Abstract
Reliable modeling of hydrocarbon oxidation relies critically on knowledge of the branching fractions (BFs) as a function of temperature (T) and pressure (p) for the products of the reaction of the hydrocarbon with atomic oxygen in its ground state, O(3P). During the past decade, we have performed in-depth investigations of the reactions of O(3P) with a variety of small unsaturated hydrocarbons using the crossed molecular beam (CMB) technique with universal mass spectrometric (MS) detection and time-of-flight (TOF) analysis, combined with synergistic theoretical calculations of the relevant potential energy surfaces (PESs) and statistical computations of product BFs, including intersystem crossing (ISC). This has allowed us to determine the primary products, their BFs, and extent of ISC to ultimately provide theoretical channel-specific rate constants as a function of T and p. In this work, we have extended this approach to the oxidation of one of the most important species involved in the combustion of aromatics: the benzene (C6H6) molecule. Despite extensive experimental and theoretical studies on the kinetics and dynamics of the O(3P) + C6H6 reaction, the relative importance of the C6H5O (phenoxy) + H open-shell products and of the spin-forbidden C5H6 (cyclopentadiene) + CO and phenol adduct closed-shell products are still open issues, which have hampered the development of reliable benzene combustion models. With the CMB technique, we have investigated the reaction dynamics of O(3P) + benzene at a collision energy (Ec) of 8.2 kcal/mol, focusing on the occurrence of the phenoxy + H and spin-forbidden C5H6 + CO and phenol channels in order to shed further light on the dynamics of this complex and important reaction, including the role of ISC. Concurrently, we have also investigated the reaction dynamics of O(1D) + benzene at the same Ec. Synergistic high-level electronic structure calculations of the underlying triplet/singlet PESs, including nonadiabatic couplings, have been performed to complement and assist the interpretation of the experimental results. Statistical (RRKM)/master equation (ME) computations of the product distribution and BFs on these PESs, with inclusion of ISC, have been performed and compared to experiment. In light of the reasonable agreement between the CMB experiment, literature kinetic experimental results, and theoretical predictions for the O(3P) + benzene reaction, the so-validated computational methodology has been used to predict (i) the BF between the C6H5O + H and C5H6 + CO channels as a function of collision energy and temperature (at 0.1 and 1 bar), showing that their increase progressively favors radical (phenoxy + H)-forming over molecule (C5H6 + CO and phenol stabilization)-forming channels, and (ii) channel-specific rate constants as a function of T and p, which are expected to be useful for improved combustion models.
Highlights
It should be noted that the theoretical results differ slightly, by up to a factor of 1.2 in the branching fractions (BFs), from those reported in our previous study[34] because of the change of the energy barrier of 3TS2, the use of the weak coupling intersystem crossing (ISC) model, and the simulation of crossed molecular beam (CMB) experiments using the collisional energy distribution rather than the average value
The experimental results were analyzed with the support of synergistic high-level quantum-chemical calculations of the underlying triplet and singlet potential energy surfaces (PESs) and statistical (RRKM/master equation (ME)) simulations on these PESs with non-adiabatic effects (i.e., ISC) taken into account in order to gain a deeper and more comprehensive understanding of the reaction mechanism and dynamics
Benzene benchmark system extends to aromatic hydrocarbons our recent combined experimental/theoretical studies[19−23,29,32−34,77] on O(3P) + C2, C3, and C4 unsaturated hydrocarbons and can serve as a gateway to more complex chemical pathways available in larger aliphatic/aromatic hydrocarbons
Summary
Since the early pioneering work of Cvetanovic in the 1950s,1−3 the reactions of ground-state atomic oxygen, O(3P), with unsaturated hydrocarbons (UHs) (alkynes, alkenes, dienes, and aromatics) have received a great deal of attention because of their importance in atmospheric chemistry[4] and especially combustion chemistry.[5−8] Initially, the effort was mainly devoted to kinetics,[3] but, starting from the early 1980s, work on dynamics under single-collision conditions was undertaken using a variety of techniques, ranging from crossed molecular beam (CMB) methods with mass spectrometric (MS) detection[9−14] to laser-based spectroscopic techniques in a cell or flow system.[15,16] the characterization of the detailed reaction mechanism, in particular the determination of the relative importance of the various competing reaction channels, has always been a challenge. Detailed comprehension of the mechanism of the combustion-relevant multichannel reactions of O(3P) with UHs requires the identification of all primary reaction products, the determination of their branching fractions (BFs), and an assessment of the role of ISC This can be achieved by combining CMB experiments (using universal soft electronionization MS detection and time-of-flight (TOF) analysis) with high-level ab initio electronic structure calculations of the triplet/singlet PESs and their couplings, and Rice−Ramsperger−Kassel−Marcus/master equation (RRKM/ME) computations of product BFs including ISC.[19−23] We emphasize that reliable information on product BFs as a function of temperature and predictions of channel-specific rate constants as a function of temperature and pressure are crucially needed to improve current combustion models.[19−24]
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