Direct observation of roaming radicals in the thermal decomposition of acetaldehyde.

The thermal dissociation of acetaldehyde has been studied with the reflected shock tube technique using H(D)-atom atomic resonance absorption spectrometry detection. The use of an unreversed light source yields extraordinarily sensitive H atom detection. As a result, we are able to measure both the total decomposition rate and the branching to radical versus molecular channels. This branching provides a direct measure of the contribution from the roaming radical mechanism since the contributions from the usual tight transition states are predicted by theory to be negligible. The experimental observations also provide a measure of the rate coefficient for H + CH(3)CHO. Another set of experiments employing C(2)H(5)I as an H-atom source provides additional data for this rate coefficient that extends to lower temperature. An evaluation of the available experimental results for H + CH(3)CHO can be expressed by a three-parameter Arrhenius expression as k = 7.66 x 10(-20)T(2.75) exp((-486 K)/T) cm(3) molecule(-1) s(-1) (298-1415 K). Analogous experiments employing C(2)D(5)I as a D-atom source allow for the study of the isotopically substituted reaction. The present experiments are the only direct measure for this reaction rate constant, and the results can be expressed by an Arrhenius expression as k = 5.20 x 10(-10) exp((-4430 K)/T) cm(3) molecule(-1) s(-1) (1151-1354 K). The H/D + CH(3)CHO reactions are also studied with ab initio transition-state theory, and the results are in remarkably good agreement with the current experimental data.