The quest for a detailed comprehension of elementary reactions in combustion: a crossed molecular beam study of the O(3P) reactions with unsaturated C4 hydrocarbons

In combustion, the overall chemical changeover is the result of an intricate network of elementary reactions (unimolecular, bimolecular and termolecular) which are classified according to their role (initiation, propagation, branching, termination steps) in the chain reaction. To implement a realistic combustion model, a deep knowledge of the chemical transformation at the molecular level for all the important steps is crucial. In this contribution, the capabilities of the crossed molecular beam (CMB) method with mass spectrometric (MS) detection in the study of multi-channel elementary reactions of relevance in combustion chemistry are illustrated by several examples of reactions involving atomic oxygen and C4 unsaturated hydrocarbons. In particular, the focus will be on the almost unique capability of the CMB-MS method in identifying the reactions products and their branching ratios by means of the same detection scheme and under well-controlled conditions. As expected, the initial steps of the reactions between atomic oxygen and C4 unsaturated hydrocarbons are similar to that of the other reactions that involve O and smaller unsaturated hydrocarbons, as they all feature the attack of the electrophilic O atom to the density of the multiple bond(s). However, the much more complex molecular structure leads to an increase of the possible reaction channels. Intersystem crossing (ISC) from the triplet potential energy surface to the lower energy singlet one via intersystem crossing is significant and opens, also in this case, several reaction channels which are not otherwise available. Several pairs of products formed after ISC are closed-shell molecules, rather than open-shell radicals. Clearly, this observation has strong implications for combustion modeling, because ISC shifts part of the reactive flux toward terminating rather than propagation or branching steps.