PEROXY RADICAL KINETICS RESULTING FROM THE OH-INITIATED OXIDATION OF 1,3-BUTADIENE, 2,3-DIMETHYL-1,3-BUTADIENE AND ISOPRENE

The laser flash photolysis/UV absorption spectrometry technique has be en used to investigate the kinetics of the peroxy radical permutation reactions (i.e. self and cross reactions) arising from the OH-initiate d oxidation of isoprene (2-methyl-1,3-butadiene), and of the simpler, but related conjugated dienes, 1,3-butadiene and 2,3-dimethyl-1,3-buta diene, The results of the two simpler systems are analysed to provide values of the rate coefficients for the 6 peroxy radical permutation r eactions of the three types of isomeric peroxy radical produced in eac h system (T = 298 K, P = 760 Torr). The rate coefficients are all sign ificantly larger than values estimated previously by extrapolation of structure-reactivity relationships based on the kinetics of a limited dataset of simpler radicals containing similar structural features. Th e results are discussed in terms of trends in self and cross reaction reactivity of primary, secondary and tertiary peroxy radicals containi ng combinations of allyl, beta-hydroxy and delta-hydroxy functionaliti es. Since the peroxy radicals formed in these systems are structurally very similar to those formed in the isoprene system, the kinetic para meters derived from the results of the simpler systems are used to ass ist the assignment of kinetic parameters to the 21 permutation reactio ns of the six types of isomeric peroxy radical generated in the isopre ne system. Kinetic models describing the OH-initiated degradation of a ll three conjugated dienes to first generation products in the absence of NOx are recommended, which are also consistent with available end product studies. The model for isoprene is considered to be a further improvement on that suggested previously for its OH-initiated oxidatio n in the absence of NOx. The mechanism is further extended to include chemistry applicable to 'NOx-present' conditions, and calculated produ ct yields are compared with those reported in the literature.