Crossed Molecular Beam Dynamics Studies of the O(3P) + Allene Reaction: Primary Products, Branching Ratios, and Dominant Role of Intersystem Crossing

We report on the determination of primary products and their branching ratios for the combustion relevant O(3P)+allene reaction by the crossed molecular beams method with soft electron-ionization mass-spectrometric detection at a collision energy of 39.3 kJ/mol. We have explored the reaction dynamics of the open channels leading to C2H4+CO, C2H2+H2CO, C2H3+HCO, CH2CCHO+H, and CH2CO+CH2. Because some of the observed products can only be formed via intersystem crossing (ISC) from triplet to singlet potential energy surfaces, from the product branching ratios we have inferred the extent of ISC. The conclusion is that the O(3P)+allene reaction proceeds mostly (>90%) via ISC. This observation poses the question of how important it is to consider nonadiabatic effects for this and other similar systems involved in combustion chemistry. Another important conclusion is that the interaction of atomic oxygen with allene breaks apart the three-carbon atom chain, mostly producing CO and ethylene.