Radical equilibrium

The free radical equilibrium between PINO and hydroxylamines and phenols with known BDE of the O—H bonds was used for to estimate the BDE of the O—H bond of PINO and other hydroxylamines [90], The values of BDE (kJ mol ) are given below. 

Figure 25.3 The reaction zone configuration used in the present analysis. On the left side solid lines for T, T02, and YcH.4 represent the outer solution, and the dashed lines show profiles resulting from finite reaction rates in the oxygen-consumption layer. The right side corresponds to an expanded view of the regions around in the left sketch, represented by a single fine there, showing the structure of the radical-equilibrium and fuel-consumption layers A — location of fuel and radical layers, B — oxidation layer, C — radical-equilibration layer, and D — fuel-consumption layer...

Figure 2. Peroxy acetyl radical equilibrium chemistry and concentrations derived for a 4-day period during afield study in rural Pennsylvania. The two curves give model predicted concentrations for two different scenarios. (Adapted with permission from reference 8. Copyright 1991 American Geophysical Union.)...

The phosphate-adduct radical is also formed, when the reaction is initiated by S04 [reaction (18)] in the presence of phosphate ions (Behrens et al. 1988). This may either be due to an Sn2 substitution reaction [reaction (19)] or a reaction of the phosphate ion with the radical cation [reaction (17)] formed either by an elimination of S042- plus H+ [reaction (20)] and subsequent protonation of the N(3)-centered radical [equilibrium (22)] or by S042- elimination [reaction (21)], as envisaged originally. The reaction of the radical cation with phosphate would then give rise to the observed radical [reaction (23)]. 

The formation of phosphine 81 and dimer 80 in the decomposition of 77 does not necessarily prove a radical equilibrium 78a 78b, although phosphoranyl radical 78a was also accessible by treating the open ring lithium compound 82 with less than equivalent amounts of bromine 97). Excess bromine leads to spirophosphonium salt 34-Br which can again be reduced to radical 78 with potassium or sodium metal or electrolytically. The investigation of a series of substituted spirophosphoranyl radicals of that sort containing in all four 4,4 -95) or 5,5 -positions98) methyl,... 

Results of studies of the tetrofluorohydrozine-difluoromino radical equilibrium (N2p4 2 NF2) are reviewed good agreement of the four diverse methods indicates = 20.0 1.0 kcal. A... 

Baldwin et al. qualified the absolute accuracy of their results by noting that the calculated values, as we have seen, are dependent on values of alkyl radical decomposition rates and the alkyl/alkylperoxy radical equilibrium constant. The work of Hughes et al. [122] now means that the data of Baldwin et al. can be appropriately scaled to give the results shown in Table 2.10. 

Since the early 1960s, the key to this thermokinetic switch has been recognized to involve the alkyl/alkylperoxy radical equilibrium. 

The parent cyclopropenyl radical is yet to be observed directly. Various theoretical studies suggest that the highly symmetric 03 form 384 is subject to Jahn-Teller distortion and is consequently much less stable than the low symmetry non-planar Cg a radical equilibrium structure 385. Three equivalent conformations are possible for 385 and these can interconvert (AH 12-30 kJ mol" via a non-planar allylic transition... 

Some of the properties of phenoselenazines, including polarography, electric resistivity, ESR, NMR, crystal structure, and molecular conformation, nuclear spin coupling constants, the cation radical equilibrium constant and the heat of dimerization, the ESR spectrum and mechanism of photoreduction, and the semiconduction behavior of phenoselenazine-iodine complexes, have been reported. 

Cyclo[di(l,4-phenyleneethylene)] can also be made to undergo a radical equilibrium polymerization of this kind. At high temperatures, scission of the cyclic dimer occurs to form xylylene, which then polymerizes when the temperature is sufficiently low. A concentration of (5-10) x 10 " mol free electrons/mol xylylene units has been obtained by electron spin resonance measurements. The degree of polymerization of 2000-4000 calculated from this data agrees well with that found by molecular weight determinations ... 

Example 5.2 Suppose that there is free-radical equilibrium polymerization with termination by disproportionation alone. 

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