Crossed-beam and theoretical studies of multichannel nonadiabatic reactions: branching fractions and role of intersystem crossing for O(3P) + 1,3-butadiene.

ABSTRACT

Atomic oxygen reactions can contribute significantly to the oxidation of unsaturated aliphatic and aromatic hydrocarbons. The reaction mechanism is started by electrophilic O atom addition to the unsaturated bond(s) to form "chemically activated" triplet oxy-intermediate(s), which can evolve adiabatically on the triplet potential energy surface (PES) and nonadiabatically via intersystem crossing on the singlet PES, forming intermediates that undergo unimolecular decomposition to a variety of bimolecular product channels. Here, we apply a combined crossed molecular beam (CMB)-theoretical approach to the study of the O(3P) + 1,3-butadiene reaction. Although the kinetics of this reaction have been extensively investigated, little is known about the primary products and their branching fractions (BFs). In the present work, a total of eight product channels were observed and characterized in a CMB experiment at a collision energy of 32.6 kJ mol-1. Synergic ab initio transition-state theory-based master equation simulations coupled with nonadiabatic transition-state theory on coupled triplet/singlet PESs were employed to compute the product BFs and assist the interpretation of the CMB experimental results. The good agreement found between the theoretical predictions and CMB experiments supported the use of the adopted methodology for the prediction of channel-specific rate constants as a function of temperature and pressure suitable to be used for the kinetic modeling of 1,3-butadiene oxidation and of systems where 1,3-butadiene is an important intermediate.