Combined Experimental and Theoretical Studies of the O(3P)+1-Butene Reaction Dynamics: Primary Products, Branching Fractions, and Role of Intersystem Crossing

Information on the detailed mechanism and dynamics (primary products, branching fractions (BFs), and channel specific rate constants as a function of temperature) for many important combustion reactions of O(P-3) with unsaturated hydrocarbons is still lacking. We report synergistic experimental/theoretical studies on the mechanism and dynamics of the O(P-3) + 1-C4H8 (1-butene) reaction by combining crossed molecular beam (CMB) experiments with soft electron ionization mass-spectrometric detection and timeof-flight analysis at 10.5 kcal/mol collision energy (E-c) to high-level ab initio electronic structure calculations of the underlying triplet and singlet potential energy surfaces (PESs) and statistical Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) computations of BFs including intersystem crossing (ISC). The reactive interaction of O(P-3) with 1-butene is found to mainly break apart the 4-carbon atom chain, leading to the radical product channels ethyl + vinoxy (BF = 0.34 +/- 0.11), methyl + C3H5O (BF = 0.28 +/- 0.09), formyl + propyl (BF = 0.17 +/- 0.05), ethyl + acetyl (BF = 0.014 +/- 0.007), and butanal radical (ethylvinoxy) + H (BF = 0.013 +/- 0.004), and molecular product channels formaldehyde + propenylidene/propene (BF = 0.15 +/- 0.05) and butenone (ethyl ketene) + H-2 (BF = 0.037 +/- 0.015). As some of these products can only be formed via ISC from triplet to singlet PESs, from BFs an extent of ISC of 50% is inferred. This value is significantly larger than that recently observed for O(P-3) + propene (22%) at similar E-c, underlying the question of how important it is to consider nonadiabatic effects for these and similar combustion reactions. Comparison of the derived BFs with those of statistical (RRKM/ME) simulations on the ab initio coupled triplet/singlet PESs shows good agreement, warranting the use of the RRKM/ME approach to provide information on the variation of BFs with temperature and to derive channel specific rate constants as a function of temperature (T) and pressure (p). Notably, ISC is predicted to decrease strongly with increasing temperature (from about 70% at 300 K to 46% at E-c = 10.5 kcal/mol, and about 1% at 2000 K). The present results lead to a detailed understanding of the complex reaction mechanism of O(P-3) + 1-butene and, by providing channel specific rate constants as a function of T and p, should facilitate the improvement of current fossil-fuel (1-butene) as well as biofuel (1-butanol) combustion models.