Nitric oxide reaction mechanisms rate constants

Peroxyl radicals are not only ones, which are able to react with ubihydroquinones. Poderoso et al. [245] showed that the short-chain ubihydroquinones Q0 and Q2 are oxidized by nitric oxide with the rate constants of 0.49 x 104 and 1.6x 1041 mol-1 s 1, respectively. The reaction apparently proceeded by one-electron transfer mechanism because the formation of intermediate semiquinone radicals has been registered. 

Herod et al. [50] has studied the kinetics and mechanism of oxyhemoglobin (Hb02) and oxymyoglobin (Mb02) oxidation by nitric oxide. At pH 7.0 the rate constants for these reactions were equal to 43.6+0.5 x 1061 mol 1 s 1 for Mb02 and 89 + 3 x 106 1 mol-1 s-1 for Hb02. It has been suggested that these reactions proceed via the formation of intermediate peroxynitrito complexes, which were rapidly decomposed to the Met-form of proteins, for example ... 

The oxidation of NO to NO2, which is an important step in the manufacture of nitric acid by the ammonia-oxidation process, is an unusual reaction in having an observed third-order rate constant (k o in ( rm) = kso Oc02) which decreases with increase in temperature. Show how the order and sign of temperature dependence could be accounted for by a simple mechanism which involves the formation of (NO)2 in a rapidly established equihbrium, followed by a relatively slow bimolecular reaction of (NO)2 with O2 to form NO2. 

The first mechanism implies k19 = k5AK11A 11A the second leads to ki9 = k2iKlm- . Independent evidence suggests the existence of both intermediate species in the nitric oxide-oxygen system, and both mechanisms involve entirely reasonable collision complexes. In both, the equilibrium step is rapid, and the overall kinetics are third order. Theoretical calculations based on the activated complex theory were made by assuming a true termolecular reaction the predicted rates agree well with experiment.161 The experimental rate constants are summarized in Tables 4-3 and 4-4. 

The investigations of early workers107 -252-269-423-426-426 -428,437 on the nitric oxide-chlorine system, while not immediately understandable, were concordant in their results. These results were critically reviewed and extended by Welinsky and Taylor,446 who recalculated the apparently inconsistent results of Trautz and co-workers and showed that the anomalous results they obtained in the presence of excess nitric oxide were due to incorrect analysis. Trautz had concluded that, in accordance with his general kinetic theory, the reaction proceeded by two consecutive steps, reactions (2) and (3). However, Welinsky and Taylor felt there was no basis for assuming any mechanism other than a single termolecular step. In Figure 5-1 values are shown for the third-order rate constant 1 for the reaction... 

The electronic structure of the vinoxy radical is usually considered a resonance structure between the two localized electronic states CH2=CHO° and °CH2-CHO. Both quantum mechanical ab initio calculations174"178 182 and microwave spectroscopy172,173 indicate a dominant contribution of the °CH2-CHO form. In consequence, the nitroso-compound ON-CH2CHO is expected to be a major product of the addition reaction of nitric oxide to the vinoxy radical. Delbos et al %2 carried out measurements of the rate constant for the reaction... 

A more detailed analysis of the radical mechanisms has been presented . Generally, all three processes show first-order kinetics but Ej reactions do not exhibit an induction period and are unaffected by radical inhibitors such as nitric oxide, propene, cyclohexene or toluene. For the non-chain mechanism, the activation energy should be equivalent to the homolytic bond dissociation energy of the C-X bond and within experimental error this requirement is satisfied for the thermolysis of allyl bromide For the chain mechanism, a lower activation energy is postulated, hence its more frequent occurrence, as the observed rate coefficient is now a function of the rate coefficients for the individual steps. Most alkyl halides react by a mixture of chain and E, mechanisms, but the former can be suppressed by increasing the addition of an inhibitor until a constant rate is observed. Under these conditions a mass of reliable reproducible data has been compiled for Ej processes. Necessary conditions for this unimolecular mechanism are (a) first-order kinetics at high pressures, (b) Lindemann fall-off at low pressures, (c) the absence of induction periods and the lack of effect of inhibitors and d) the absence of stimulation of the reaction in the presence of atoms or radicals. 

The autocatalytic oxidation reactions of nitric acid have been studied by Bazsa et al They have examined the oxidation of formaldehyde, and suggest a rate-determining step involving H2C(OH)2 and N2O4. A much more complex system is the oxidation of bromide. The reaction has been studied in both forward and backward directions, and has been found to exhibit bistability when studied in a continuous-flow stirred tank reactor. A detailed mechanism has been proposed [see Eqs. (25)-(31)] together with values for the rate constants. The reverse reaction, the oxidation of nitrous acid by bromine, gives rate law (24), a much simpler... 

NH3, H2O) and nitric oxide, which all result in the formation of the [Ru (NHslsNO ] product, were reinvestigated in acidic aqueous medium in order to clarify the underlying reaction mechanism (94). As expected, the second-order rate constants obtained in this study for the substitution of various X in [Ru (NH3)5X] by nitric oxide are all much higher than those found for the substitution reactions involving other entering ligands. Importantly, the substitution of Cl by NO is approximately as fast as the displacement of NH3 (0.75 0.03 and 0.3 0.01 s, respectively), but both reactions are much slower than... 

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