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Mise au point a partir du modele de cinetique chimique: travaux de divers laboratoires, developpement futur du modele

Using the technique of molecular modulation spectrometry, the rate constant for methyl radical recombination has been measured to be (4.0 ± 0.9)× 10–11 cm3 molecule–1 s–1 between 250 and 450 K. The rate constant for perdeuteromethyl radicals is identical (within 20 %) at room temperature. With 0.6 nm resolution the effective peak absorption cross-section for methyl radicals at 216 nm is (3.2 ± 0.6)× 10–17 cm2, consistent with an absolute cross-section of (4.2 ± 0.8)× 10–17 cm2. With a resolution of 1.0 nm the observed maximum cross-section for CD3 absorption, 3.0 × 10–17, is consistent with an absolute cross-section of (1.0 ± 0.2)× 10–16 cm2.

Using the technique of molecular modulation spectrometry, we have found transient ultraviolet absorptions which we assign to ethyl, isopropyl and t-butyl radicals. The spectra are broad continua with maxima near 250, 233 and 230 nm, respectively. From the rate constants of their mutual reactions, measured by monitoring the radicals in the ultraviolet, and the disproportionation/recombination ratios found from separate experiments, the rates of recombination are measured at room temperature to be: ethyl, (1.3 ± 0.3)× 10–11 cm3 molecule–1 s–1; isopropyl, (8.3 ± 2.0)× 10–12 cm3 molecule–1 s–1; t-butyl, (4.0 ± 1.0)× 10–12 cm3 molecule–1 s–1. The t-butyl rate falls by a factor of 2 as the temperature is increased to 423 K. These results are compared with other recent gas- and liquid-phase determinations, and we conclude that the currently accepted thermochemistry of larger alkyl radicals and our results are incompatible.