Crossed-Beams and Theoretical Studies of the Multichannel Reaction O(P) + 1,2-Butadiene (Methylallene): Product Branching Fractions and Role of Intersystem Crossing.

The reactions of ground state oxygen atoms, O(P), with unsaturated hydrocarbons (UHs) are relevant in the oxidation in different environments. They are usually multichannel reactions that exhibit a variety of competing product channels, some of which occur adiabatically on the entrance triplet potential energy surface (PES), while others occur nonadiabatically on the singlet PES that can be accessed via (ISC). ISC plays a key role on the mechanism of these reactions, impacting greatly the product yields. Identification of all primary reaction products, determination of their branching fractions (BFs), and assessment of the role of ISC is central for understanding the mechanism of these reactions. This goal can be best achieved combining crossed-molecular-beam (CMB) experiments with universal, ionization, mass-spectrometric detection and time-of-flight analysis to high-level electronic structure calculations of triplet/singlet PESs and Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) computations of product BFs with inclusion of ISC effects. Over the years this combined approach was found to be rewarding and successful for O(P) reactions with the simplest alkynes, alkenes, and dienes containing two, three, or four carbon atoms. Here, we report the full experimental and theoretical work on the reaction O(P) + 1,2-butadiene that permits us to explore how the mechanism and product distribution vary when moving from O(P) + allene (propadiene) to O(P) + methylallene (1,2-butadiene) and when comparing this system to related C4 unsaturated systems, namely O(P) + 1-butene and O(P) + 1,3-butadiene. In the present CMB experiments at the collision energy of 41.8 kJ/mol we have observed and characterized nine different product channels. Synergistic transition-state theory-based master equation simulations coupled with nonadiabatic transition-state theory on the coupled triplet/singlet PESs were used for computing the product BFs and assisting the interpretation of the experimental results. Theoretical predictions and experimental results were found to be in overall good agreement. The finding of this work can be useful for the kinetic modeling of the oxidation of 1,2-butadiene and of systems involving 1,2-butadiene as an important intermediate.