Crossed-Beam and Theoretical Studies of the O(³P, ¹D) + Benzene Reactions: Primary Products, Branching Fractions, and Role of Intersystem Crossing

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( 3 P). During the past decade, we have performed in-depth investigations of the reactions of O( 3 P) 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 (C 6 H 6 ) molecule. Despite extensive experimental and theoretical studies on the kinetics and dynamics of the O( 3 P) + C 6 H 6 reaction, the relative importance of the C 6 H 5 O (phenoxy) + H open-shell products and of the spin-forbidden C 5 H 6 (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( 3 P) + benzene at a collision energy ( E c ) of 8.2 kcal/mol, focusing on the occurrence of the phenoxy + H and spin-forbidden C 5 H 6 + 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( 1 D) + benzene at the same E c . 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( 3 P) + benzene reaction, the so-validated computatio...

Crossed-beam and theoretical studies of multichannel nonadiabatic reactions: branching fractions and role of intersystem crossing for O(3P) + 1,3-butadiene

“…The system reactivity was investigated using the ab initio transition state theorybased master equation methodology 60 we have used in recent years to study several reactions of addition of O( 3 P) to unsaturated hydrocarbons [20][21][22]25,26,33 and benzene. 27,28 This computational procedure is composed of three steps: investigation of the PES through electronic structure calculations, determination of the rate constants of elementary steps using the form of transition state theory (TST) that is most suitable for the specic reaction channel (variational or conventional), and determination of rate constants, solving the master equation. Details are reported below for each step.…”

“…The minimum energy crossing point (MECP) between triplet and singlet PESs was determined at Level 1 of theory using the methodology recently implemented in EStokTP. 28,63 All Level 0 and Level 1 electronic structure calculations were performed with Gaussian 09, 64 while DF-MP2, CCSD(T), and CASPT2 calculations were performed with Molpro. 65…”

“…During the oxidation of UHs and AHs (aromatic hydrocarbons) by O( 3 P), the “chemically activated” triplet oxy-intermediate(s) formed following electrophilic O atom addition to the unsaturated bond(s), can evolve adiabatically on the triplet potential energy surface (PES) and/or nonadiabatically ( via intersystem crossing, ISC) on the singlet PES, undergoing unimolecular decomposition to a variety of bimolecular product channels. In recent years, this picture has been documented in several multichannel nonadiabatic reactions of O( 3 P) with 2C, 3C, and 4C UHs, such as ethylene, 16–19 propene, 20 propyne, 21,22 allene, 23,24 1-butene, 25 1,2-butadiene, 26 and AHs, such as benzene, 27,28 by combining, in a synergistic fashion, crossed-molecular-beam (CMB) experiments with theoretical calculations of the underlying triplet/singlet PESs and statistical calculations of product branching fractions (BFs), taking into account ISC. The theoretical and experimental predictions 20,28 were found to be in good agreement also with several kinetic studies on various O( 3 P) reactions with UHs using product detection by ionization with tunable vacuum ultraviolet synchrotron radiation.…”

“…In recent years, this picture has been documented in several multichannel nonadiabatic reactions of O( 3 P) with 2C, 3C, and 4C UHs, such as ethylene, 16–19 propene, 20 propyne, 21,22 allene, 23,24 1-butene, 25 1,2-butadiene, 26 and AHs, such as benzene, 27,28 by combining, in a synergistic fashion, crossed-molecular-beam (CMB) experiments with theoretical calculations of the underlying triplet/singlet PESs and statistical calculations of product branching fractions (BFs), taking into account ISC. The theoretical and experimental predictions 20,28 were found to be in good agreement also with several kinetic studies on various O( 3 P) reactions with UHs using product detection by ionization with tunable vacuum ultraviolet synchrotron radiation. 29,30 The capability of the developed model to reconcile CMB and kinetic thermal experiments, which differ significantly for operative conditions, supported the theoretical foundations of the model and allowed us to obtain a qualitative interpretation of the reactive dynamics of these systems that we used to develop rate rules for the whole O( 3 P) + alkene reaction class.…”

Reactions O(3P, 1D) + HCCCN(X1Σ+) (Cyanoacetylene): Crossed-Beam and Theoretical Studies and Implications for the Chemistry of Extraterrestrial Environments

“…Specifically, we have analyzed several examples of reactions of O­( 3 P) with unsaturated hydrocarbons: acetylene, ethylene, − propene, , propyne, , allene, 1-butene, 1,2-butadiene, 1,3-butadiene, and, more recently, also small aromatic compounds (benzene , and pyridine). We have seen that oxygen atoms are even more effective than we thought in inducing the breakup of C–C bonds and in degrading the hydrocarbons directly toward CO or CO precursors because of intersystem crossing (ISC) to the underlying singlet potential energy surface (PES). − However, at the same time, the reactions of O­( 3 P) with organic molecules allow for the formation of other complex molecular species , that can, in turn, foster the chemical growth toward complexity. All those processes, indeed, can form also new O-containing organic molecules (e.g., ketene, phenol, butenone) or O-containing radicals that can further react, leading to the formation of other O-rich organic molecules.…”