Displacement of the Maximum in the Concentration-Time Diagram of Uni-bi-, Bi-uni-, and Bi-bimolecular Consecutive Reactions

Talât-Erben, M.

doi:10.1063/1.1743267

Cited by 4 publications

(2 citation statements)

References 2 publications

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“…Therefore, since the maximum concentration of the methyl radical may be calculated from the initial conditions of the experiment, both the absorption cross section and the rate coeficient for recombination may be determined in a single experiment. If the rate of dissociation of the precursor is comparable to the rate of recombination eq 1a becomes more complex, , but the above conclusion is still valid. If an alternate approach is used to produce the methyl radicals, for example a precursor with an unknown yield or simple photolysis, then it may not be possible to calculate the initial methyl concentration and eq 1a must be rewritten to give In experiments of this type, the cross section must be obtained from other measurements.…”

Section: Cross Section For Optical Absorptionmentioning

confidence: 93%

Recombination of Methyl Radicals. 2. Global Fits of the Rate Coefficient

Hessler

Ogren

1996

J. Phys. Chem.

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The temperature-and pressure-dependent behavior of the cross section for optical absorption by the methyl radical is carefully considered, so we may define a criterion for selecting and correcting measurements of the rate coefficient for the recombination of methyl radicals, CH 3 + CH 3 f C 2 H 6 . The low-temperature data of Slagle et al., Hippler et al., and Walter et al. and the high-temperature data of Glänzer et al., Hwang et al., and our latest results (previous paper in this issue) are used to define a data set which contains 217 points. Subsets of isothermal data show that the temperature dependence of the high-pressure rate coefficient may be described by the simple exponential function A ∞ exp{-T(K)/T ∞ }. Four different formulations for the pressuredependent behavior in the falloff region are used for the global fits: (1) the asymmetric Lorentzian broadening function of Gilbert et al.; (2) the Gaussian broadening function of Wang and Frenklach; (3) the empirical "a equation" introduced by Gardiner; (4) the extension of Lindemann's expression suggested by Oref. All formulations reproduce the data, but Oref's "J equation" produces the least correlation between the best-fit parameters, the least uncertainty in these parameters, and the smallest uncertainty in the predictions. These results are k ∞ (T) ) 8.78 × 10 -11 exp{-T(K)/723} cm 3 s -1 , k 0 (T) ) 9.04 × 10 -27 cm 6 s -1 , and J exp (T) ) {exp[T(K)/268] -1} 2 .

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Section: Cross Section For Optical Absorptionmentioning

confidence: 93%

Recombination of Methyl Radicals. 2. Global Fits of the Rate Coefficient

Hessler

Ogren

1996

J. Phys. Chem.

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“…Either of these cases would produce a kinetic system in which an irreversible second-order or pseudo second-order process follows an irreversible pseudo first-order process. TALAT-ERBEN (26) has shown that in such systems k~tma x decreases sharply and (B)max/(A)0 decreases gradually as (A)o is increased. Neither of these effects occurs at high pH', while the evidence for changes in kltma x or Emax/(A)0 at low pH' is marginal.…”

Section: Discussionmentioning

confidence: 99%

The air oxidation of 4,6-di (2-phenyl-2-propyl) pyrogallol. Spectroscopic and kinetic studies of the intermediates

Abrash

1977

Carlsberg Res. Commun.

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Two colored extinction bands, one with ~'max near 520 nm and the other near 770 nm, form and decay during air oxidations of 4,6-di(2-phenyl-2-propyl) pyrogallol in alkaline methanol. The 770 nm band appears to be due to the semiquinone monoanion while the extinction near 500 nm is thought to be due to both the semiquinone and 3-hydroxy-o-quinone. Although the rates of formation and decay of the two bands are indistinguishable in the presence of excess dissolved oxygen, the 770 nm band disappears slightly before the 500 nm extinction when the pyrogallol reactant is initially in excess of the dissolved oxygen. Except at high methoxide concentrations, the kinetics indicate a system of two consecutive reactions with pseudo first-order rate constants k~ and k 2. The pH' dependence of k~ indicates that the neutral pyrogallol, its monoanion and dianion all react, the rate increasing with the negative charge of the species. The k2/k ~ ratio is constant at 5 from pH' 11 to 16. Extinction coefficients at both wavelengths increase with pH'. The 770 nm extinction disappears immediately after acidification while the 500 nm extinction decays at a rate consistent with oxidation. The rate constants k~ and k2 are insensitive to changes in pyrogallol concentrations, indicating an absence of irreversible semiquinone disproportionations. Increased oxygen concentration raises k~ and k2 to the same extent.

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