Over the past ten years or so, great advances in our understanding of the dynamics of elementary (bimolecular) polyatomic reactions in the gas-phase have occurred. This has been made possible by critical improvements (a) in crossed molecular beam (CMB) instruments with rotating mass spectrometric detection and time-of-flight analysis, especially following the implementation of soft ionization (by tunable low energy electrons or vacuum-ultraviolet synchrotron radiation) for product detection with increased sensitivity and universal detection power, and (b) in REMPI-slice velocity map ion imaging (VMI) detection techniques in pulsed CMB experiments for obtaining product pair-correlated information through high-resolution measurements directly in the center of mass system. The improved universal CMB method is permitting us to identify all primary reaction products, characterize their formation dynamics, and determine the branching ratios (BRs) for multichannel non-adiabatic reactions, such as those of ground state oxygen atoms, O(P), with unsaturated hydrocarbons (alkynes, alkenes, dienes). The improved slice VMI CMB technique is permitting us to explore at an unprecedented level of detail, through pair-correlated measurements, the reaction dynamics of a prototype polyatomic molecule such as CH (and isotopologues) in its ground state with a variety of important X radicals such as F, Cl, O, and OH. In this review, we highlight this recent progress in the field of CMB reaction dynamics, with an emphasis on the experimental side, but with the related theoretical work, at the level of state-of-the-art calculations of both the underlying potential energy surfaces and the reaction dynamics, noted throughout. In particular, the focus is (a) on the effect of molecular complexity and structure on product distributions, branching ratios and role of intersystem crossing for the multichannel, addition-elimination reactions of unsaturated hydrocarbons with O atoms, and (b) on the very detailed dynamics of the abstraction reactions of ground-state methane (and isotopologues) with atoms (F, Cl, O) and diatoms (OH), with inclusion of also rotational mode specificity in the vibrationally excited methane reactions.

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UV excitation of the CH2OO Criegee intermediate across most of the broad span of the (B 1A′)–(X 1A′) spectrum results in prompt dissociation to two energetically accessible asymptotes: O (1D) + H2CO (X 1A1) and O (3P) + H2CO (a 3A′′). Dissociation proceeds on multiple singlet potential energy surfaces that are coupled by two regions of conical intersection (CoIn). Velocity map imaging (VMI) studies reveal a bimodal total kinetic energy release (TKER) distribution for the O (1D) + H2CO (X 1A1) products with the major and minor components accounting for ca. 40% and ca. 20% on average of the available energy (E avl), respectively. The unexpected low TKER component corresponds to highly internally excited H2CO (X 1A1) products accommodating ca. 80% of E avl. Full dimensional trajectory calculations suggest that the bimodal TKER distribution of the O (1D) + H2CO (X 1A1) products originates from two different dynamical pathways: a primary pathway (69%) evolving through one CoIn region to products and a smaller component (20%) sampling both CoIn regions enroute to products. Those that access both CoIn regions likely give rise to the more highly internally excited H2CO (X 1A1) products. The remaining trajectories (11%) dissociate to O (3P) + H2CO (a 3A′′) products after traversing through both CoIn regions. The complementary experimental and theoretical investigation provides insight on the photodissociation of CH2OO via multiple dissociation pathways through two regions of CoIn that control the branching and energy distributions of products.

The extent of intersystem crossing in the O( 3 P) + C 6 H 6 reaction, a prototypical system for spin-forbidden reactions in oxygenated aromatic molecules, is theoretically evaluated for the first time. Calculations are performed using nonadiabatic transition-state theory coupled with stochastic master equation simulations and Landau–Zener theory. It is found that the dominant intersystem crossing pathways connect the T2 and S0 potential energy surfaces through at least two distinct minimum-energy crossing points. The calculated channel-specific rate constants and intersystem crossing branching fractions differ from previous literature estimates and provide valuable kinetic data for the investigation of benzene and polycyclic aromatic hydrocarbons oxidation in interstellar, atmospheric, and combustion conditions. The theoretical results are supported by crossed molecular beam experiments with electron ionization mass-spectrometric detection and time-of-flight analysis at 8.2 kcal/mol collision energy. This system is a suitable benchmark for theoretical and experimental studies of intersystem crossing in aromatic species.

Information on the detailed mechanism and dynamics (primary products, branching fractions (BFs), and channel specific rate constants as a function of temperature) for many important combustion reactions of O( 3 P) with unsaturated hydrocarbons is still lacking. We report synergistic experimental/theoretical studies on the mechanism and dynamics of the O( 3 P) + 1-C 4 H 8 (1-butene) reaction by combining crossed molecular beam (CMB) experiments with soft electron ionization mass-spectrometric detection and timeof-flight analysis at 10.5 kcal/mol collision energy (E c ) to highlevel ab initio electronic structure calculations of the underlying triplet and singlet potential energy surfaces (PESs) and statistical Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) computations of BFs including intersystem crossing (ISC). The reactive interaction of O( 3 P) with 1-butene is found to mainly break apart the 4-carbon atom chain, leading to the radical product channels ethyl + vinoxy (BF = 0.34 ± 0.11), methyl + C 3 H 5 O (BF = 0.28 ± 0.09), formyl + propyl (BF = 0.17 ± 0.05), ethyl + acetyl (BF = 0.014 ± 0.007), and butanal radical (ethylvinoxy) + H (BF = 0.013 ± 0.004), and molecular product channels formaldehyde + propenylidene/propene (BF = 0.15 ± 0.05) and butenone (ethyl ketene) + H 2 (BF = 0.037 ± 0.015). As some of these products can only be formed via ISC from triplet to singlet PESs, from BFs an extent of ISC of 50% is inferred. This value is significantly larger than that recently observed for O( 3 P) + propene (22%) at similar E c , underlying the question of how important it is to consider nonadiabatic effects for these and similar combustion reactions. Comparison of the derived BFs with those of statistical (RRKM/ME) simulations on the ab initio coupled triplet/singlet PESs shows good agreement, warranting the use of the RRKM/ME approach to provide information on the variation of BFs with temperature and to derive channel specific rate constants as a function of temperature (T) and pressure (p). Notably, ISC is predicted to decrease strongly with increasing temperature (from about 70% at 300 K to 46% at E c = 10.5 kcal/mol, and about 1% at 2000 K). The present results lead to a detailed understanding of the complex reaction mechanism of O( 3 P) + 1-butene and, by providing channel specific rate constants as a function of T and p, should facilitate the improvement of current fossil-fuel (1-butene) as well as biofuel (1-butanol) combustion models.

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...

The electronic spectrum of methyl vinyl ketone oxide (MVK-oxide), a four-carbon Criegee intermediate derived from isoprene ozonolysis, is examined on its second π* ← π transition, involving primarily the vinyl group, at UV wavelengths (λ) below 300 nm. A broad and unstructured spectrum is obtained by a UV-induced ground state depletion method with photoionization detection on the parent mass (m/z 86). Electronic excitation of MVK-oxide results in dissociation to O (1D) products that are characterized using velocity map imaging. Electronic excitation of MVK-oxide on the first π* ← π transition associated primarily with the carbonyl oxide group at λ > 300 nm results in a prompt dissociation and yields broad total kinetic energy release (TKER) and anisotropic angular distributions for the O (1D) + methyl vinyl ketone products. By contrast, electronic excitation at λ ≤ 300 nm results in bimodal TKER and angular distributions, indicating two distinct dissociation pathways to O (1D) products. One pathway is analogous to that at λ > 300 nm, while the second pathway results in very low TKER and isotropic angular distributions indicative of internal conversion to the ground electronic state and statistical unimolecular dissociation.

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