Crossed molecular beam reactive scattering: from simple triatomic to multichannel polyatomic reactions

Spiers Memorial Lecture: New directions in molecular scattering

The field of molecular scattering is reviewed as it pertains to gas-gas as well as gas-surface chemical reaction dynamics. We emphasize the importance of collaboration of experiment and theory, from which new directions of research are being pursued on increasingly complex problems. We review both experimental and theoretical advances that provide the modern toolbox available to molecular-scattering studies. We distinguish between two classes of work. The first involves simple systems and uses experiment to validate theory so that from the validated theory, one may learn far more than could ever be measured in the laboratory. The second class involves problems of great complexity that would be difficult or impossible to understand without a partnership of experiment and theory. Key topics covered in this review include crossed-beams reactive scattering and scattering at extremely low energies, where quantum effects dominate. They also include scattering from surfaces, reactive scattering and kinetics at surfaces, and scattering work done at liquid surfaces. The review closes with thoughts on future promising directions of research.

Theoretical Insights into the Dynamics of Gas-Phase Bimolecular Reactions with Submerged Barriers

Much attention has been paid to the dynamics of both activated gas-phase bimolecular reactions, which feature monotonically increasing integral cross sections and Arrhenius kinetics, and their barrierless capture counterparts, which manifest monotonically decreasing integral cross sections and negative temperature dependence of the rate coefficients. In this Perspective, we focus on the dynamics of gas-phase bimolecular reactions with submerged barriers, which often involve radicals or ions and are prevalent in combustion, atmospheric chemistry, astrochemistry, and plasma chemistry. The temperature dependence of the rate coefficients for such reactions is often non-Arrhenius and complex, and the corresponding dynamics may also be quite different from those with significant barriers or those completely dominated by capture. Recent experimental and theoretical studies of such reactions, particularly at relatively low temperatures or collision energies, have revealed interesting dynamical behaviors, which are discussed here. The new knowledge enriches our understanding of the dynamics of these unusual reactions.

Li + HF and Li + HCl Reactions Revisited I: QCT Calculations and Simulation of Experimental Results

The Li + HF and Li + HCl reactions share some common features. They have the same kinematics, relatively small barrier heights, bent transition states, and are both exothermic when the zero point energy is considered. Nevertheless, the pioneering crossed beam experiments by Lee and co-workers in the 80s (Becker et al., J. Chem. Phys. 1980, 73, 2833) revealed that the dynamics of the two reactions differ significantly, especially at low collision energies. In this work, we present theoretical simulations of their results in the laboratory frame (LAB), based on quasiclassical trajectories and obtained using accurate potential energy surfaces. The calculated LAB angular distributions and time-of-flight spectra agree well with the raw experimental data, although our simulations do not reproduce the experimentally derived center-of-mass (CM) differential cross section and velocity distributions. The latter were derived by forward convolution fitting under the questionable assumption that the CM recoil velocity and scattering angle distribution were uncoupled, while our results show that the coupling between them is relevant. Some important insights into the reaction mechanism discussed in the article by Becker et al. had not been contrasted with those that can be extracted from the theoretical results. Among them, the correlation between the angular momenta involved in the reactions has also been examined. Given the kinematics of both systems, the reagent orbital angular momentum, l , is almost completely transformed into the rotation of the product diatom, j'. However, contrary to the coplanar mechanism proposed in the original paper, we find that the initial and final relative orbital angular momenta are not necessarily parallel. Both reactions are found to be essentially direct, although about 15% of the LiFH complexes live longer than 200 fs.

A crossed molecular beams and computational study on the unusual reactivity of banana bonds of cyclopropane (c-C3H6; ) through insertion by ground state carbon atoms (C(3Pj))

The mechanism and chemical dynamics of the reaction of ground electronic state atomic carbon C(³P_j) with cyclopropane c-C₃H₆ have been explored by combining crossed molecular beams experiments with electronic structure calculations of the pertinent triplet C₄H₆ potential energy surface and statistical computations of product branching ratios under single-collision conditions. The experimental findings suggest that the reaction proceeds via indirect scattering dynamics through triplet C₄H₆ reaction intermediate(s) leading to C₄H₅ product(s) plus atomic hydrogen via a tight exit transition state, with the overall reaction exoergicity evaluated as 231 ± 52 kJ mol^-1. The calculations indicate that C(³P_j) can easily insert into one of the three equivalent C-C 'banana' bonds of cyclopropane overcoming a low barrier of only 2 kJ mol^-1 following the formation of a van der Waals reactant complex stabilized by 15 kJ mol^-1. The carbon atom insertion into one of the six C-H bonds is also feasible via a slightly higher barrier of 5 kJ mol^-1. These results highlight an unusual reactivity of cyclopropane's banana C-C bonds, which behave more like unsaturated C-C bonds with a π-character than saturated σ C-C bonds, which are known to be generally unreactive toward the ground electronic state atomic carbon such as in ethane (C₂H₆). The statistical theory predicts the overall product branching ratios at the experimental collision energy as 50% for 1-butyn-4-yl, 33% for 1,3-butadien-2-yl, i-C₄H₅, and 11% for 1,3-butadien-1-yl, n-C₄H₅, with i-C₄H₅ (230 kJ mol^-1 below the reactants) favored by the C-C insertion providing the best match with the experimentally observed reaction exoergicity. The C(³P_j) + c-C₃H₆ reaction is predicted to be a source of C₄H₅ radicals under the conditions where its low entrance barriers can be overcome, such as in planetary atmospheres or in circumstellar envelopes but not in cold molecular clouds. Both i- and n-C₄H₅ can further react with acetylene eventually producing the first aromatic ring and hence, the reaction of the atomic carbon with c-C₃H₆ can be considered as an initial step toward the formation of benzene.

Unsaturated Dinitriles Formation Routes in Extraterrestrial Environments: A Combined Experimental and Theoretical Investigation of the Reaction between Cyano Radicals and Cyanoethene (C2H3CN)

The reaction between cyano radicals (CN, X²Σ⁺) and cyanoethene (C₂H₃CN) has been investigated by a combined approach coupling crossed molecular beam (CMB) experiments with mass spectrometric detection and time-of-flight analysis at a collision energy of 44.6 kJ mol^–1 and electronic structure calculations to determine the relevant potential energy surface. The experimental results can be interpreted by assuming the occurrence of a dominant reaction pathway leading to the two but-2-enedinitrile (1,2-dicyanothene) isomers (E- and Z-NC–CH=CH–CN) in a H-displacement channel and, to a much minor extent, to 1,1-dicyanoethene, CH₂C(CN)₂. In order to derive the product branching ratios under the conditions of the CMB experiments and at colder temperatures, including those relevant to Titan and to cold interstellar clouds, we have carried out RRKM statistical calculations using the relevant potential energy surface of the investigated reaction. We have also estimated the rate coefficient at very low temperatures by employing a semiempirical method for the treatment of long-range interactions. The reaction has been found to be barrierless and fast also under the low temperature conditions of cold interstellar clouds and the atmosphere of Titan. Astrophysical implications and comparison with literature data are also presented. On the basis of the present work, 1,2-dicyanothene and 1,1-dicyanothene are excellent candidates for the search of dinitriles in the interstellar medium.

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

Atomic oxygen reactions can contribute significantly to the oxidation of unsaturated aliphatic and aromatic hydrocarbons. The reaction mechanism is started by electrophilic O atom addition to the unsaturated bond(s) to...

A chemical dynamics study of the reaction of the methylidyne radical (CH, X2Π) with dimethylacetylene (CH3CCCH3, X1A1g)

The gas-phase reaction of the methylidyne (CH; X²Π) radical with dimethylacetylene (CH₃CCCH₃; X¹A_1g) was studied at a collision energy of 20.6 kJ mol^-1 under single collision conditions with experimental results merged with ab initio calculations of the potential energy surface (PES) and ab initio molecule dynamics (AIMD) simulations. The crossed molecular beam experiment reveals that the reaction proceeds barrierless via indirect scattering dynamics through long-lived C₅H₇ reaction intermediate(s) ultimately dissociating to C₅H₆ isomers along with atomic hydrogen with atomic hydrogen predominantly released from the methyl groups as verified by replacing the methylidyne with the D1-methylidyne reactant. AIMD simulations reveal that the reaction dynamics are statistical leading predominantly to p28 (1-methyl-3-methylenecyclopropene, 13%) and p8 (1-penten-3-yne, 81%) plus atomic hydrogen with a significant amount of available energy being channeled into the internal excitation of the polyatomic reaction products. The dynamics are controlled by addition to the carbon-carbon triple bond with the reaction intermediates eventually eliminating a hydrogen atom from the methyl groups of the dimethylacetylene reactant forming 1-methyl-3-methylenecyclopropene (p28). The dominating pathways reveal an unexpected insertion of methylidyne into one of the six carbon-hydrogen single bonds of the methyl groups of dimethylacetylene leading to the acyclic intermediate, which then decomposes to 1-penten-3-yne (p8). Therefore, the methyl groups of dimethylacetylene effectively 'screen' the carbon-carbon triple bond from being attacked by addition thus directing the dynamics to an insertion process as seen exclusively in the reaction of methylidyne with ethane (C₂H₆) forming propylene (CH₃C₂H₃). Therefore, driven by the screening of the triple bond, one propynyl moiety (CH₃CC) acts in four out of five trajectories as a spectator thus driving an unexpected, but dominating chemistry in analogy to the methylidyne - ethane system.

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

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 (E_c) 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 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(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.

Advances and New Challenges to Bimolecular Reaction Dynamics Theory

Dynamics of bimolecular reactions in the gas phase are of foundational importance in combustion, atmospheric chemistry, interstellar chemistry, and plasma chemistry. These collision-induced chemical transformations are a sensitive probe of the underlying potential energy surface(s). Despite tremendous progress in past decades, our understanding is still not complete. In this Perspective, we survey the recent advances in theoretical characterization of bimolecular reaction dynamics, stimulated by new experimental observations, and identify key new challenges.

Experimental and theoretical studies of the N(2D) + H2 and D2 reactions

This study reports the first kinetic measurements of the N(²D) + H₂, D₂ reactions below 200 K.

A global ab initio potential energy surface and dynamics of the proton-transfer reaction: OH− + D2 → HOD + D−

The reaction mechanisms of OH⁻ + D₂ → HOD + D⁻ were first revealed by theory, based on an accurate full-dimensional PES.

Stereodynamical control of product branching in multi-channel barrierless hydrogen abstraction of CH3OH by F

Comprehensive dynamical simulations of a prototypical multi-channel reaction on a globally accurate potential energy surface show that the non-statistical product branching is dictated by unique stereodynamics in the entrance channels.

Hydrogen abstraction from methanol (CH₃OH) by F atoms presents an ideal proving ground to investigate dynamics of multi-channel reactions, because two types of hydrogen can be abstracted from the methanol molecule leading to the HF + CH₃O and HF + CH₂OH products. Using the quasi-classical trajectory approach on a globally accurate potential energy surface based on high-level ab initio calculations, this work reports a comprehensive dynamical investigation of this multi-channel reaction, yielding measurable attributes including integral and differential cross sections, as well as branching ratios. It is shown that while complex-forming and direct mechanisms coexist at low collision energies, these barrierless reaction channels are dominated at high energies by the direct mechanism, in which the reaction is only possible for trajectories entering into the respective dynamical cones of acceptance. Perhaps more importantly, the non-statistical product branching is found to be dictated by unique stereodynamics in the entrance channels.

Combined Experimental-Theoretical Study of the OH + CO → H + CO2 Reaction Dynamics

A combined experimental-theoretical study is performed to advance our understanding of the dynamics of the prototypical tetra-atom, complex-forming reaction OH + CO → H + CO₂, which is also of great practical relevance in combustion, Earth's atmosphere, and, potentially, Mars's atmosphere and interstellar chemistry. New crossed molecular beam experiments with mass spectrometric detection are analyzed together with the results from previous experiments and compared with quasi-classical trajectory (QCT) calculations on a new, full-dimensional potential energy surface (PES). Comparisons between experiment and theory are carried out both in the center-of-mass and laboratory frames. Good agreement is found between experiment and theory, both for product angular and translational energy distributions, leading to the conclusion that the new PES is the most accurate at present in elucidating the dynamics of this fundamental reaction. Yet, small deviations between experiment and theory remain and are presumably attributable to the QCT treatment of the scattering dynamics.

Laboratory Studies of Methane and Its Relationship to Prebiotic Chemistry

To examine how prebiotic chemical evolution took place on Earth prior to the emergence of life, laboratory experiments have been conducted since the 1950s. Methane has been one of the key molecules in these investigations. In earlier studies, strongly reducing gas mixtures containing methane and ammonia were used to simulate possible reactions in the primitive atmosphere of Earth, producing amino acids and other organic compounds. Since Earth's early atmosphere is now considered to be less reducing, the contribution of extraterrestrial organics to chemical evolution has taken on an important role. Such organic molecules may have come from molecular clouds and regions of star formation that created protoplanetary disks, planets, asteroids, and comets. The interstellar origin of organics has been examined both experimentally and theoretically, including laboratory investigations that simulate interstellar molecular reactions. Endogenous and exogenous organics could also have been supplied to the primitive ocean, making submarine hydrothermal systems plausible sites of the generation of life. Experiments that simulate such hydrothermal systems where methane played an important role have consequently been conducted. Processes that occur in other Solar System bodies offer clues to the prebiotic chemistry of Earth. Titan and other icy bodies, where methane plays significant roles, are especially good targets. In the case of Titan, methane is both in the atmosphere and in liquidospheres that are composed of methane and other hydrocarbons, and these have been studied in simulation experiments. Here, we review the wide range of experimental work in which these various terrestrial and extraterrestrial environments have been modeled, and we examine the possible role of methane in chemical evolution. Key Words: Methane-Interstellar environments-Submarine hydrothermal systems-Titan-Origin of life. Astrobiology 17, 786-812.

Dynamical importance of van der Waals saddle and excited potential surface in C(1D)+D2 complex-forming reaction

Encouraged by recent advances in revealing significant effects of van der Waals wells on reaction dynamics, many people assume that van der Waals wells are inevitable in chemical reactions. Here we find that the weak long-range forces cause van der Waals saddles in the prototypical C(¹D)+D₂ complex-forming reaction that have very different dynamical effects from van der Waals wells at low collision energies. Accurate quantum dynamics calculations on our highly accurate ab initio potential energy surfaces with van der Waals saddles yield cross-sections in close agreement with crossed-beam experiments, whereas the same calculations on an earlier surface with van der Waals wells produce much smaller cross-sections at low energies. Further trajectory calculations reveal that the van der Waals saddle leads to a torsion then sideways insertion reaction mechanism, whereas the well suppresses reactivity. Quantum diffraction oscillations and sharp resonances are also predicted based on our ground- and excited-state potential energy surfaces.

It is commonly held that van der Waals wells are inevitable in chemical reactions. Here, the authors show that weak van der Waals forces in the entrance channel of a prototypical complex-forming reaction cause a van der Waals saddle instead, with different dynamical effects from a well at low collision energies.

Recent advances in quantum scattering calculations on polyatomic bimolecular reactions

This review surveys quantum scattering calculations on chemical reactions of polyatomic molecules in the gas phase published in the last ten years.

Crossed beam polyatomic reaction dynamics: recent advances and new insights

This review summarizes the developments in polyatomic reaction dynamics, focusing on reactions of unsaturated hydrocarbons with O-atoms and methane with atoms/radicals.

Control of chemical reactivity by transition-state and beyond

It has been long established that the transition state for an activated reaction controls the overall reactivity, serving as the bottleneck for reaction flux. However, the role of the transition state in regulating quantum state resolved reactivity has only been addressed more recently, thanks to advances in both experimental and theoretical techniques. In this perspective, we discuss some recent advances in understanding mode-specific reaction dynamics in bimolecular reactions, mainly focusing on the X + H2O/CH4 (X = H, F, Cl, and O(3P)) systems, extensively studied in our groups. These advances shed valuable light on the importance of the transition state in mode-specific and steric dynamics of these prototypical reactions. It is shown that many mode-specific phenomena can be understood in terms of a transition-state based model, which assumes in the sudden limit that the ability of a reactant mode for promoting the reaction stems from its coupling with the reaction coordinate at the transition state. Yet, in some cases the long-range anisotropic interactions in the entrance (or exit) valley, which govern how the trajectories reach (or leave) the transition state, also come into play, thus modifying the reactive outcomes.

Isomer-Specific Chemistry in the Propyne and Allene Reactions with Oxygen Atoms: CH3CH + CO versus CH2CH2 + CO Products

We report direct experimental and theoretical evidence that, under single-collision conditions, the dominant product channels of the O((3)P) + propyne and O((3)P) + allene isomeric reactions lead in both cases to CO formation, but the coproducts are singlet ethylidene ((1)CH3CH) and singlet ethylene (CH2CH2), respectively. These data, which settle a long-standing issue on whether ethylidene is actually formed in the O((3)P) + propyne reaction, suggest that formation of CO + alkylidene biradicals may be a common mechanism in O((3)P) + alkyne reactions, in contrast to formation of CO + alkene molecular products in the corresponding isomeric O((3)P) + diene reactions, either in combustion or other gaseous environments. These findings are of fundamental relevance and may have implications for improved combustion models. Moreover, we predict that the so far neglected (1)CH3CH + CO channel is among the main reaction routes also when the C3H4O singlet potential energy surface is accessed from the OH + C3H3 (propargyl) entrance channel, which are radical species playing a key role in many combustion systems.

Recent Advances in Quantum Dynamics of Bimolecular Reactions

In this review, we survey the latest advances in theoretical understanding of bimolecular reaction dynamics in the past decade. The remarkable recent progress in this field has been driven by more accurate and efficient ab initio electronic structure theory, effective potential-energy surface fitting techniques, and novel quantum scattering algorithms. Quantum mechanical characterization of bimolecular reactions continues to uncover interesting dynamical phenomena in atom-diatom reactions and beyond, reaching an unprecedented level of sophistication. In tandem with experimental explorations, these theoretical developments have greatly advanced our understanding of key issues in reaction dynamics, such as microscopic reaction mechanisms, mode specificity, product energy disposal, influence of reactive resonances, and nonadiabatic effects.

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

Comprehension of the detailed mechanism of O((3)P) + 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((3)P) + propene reaction by combining crossed molecular beams experiments using mass spectrometric detection at 9.3 kcal/mol collision energy (Ec) 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 Ec's and temperatures higher than 1000 K, main products are found to be formaldehyde (H2CO) and triplet ethylidene ((3)CH3CH) 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.

A nine-dimensional global potential energy surface for NH4(X2A1) and kinetics studies on the H + NH3 ↔ H2 + NH2 reaction

A nine-dimensional global potential energy surface (PES) for the NH₄ system is developed from ∼10⁵ high-level ab initio points and the hydrogen abstraction kinetics on the PES agree with experiment.

Combined crossed beam and theoretical studies of the N(2D) + C2H4 reaction and implications for atmospheric models of Titan

The dynamics of the H displacement channels in the reaction N((2)D) + C(2)H(4) have been investigated by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at two different collision energies (17.2 and 28.2 kJ/mol). The interpretation of the scattering results is assisted by new electronic structure calculations of stationary points and product energetics for the C(2)H(4)N ground state doublet potential energy surface. RRKM statistical calculations have been performed to derive the product branching ratio under the conditions of the present experiments and of the atmosphere of Titan. Similarities and differences with respect to a recent study performed in crossed beam experiments coupled to ionization via tunable VUV synchrotron radiation are discussed (Lee, S.-H.; et al. Phys. Chem. Chem. Phys.2011, 13, 8515-8525). Implications for the atmospheric chemistry of Titan are presented.

Intersystem crossing and dynamics in O(3P) + C2H4 multichannel reaction: experiment validates theory

The O((3)P) + C(2)H(4) reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH(2)CHO, H + CH(3)CO, H(2) + CH(2)CO, CH(3) + HCO, CH(2) + CH(2)O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH(3) + HCO and H + CH(2)CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.