(673c) Evidence for Roaming Radical Mechanisms in Thermal Systems: High Temperature Shock Tube Studies

The dissociation of a molecule by simple bond cleavage generally leads to two radical fragments. However, as the radicals separate they can roam, sampling large volumes of orientation space. This roaming occurs at large radical-radical separations (> 3 Å). During this roaming, the radicals may sample orientations where there is a barrierless path leading to the abstraction of a radical from one radical to the other and thereby forming two stable molecules. Most commonly, this abstraction involves the transfer of an H atom from one radical to another.^1,2 An example for this roaming process occurs in acetaldehyde, CH₃CHO.³ Acetaldehyde undergoes C-C bond fission giving CH₃ and HCO. However with a roaming mechanism, the CH₃ can abstract the H atom in HCO thereby yielding two stable molecular products CH₄ and CO. In order to unambiguously determine the branching ratios leading to roaming products we have performed thermal experiments on a series of molecules under high temperature conditions with the ANL flash photolysis shock tube. The high sensitivity H-atom ARAS detection technique used with the shock tube provides an excellent diagnostic for quantitative measurements of the roaming fraction. Experiments have been performed on acetaldehyde, propane, neo-pentane, iso-butane and dimethyl ether at high temperatures (1200 ? 1800 K). Typically the roaming radical mechanism in these molecules contribute anywhere from 10-22% to the total dissociation flux at high temperatures.

Acknowledgements

This work was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract No. DE-AC02-06CH11357.

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