DIRECT MEASUREMENTS OF THE PEROXY HYDROPEROXY RADICAL ISOMERIZATION, A KEY STEP IN HYDROCARBON COMBUSTION

The rate coefficient for an alkyl peroxy (RO2) --> alkyl hydroperoxy radical isomerisation reaction has been measured directly for the first time (with R = neo-pentyl). The alkyl radical was generated by the 248 nm pulsed photolysis of neo-pentyl iodide and the OH product of the peroxy radical reaction sequence was detected by laser-induced fluorescence. The time constant for the growth of OH varies with [O2], as the rate determining step in the reaction sequence changes. This variation permits the rate coefficients for peroxy radical isomerisation (k3) and decomposition (k-2) to be determined. At 700 K, k3 = 1.24 x 10(3) s-1 and k-2 = 9.1 X 10(3) s-1. The value obtained for k3 is more than an order of magnitude smaller than that obtained by Baldwin, Hisham and Walker using indirect methods. It is demonstrated that the present experimental results are compatible with those of Baldwin, Hisham and Walker and the discrepancy arises from their use of an inaccurate estimate of the equilibrium constant for formation of the peroxy radical.