The ethylperoxy radical spectrum and rate constant for mutual interaction measured by flash photolysis and kinetic spectroscopy

Abstract

Intrinsic spectral and kinetic parameters have been measured for the ethylperoxy radical, which was formed in the gas phase by the flash photolysis of azoethane in the presence of an adequate excess of oxygen. Absolute values of the extinction coefficient €(A) were derived from complementary measurements of the yield of nitrogen and the absorbance of an equivalent concentration of ethylperoxy radicals. The absorption spectrum is broad, structureless and comparatively weak; ~(236) = 1.02 X lo3 liter mole-' cm-I a t the maximum, and the oscillator strength is 3.4 X This spectrum resembles the spectrum of the methylperoxy radical closely in form, but it is less intense; the ratio of the values of oscillator strength is 0.5. The bimolecular reactions of mutual interaction of ethylperoxy radicals are not exclusively terminating, and ethoxy and hydroperoxy radicals are formed in kinetically significant quantities. A computer program was designed to simulate the rise and fall of the concentration of each radical species, and to perform the related kinetic analysis. This program predicted that a second-order plot of the decline of the absorbance of the ethylperoxy radical during the dark period would not show a significant departure from linearity, a conclusion which was confirmed by experiment. Accordingly, the gradient of each such plot yielded a value of k ' / t ( X ) , where k' is the apparent value of the rate constant for the collective reactions of mutual interaction. This rate constant was evaluated from the product of corresponding values of k'/c(X) and €(A); individual values are independent of the wavelength of measurement, and the mean value is k' = (6.6 f 0.5) X loT liter mole-'sec-'. Further kinetic analysis yielded the corresponding absolute value: k = (6.0 f 0.6) X lo7 liter mole-' sec-l. This value fits the pattern of a relationship between rate constant and structure shown by the methylperoxy, isopropylperoxy, and tert -butylperoxy radicals. Adequate sensitivity for the characterization of the spectrum of the ethylperoxy radical was achieved by the use of a pulsed xenon arc as the monitoring light source in conjunction with a dual beam detection system with twin cells and balanced photomultipliers, and the apparatus is described in detail.

The acetyl radicals CH3CO· and CD3CO· studied by flash photolysis and kinetic spectroscopy

“…This was achieved by the use of a pulsed analytical light source in conjunction with a dual-beam optical system and balanced photomultipliers to allow differential photoelectric detection. A detailed description has been given in our account of an analogous study of the ethylperoxy radical [4], and only the salient features will be mentioned here.…”

“…Carbon monoxide of 99.99% purity and argon of 99.9995% purity were supplied by the Matheson Company and were used without further purification. n -Pentane from Matheson, Coleman, and Bell Manufacturing Chemists was purified, analyzed, and stored as described elsewhere [6].…”

“…and both ethyl and propionyl radicals may contribute to the absorbance recorded in Figure 8. The ethyl radical spectrum is weak, broad, and structureless, with ECH3CH2(247) = 4.8 x 102 L/mol.cm at its maximum [6];…”

“…A search was made for the absorbance of the acetyl radical after the photoflash had become extinct, but it proved to be too weak to be detected. Accordingly a computer program was designed to calculate the concentration of the acetyl and methyl radicals over the period of interest, using a mechanism comprising reaction (1)- (6). The corresponding rate equations were integrated by a fourthorder Runge-Kutta procedure analogous to the one that we used for the interaction of the ethyl radical and the oxygen molecule [4].…”

“…Accordingly a computer program was designed to calculate the concentration of the acetyl and methyl radicals over the period of interest, using a mechanism comprising reaction (1)- (6). The corresponding rate equations were integrated by a fourthorder Runge-Kutta procedure analogous to the one that we used for the interaction of the ethyl radical and the oxygen molecule [4]. The values of k z , k3, k q , and k6 are given elsewhere in this paper; we chose k 5 = 1 s-l as representative of the values reviewed by Gandini and Hackett [3], but the computation is insensitive to the value of this rate constant.…”