Gas-Phase Kinetics of the Hydroxyl Radical Reaction with Allene: Absolute Rate Measurements at Low Temperature, Product Determinations, and Calculations

The gas phase reaction of the hydroxyl radical with allene has been studied theoretically and experimentally in a continuous supersonic flow reactor over the range 50 ≤ <i>T</i>/K ≤ 224. This reaction has been found to exhibit a negative temperature dependence over the entire temperature range investigated, varying between (0.75 and 5.0) × 10^–11 cm³ molecule^–1 s^–1. Product formation from the reaction of OH and OD radicals with allene (C₃H₄) has been investigated in a fast flow reactor through time-of-flight mass spectrometry, at pressures between 0.8 and 2.4 Torr. The branching ratios for adduct formation (C₃H₄OH) in this pressure range are found to be equal to 34 ± 16% and 48 ± 16% for the OH and OD + allene reactions, respectively, the only other channel being the formation of CH₃ or CH₂D + H₂CCO (ketene). Moreover, the rate constant for the OD + C₃H₄ reaction is also found to be 1.4 times faster than the rate constant for the OH + C₃H₄ reaction at 1.5 Torr and at 298 K. The experimental results and implications for atmospheric chemistry have been rationalized by quantum chemical and RRKM calculations

Relevance of the Channel Leading to Formaldehyde + Triplet Ethylidene in the O(³P) + Propene Reaction under Combustion Conditions

Comprehension of the detailed mechanism of O­(³P) + unsaturated hydrocarbon reactions is complicated by the existence of many possible channels and intersystem crossing (ISC) between triplet and singlet potential energy surfaces (PESs). We report synergic experimental/theoretical studies of the O­(³P) + propene reaction by combining crossed molecular beams experiments using mass spectrometric detection at 9.3 kcal/mol collision energy (<i>E</i>_c) with high-level ab initio electronic structure calculations of the triplet PES and RRKM/master equation computations of branching ratios (BRs) including ISC. At high <i>E</i>_c’s and temperatures higher than 1000 K, main products are found to be formaldehyde (H₂CO) and triplet ethylidene (³CH₃CH) formed in a reaction channel that has never been identified or considered significant in previous kinetics studies at 300 K and that, as such, is not included in combustion kinetics models. Global and channel-specific rate constants were computed and are reported as a function of temperature and pressure. This study shows that BRs of multichannel reactions useful for combustion modeling cannot be extrapolated from room-temperature kinetics studies

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

Despite extensive kinetics/theoretical studies, information on the detailed mechanism (primary products, branching ratios (BRs)) for many important combustion reactions of O­(³P) with unsaturated hydrocarbons is still lacking. We report synergic experimental/theoretical studies on the mechanism of the O­(³P) + C₃H₆ (propene) reaction by combining crossed-molecular-beam experiments with mass spectrometric detection at 9.3 kcal/mol collision energy (<i>E</i>_c) with high-level <i>ab initio</i> 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(³P) 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, CH₃CHCHO (7%) and CH₃COCH₂ (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 H₂ 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(³P) + ethylene (∼50%) and O­(³P) + allene (∼90%) at similar <i>E</i>_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 <i>E</i>_c. ISC is estimated to decrease from 60% to 20% with increasing <i>E</i>_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