Metastability-disproportionating reactions

Castelliz et al found that an iron rich wiistite phase would occur during the disproportionating process, namely near stoichiometric FeO. According to the studies of Fischer et disproportionating of wiistite divides into two steps First it changes to iron rich wiistite phase accompanied by the formation of Fe304 second, this iron rich wiistite disproportionates to Fe304 and Fe. Chemical equations of the 

Greenwood et studied the disproportionation behavior of Fei xO at temperature range of 27°C-427°C by Mossbauer techniques and proposed that the disproportionation of Fei xO is related with temperature. At temperature below 227°C, the disproportionating reaction is in progress with Eqs. (3.8) and (3.9) at above this temperature, Fei xO disproportionates to Fo304 and Fe because the reaction (3.8) is so fast that it could not be observed. Stolen observed by experiment that the reaction (3.8) occurs at a temperatme of about 197°C and that is consistent with the result of Greenwood. 

Pattek-Janczyki studied the phase changes of wiistite before or after the annealing by microscope and microprobe, and proposed a possible model of the formation process of active sites of catalyst during the initial activation stage. The model assumed (i) The reaction (3.8) is regarded as a transform reaction from elementary state to intermediate state, then intermediate state transforms to 

10 Gibbs free energies of phetses during the disproportionation of wiistite 

11 Nucleation model of catalyst (Solid line represents disproportionating reaction, and the initial magnetite is not in) 

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A method for the synthesis of Al and Ga clusters has been described by Schnepf and Schnockel (2002) which is based on the preparation of the gaseous monohalides subsequently isolated in metastable solutions at — 78°C. The halogen atoms are substituted by bulky groups and, in a parallel disproportionation reaction, large Al (or Ga) clusters are formed. 

Before closing this section it should be pointed out that the formation of most hydrides of intermetallic compounds is metastable vith respect to disproportionation [35]. Again, taking LaNis as an example, the disproportionation reaction ... 

Figure 3.9 indicates that FeO is a metastable phase substance in thermodynamics. FeO is stable only at temperatures above 570°C and oxygen partial pressure of —26 < lgPo2 < —6(Po2 in bar), below this temperature FeO caimot form in thermodynamics. When it is cooled down from high temperatures, FeO will undergo disproportionating reaction to form the stable products, Fe and Fes04, that is. 

The ion Cu" is extremely labile. Rate constants for the formation of maleate or fumarate complexes are =10 M s Ref. 281. It can be prepared in an acid perchlorate solution by reaction of Cu with a one-electron reducing agent such as Cr, or Eu Ref. 282. Although there is a marked tendency for disproportionation, solutions of Cu are metastable for hours in the absence of oxygen, particularly when concentrations of Cu(I) are low and the acidity is high. Espenson has capitalized on this to study the rates of reduction by Cu of some oxidants, particularly those of Co(III), Table 5.7 (see Prob. 6(c) Chap. 5). 

When the amino group is fully deprotonated, the rate of the H-transfer is 1.5 x 10s s4, but also around pH 7 the reaction is still fast, at the ms timescale (for a quantum mechanical study see Rauk et al. 2001). Upon the decay of the amnioal-kyl radicals formed in reaction (35) ammonia as formed in a yield that points to disproportionation as the major process (Zhao et al. 1997). The fact that the ami-noalkyl radical is the thermodynamically favored species does not mean that the repair of DNA radicals by GSH (Chap. 12.11) is not due to its action as a thiol. As with many other examples described in this book, the much faster kinetics that lead to a metastable intermediate (here the formation of the thiyl radical) rather than the thermodynamics as determined by the most stable species (here the aminoalkyl radical) determine the pathway the the reaction. In fact, the C-H BDE of the peptide linkage is lower than the S-H BDE and repair of DNA radicals by peptides, e.g., proteins would be thermodynamically favored over a repair by thiols but this reaction is retarded kinetically (Reid et al. 2003a,b). 

When the CO disproportionation is catalyzed by cobalt, some ordered metastable structures are detected inside the active metal nanoparticles after the reaction. These structures are regular thin (approximately 5 atoms in thickness) alternating cobalt layers of different crystallographic modifications (Figure 4.17). Note that the appearance of such structures at thermodynamically equilibrium states of the catalyst substance is contrary to the Gibbs phase rule for the phase equilibria in solids. Thus, the metastable layered structures may be considered an analogue of spatial dissipative structures. 

Reaction of BF3 with B at 1850 °C generates the reactive intermediate species boron monofluoride, BF, which cannot be isolated. Cocondensation of BF with BF3 yields B2F4, a metastable lower fluoride of boron, and BF2-B(F) BF2, which is unstable and disproportionates according to equation (46). Mixtures of boron trihalides undergo exchange reactions, presumably via a four-centered... 

The equilibrium constant of reaction (1), K = [Cu ][Cu ]/[Cu ], is of the order of 10 thus, only vanishingly small concentrations of aquo-copper(I) species can exist at equilibrium. However, in the absence of catalysts for the disproportionation—such as glass surfaces, mercury, red copper(I) oxide (7), or alkali (311)—equilibrium is only slowly attained. Metastable solutions of aquocopper(I) complexes may be generated by reducing copper(II) salts with europium(II) (113), chromium(II), vanadium(II) (113, 274), or tin(II) chloride in acid solution (264). The employment of chromium(II) as reducing agent is best (113), since in most other cases further reduction to copper metal is competitive with the initial reduction (274). 

Hayatsu et al. (1980b) have shown, however, that carbyne forms metastably at much lower temperatures (520-620 K), by catalytic disproportionation of CO (to COj and C) on a chromite or olivine catalyst. This reaction is much slower... 

For example, at 10 atm, 50% of the CO should have transformed to CH at 590 K, but this reaction is very slow in the absence of catalysts and so CO may instead disproportionate to CO2 and elemental carbon. This reaction should be 50% complete at 520 K. It yields carbynes rather than graphite, if a chromite catalyst is present. At 400 K, clays form from anhydrous silicates, catalyzing hydrogenation of CO to complex organic cxrmpounds. The dashed line shows the temperatures at which 1 % of the CO transforms to a typical alkane, C20H42, under metastable conditions (the lines for most other alkanes, from CjHg upward, are very similar). 

In contrast to SCI2 which, though reactive, is metastable in the liquid phase, SeCl2 has been known previously only in the gas phase in the presence of chlorine 44, 45). Until our current investigation the only known chemistry of SeCU was the disproportionation to Se2Cl2 and SeCb in the liquid or solid states and the reaction with CI2 to give SeCb. 

The mechanism of diphenol oxidation may also include free radical formation, either in the enzymatic reaction itself or in subsequent spontaneous disproportionations or cleavages (32,38) The influence of diphenolic radicals on fi carbon reactivity has not been well delineated. The metastable radicals are not easily studied in solution or wet solids such as cuticle because they are present at very low concentrations and undergo rapid nonenzymatic reactions. 

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