Relevance of the Channel Leading to Formaldehyde plus Triplet Ethylidene in the O(P-3) + Propene Reaction under Combustion Conditions

Comprehension of the detailed mechanism of O(P-3) + unsaturated hydrocarbon reactions is complicated by the existence of many possible channels and intersystem crossing (ISC) between triplet and singlet potential energy surfaces (PESs). We report synergic experimental/theoretical studies of the O(P-3) + propene reaction by combining crossed molecular beams experiments using mass spectrometric detection at 9.3 kcal/mol collision energy (E-c) with high-level ab initio electronic structure calculations of the triplet PES and RRKM/master equation computations of branching ratios (BRs) including ISC. At high E-c's and temperatures higher than 1000 K, main products are found to be formaldehyde (H2CO) and triplet ethylidene ((CH3CH)-C-3) formed in a reaction channel that has never been identified or considered significant in previous kinetics studies at 300 K and that, as such, is not included in combustion kinetics models. Global and channel-specific rate constants were computed and are reported as a function of temperature and pressure. This study shows that BRs of multichannel reactions useful for combustion modeling cannot be extrapolated from room-temperature kinetics studies.