Experimental and Theoretical Studies on the Dynamics of the O(³P) + Propene Reaction: Primary Products, Branching Ratios, and Role of Intersystem Crossing

Abstract

Despite extensive kinetics/theoretical studies, information on the detailed mechanism (primary products, branching ratios (BRs)) for many important combustion reactions of O( 3 P) with unsaturated hydrocarbons is still lacking. We report synergic experimental/theoretical studies on the mechanism of the O( 3 P) + C 3 H 6 (propene) reaction by combining crossed-molecular-beam experiments with mass spectrometric detection at 9.3 kcal/mol collision energy ( E c ) with high-level ab initio electronic structure calculations of underlying triplet/singlet potential energy surfaces (PESs) and statistical (RRKM/Master Equation) computations of BRs including intersystem crossing (ISC). The reactive interaction of O( 3 P) with propene is found to mainly break apart the three-carbon atom chain, producing the radical products methyl + vinoxy (32%), ethyl + formyl (9%), and molecular products ethylidene/ethylene + formaldehyde (44%). Two isomers, CH 3 CHCHO (7%) and CH 3 COCH 2 (5%), are also observed from H atom elimination, reflecting O atom attack to both terminal and central C atoms of propene. Some methylketene (3%) is also formed following H 2 elimination. As some of these products can only be formed via ISC from triplet to singlet PESs, from BRs an extent of ISC of about 20% is inferred. This value is significantly lower than recently observed in O( 3 P) + ethylene (∼50%) and O( 3 P) + allene (∼90%) at similar E c, posing the question of how important it is to consider nonadiabatic effects for these and similar combustion reactions. Comparison of the derived BRs with those from recent kinetics studies at 300 K and statistical predictions provides information on the variation of BRs with E c . ISC is estimated to decrease from 60% to 20% with increasing E c . The present results lead to a detailed understanding of the complex reaction mechanism of O + propene and should facilitate the development of improved models of hydrocarbon combustion.