Direct measurements of the neopentyl peroxy-hydroperoxy radical isomerisation over the temperature range 660–750 K

Abstract

The rate constant for the isomerisation reaction neo-C5H11O2→C5H10OOH (k3) has been determined directly over the temperature range 660–750 K. neo-C5H11I was photolysed at 248 nm using a KrF laser in the presence of O2 and He. The alkyl radical generated in the photolysis reacts with O2 to form the peroxy radical which then isomerises to the hydroperoxy radical. Subsequent rapid reactions lead to the generation of OH, which was detected by laser induced fluorescence as a function of time. At high [O2] the time constant, λ+, for the build up of OH tends to −k3. As [O2] decreases, earlier reactions in the peroxy radical chain become important and analysis of the [O2] dependence of λ+ allows both k3 and k2, the rate constant for the peroxy radical decomposition, to be determined. Data analysis shows that the results are fully compatible with the steady-state measurements of Baldwin et al except that values for k3 a factor of over ten lower than their values are obtained. The discrepancy is shown to be due to errors in the equilibrium constant, K2, they used for the (R2) reaction.C5H11+O2⇌C5H11O2An Arrhenius analysis gives(k3/s1)=1012.2−˙0.77exp{−(1.48±0.12)×104K/T} The measurements of k−2 were combined with literature data for k2 and calculated values of ΔS28 to give ΔH28(298)=142±6kJ mol−1 for the neo-C5H11 + O2 ⇌ C5H11O2 equilibrium, in satisfactory agreement with group additivity values.