproportionation equilibria need to be examined. Pseudo-equilibrium conditions may be maintained for some time, where competing oxidation-reduction systems can be involved. For example, the following kinds of reactions... 

The protonation of anion radicals and dianions derived from aromatic hydrocarbons has been studied in some detail by Hoijtink and co-workers 113-11 s). it was shown that apart from the reactions given above (Eqs. (42)—(46)) other disproportionation equilibria also play an important role. These are different for different anion radicals, making the whole picture very complex. Kinetic studies on the disproportionation of the nitrobenzene anion radical and some of its derivati-ves 116,u 7) jlave s 10wn that in aqueous solution at a pH > 11.5, reaction (41) is of great importance, whereas the protonated radical ion and the radical ion are the kinetically active species in the pH interval between 3.2 and 11.5. 

Electron transfer between 7t-radicals constitutes their disproportionation. For cation- and anion-radicals, respectively, the disproportionation equilibria are given by Eqs. (8) and (9) ... 

Factors such as solvation and, in the case of ion-radicals, the counterion, may influence the properties of radicals. It is beyond the scope of this chapter to describe ion aggregation mechanisms and the factors which govern the hyperfine splittings manifested by counterions. The subject has been reviewed, however, by a prime mover in the field.12 Suffice it to say that the association of an ion-radical with a counterion may lead to a considerable redistribution of spin within the radical with consequences for the chemistry. For example, disproportionation equilibria and persistence may be influenced by the nature of the association.68 Closely allied to the phenomenon of ion association is, of course, solvation. Whether or not an ion-pair or other ionic assemblage exists in preference to free ions depends on the extent of the solvation of the ions. Nonionic radicals are also subject to variation in properties with change in solvent principally owing to interaction of the solvent with dipolar charges within the radical. 

Thermodynamic constants have been determined for the disproportionation equilibria of diazafluorene ketyls, e.g., 112 and its 1,8-diaza isomer, in the presence of each alkali-metal counterion.368... 

The effect of the addition of water and the presence of Li on the disproportionation equilibria was also examined (Ammar and Saveant, 1973). The effect of water was observed to be a decrease in AS with little change in the entropy term. On the other hand, Li also brought about a decrease in A but the entropy increased and became positive. The latter brought about an inverse temperature effect instead of AE increasing with temperature a decrease was observed. 

Disproportionation equilibria have been studied for various systems. Cauquis and co-workers investigated by electrochemical means the matrix of equilibria corresponding to Scheme 2 for 3,7-dimethoxypheno-thiazine and its derivatives, and applied the measurement of the response of the equilibria to different conditions of basicity to the definition of a scale of basicity in acetonitrile. The disproportionation kinetics of the iron-thionine system were measured several years ago solvent effects on the disproportionation rate constant have been examined, and, lately, an indirect measurement of the synproportionation rate constant of thionine and leucothionine has been made. ... 

UHF calculation of spin densities, theoretical discussion of fluorine coupling. ) Hiickel and McLachlan calculations of spin densities. - Determination of disproportionation equilibria and thermodynamic constants of the radical anions with the di-anions and the neutral molecules for Li, Na, K and Cs gegenions. ) HUckel and McLachlan calculations of spin densities. - Determination of disproportionation equilibria and thermodynamic constants for Li, Na and Cs gegenions. ) HUckel and McLachlan calculations of spin densities. - Determination of disproportionation equilibria and thermodynamic constants for Li, Na, K, Rb and Cs gegenions. ... 

Hiickel and McLachlan calculations of spin densities. - Determination of disproportionation equilibria and thermodynamic constants for Li +, Na", K+ and Cs+ gegenions. ) McLachlan calculation of spin densities. ... 

Systematic studies of the iodine hydrolysis and disproportionation equilibria were, therefore, undertaken in past years in several laboratories, including both thermodynamic and kinetic calculations as well as experimental investigations (e. g. Bell et al., 1982 a Palmer and Lietzke, 1982). TTiese studies showed that in order to completely describe the iodine reaction equilibria, additional reactions have to be taken into account among them, the most important are ... 

If sohd AgCl is filtered off and the solvent is removed, the remaining solid contains Au, CEES and/or CEESO moieties but no Ag. Attempted recrystalhzation of this material was unsuccessful, and Au(III) was reduced to a catalytically inactive Au(0) colloid. It is observed that once Au(0) forms in the system, it is not reoxidized, leading to an irreversible inactivation of the catalyst. However, use of the correct ratios of precursors in the presence of O2, or even ambient air, produces soluble and stable Au-based catalysts, some of which retain activity for at least one week. Most Au(I) complexes, with the exception of [Au(CN)2], disproportionate in H2O and several disproportionation equilibria have been measured. Although disproportionation is less favourable kinetically in CH3CN solution, it is possible that the catalyst inactivates by hydrolysis of the Au(I) intermediate followed by its reduction to form Au(0). ... 

Methyltetraphenoxyphosphorane and the p-nitrophenoxy analog rapidly disproportionate on mixing to give all five possible phosphoranes, CH3P(OPh) (OAr)4 . All disproportionation equilibria have been characteiized along with rate constants for their dissociation to phosphonium salts and for the corresponding associations. 

Hydrocarbon Radical Anion and Dianion Alkali-Metal Compounds.— The orystal structure of (Ph—Ph), K+(tetraglyme)2 is essentially the same as that of the Rb analogue the cations are surrounded by ten oxygens of the two tetraglyme molecules. The effect of pressure on the ion-pair equilibria, solvent-separated ion-pair (s.s.i.p.)v contact ion-pair (c.i.p.), for Naph, Na+ in THF was studied as was the volume change for the conversion c.i.p. -> s.s.i.p. Disproportionation equilibria [equation (1)] were variously studied, e.g. for ArH = perylene (Pe), ... 

Wang, Y, Zhong, M., Zhu, W., Peng, C.-H., Zhang, Y., Konkolewicz, D., Bortolamei, N., Isse, A.A., Gennaro, A., Matyjaszewski, K. Reversible-deactivation radical polymerization in the presence of metallic copper. Comproportionation-disproportionation equilibria and kinetics. Macromolecules 46(10), 3793-3802 (2013)... 

Scheme 7. Thermodynamic stability (disproportionation equilibria) of flavosemiquinone tautomers. Upon deprotonation of the stable, red (but because of high acidity biologically irrelevant) cations red unstable or blue stable neutral radicals are formed according to...

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