Alkenes and peroxyl radicals are, respectively, significant primary products and important chain carriers of the oxidation of alkanes in the gas phase at relatively low temperatures,[1] below ca. 850K. The study of the addition of peroxyl radicals to alkenes is therefore necessary for a good understanding of hydrocarbon combustion in the cool flame regime. Furthermore, the resulting epoxides are high value chemical products, and their formation via the non-catalytic, gas phase autoxidation of alkenes has been of considerable recent interest.[2-5]

It has been understood for some time that the addition of peroxyl radicals to alkenes is the rate determining step in the formation of the epoxide (k₂ >> k_-1),[6] and that for a peroxyl radical attacking a series of alkenes, the activation energy for the overall reaction correlates well with the ionisation energy of the alkene ( figure 1 and figure 2 ).[6,7]

                            k1,-1              k2          O
           ROO.  +  >C=C<   <=>     ROOC-C<   ->   RO + >C-C<  reactions 1, 2

It would also be of practical use to be able to relate the rate of epoxidation to a physical property of the attacking peroxyl radical. Therefore an investigation is made of the correlation of epoxidation activation energies with three parameters: the charge transfer that occurs during the formation of the peroxyalkyl adduct, the corresponding energy decrease due to the charge transfer, and the difference between the ionisation energy of the alkene and the electron affinity of the peroxyl radical. These correlations are used to estimate epoxidation rate constants of relevance to the modelling of propene autoxidation.