Reaction dynamics of O(P-3) + propyne: I. primary products, branching ratios, and role of intersystem crossing from crossed molecular beam experiments

Abstract

We performed synergic experimental/theoretical studies on the mechanism of the O(3P) + propyne reaction by combining crossed molecular beams experiments with mass-spectrometric detection and time-of-flight analysis at 9.2 kcal/mol collision energy (Ec) with ab initio electronic structure calculations at a high level of theory of the relevant triplet and singlet potential energy surfaces (PESs) and statistical calculations of branching ratios (BRs) taking into account intersystem crossing (ISC). In this paper (I) we report the results of the experimental investigation, while the accompanying paper (II) shows results of the theoretical investigation with comparison to experimental results. By exploiting soft electron ionization detection to suppress/mitigate the effects of the dissociative ionization of reactants, products, and background gases, product angular and velocity distributions at different charge-to-mass ratios were measured. From the laboratory data angular and translational energy distributions in the center-of-mass system were obtained for the five competing most important product channels, and product BRs were derived. The reactive interaction of O(3P) with propyne under single collision conditions is mainly leading to the rupture of the three-carbon atom chain, with production of the radical products methylketenyl + atomic hydrogen (BR = 0.04), methyl + ketenyl (BR = 0.10), and vinyl + formyl (BR = 0.11) and the molecular products ethylidene/ethylene + carbon monoxide (BR = 0.74) and propandienal + molecular hydrogen (BR = 0.01). Because some of the products can only be formed via ISC from the entrance triplet to the low-lying singlet PES, we infer from their BRs an amount of ISC larger than 80%. This value is dramatically large when compared to the negligible ISC reported for the O(3P) reaction with the simplest alkyne, acetylene. At the same time, it is much larger than that (∼20%) recently observed in the related reaction of the three-carbon atom alkene, O(3P) + propene at a comparable Ec. This poses the question of how important it is to consider nonadiabatic effects and their dependence on molecular structure for this kind of combustion reactions. The prevalence of the CH3 over the H displacement channels is not explained by invoking a preference for the addition on the methyl-substituted acetylenic carbon atom, but rather it is believed to be due to the different tendencies of the two addition triplet intermediates CH3CCHO (preferentially leading to H elimination) and CH3COCH (preferentially leading to CH3 elimination) to undergo ISC to the underlying singlet PES. It is concluded that the main coproduct of the CO forming channel is singlet ethylidene (1CH3CH) rather than ground-state ethylene. By comparing the derived BRs with those very recently derived from kinetics studies at room temperature and 4 Torr we obtained information on how BRs vary with collision energy. The extent of ISC is estimated to remain essentially constant (∼85%) with increasing Ec from ∼1 to ∼10 kcal/mol. The present experimental results shed light on the mechanism of the title reaction at energies comparable to those involved in combustion and, when compared with the results from the statistical calculations on the ab initio coupled PESs (see accompanying paper II), lead to an in-depth understanding of the rather complex reaction mechanism of O + propyne. The overall results are expected to contribute to the development of more refined models of hydrocarbon combustion.