Atmospheric fate of small alkoxy radicals: recent experimental and theoretical advances

Abstract

New results concerning the kinetics and the atmospheric fate of small alkoxy radicals (ethoxy, 1-propoxy, 2-propoxy, 2-butoxy, t-butoxy, and 3-pentoxy) are reviewed here. The three main reactions of atmospheric relevance are considered to be: the bimolecular reaction with O 2, the unimolecular isomerization, and the unimolecular decomposition. Concerning the latter, significant advances have been achieved thanks to the combination of direct time resolved techniques and theoretical (ab initio and statistical) calculations. Based on a reasonable accord between theory and experiment, a few simple structure activity relationships (SAR), proposed recently by different authors, are compared and should allow a reliable prediction of the unimolecular decomposition rate constant of many alkoxys. On the other hand, new measurements concerning the reaction of a few alkoxys with O 2 confirm that the generic value of k O 2 =(8±2)×10 −15 cm 3 molecule −1 s −1 at 298 K is indeed applicable for all alkoxy radicals. Concerning the unimolecular isomerization, all recent experimental data (from indirect experiments) confirm the order of magnitude estimates proposed by Atkinson [Int. J. Chem. Kinet. 29 (1997) 99] for the isomerization rate constants but only direct absolute measurements, still lacking, will allow a reliable SAR to be developed.