Polyatomic Reaction Dynamics Research Articles

Toward a Comprehensive Understanding of Mode-Specific Dynamics of Polyatomic Reactions: A Full-Dimensional Quantum Dynamics Study of the H + NH3 Reaction.

Mode specificity not only sheds light on reaction dynamics but also opens the door for chemical reaction control. This work reports a state-of-the-art full-dimensional quantum dynamics study on the prototypical hydrogen abstraction reaction of hydrogen with ammonia, which serves as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. By taking advantage of the (3 + 1) Radau-Jacobi coordinates, the bond-specific probabilities are resolved with the reactant NH3 initiated from either a non-degenerate or degenerate stretching vibrational state. The observed different atom-specific abstraction probabilities from individual states of the degenerate pair are rationalized in the local mode representation according to the different vibrational energy deposited in each N-H bond. It is verified that the three H atoms in NH3 have the same abstraction probability as that from the degenerate pair and the linear combination of the degenerate pair gives the same reaction probability. In addition, the symmetric and asymmetric stretching modes of the reactant NH3 have comparable efficacies on driving the reaction.

Vibrational and Rotational Gaussian Binning selections on QCT calculations for the OH + D2 → HOD((vHO’, vHOD’, vOD’), J’) + D reaction

Abstract A large sample has been employed to obtain total reaction cross sections and product state distributions (translational, vibrational and rotational) for the title reaction. Quasi-Classical Trajectory (QCT) calculations and the Gaussian Binning (GB) methodology have been used on Wu-Schatz-Lendvay-Fang-Harding (WSLFH) and Neural Networks (NN) potential energy surfaces. After a first selection, Vibrational-Gaussian Binning (V-GB), (vHO’, vHOD’, vOD’), and a second selection, Rotational-Gaussian Binning (R-GB), although the preference observed (odd or even J') is not exactly followed by each vibrational excitation of HOD, there is a similar behavior to the physical laser phenomenon observed in another triatomic molecule utilized in the gas laser of CO2. The dynamics of polyatomic chemical reactions (e.g., OH + D2 → HOD* + D and its isotopic variants), of relevance in the excitation/emission processes of current and future chemical lasers, could be studied theoretically from the results obtained in this article.

Universal crossed beam imaging studies of polyatomic reaction dynamics.

The marriage between high level quantum calculations and experimental advances in laser technology, quantum state control, and detection techniques have opened the door to the study of molecular collision dynamics at a new level of detail. However, one current challenge lies in adapting these powerful strategies to address questions beyond the scope of the small ground state systems that have largely been the focus of reaction dynamics investigations to-date. For molecules with intermediate or large size (more than 6 atoms), lack of spectroscopic information and spectral congestion limit quantum state preparation, control and detection for experiment, and the large number of degrees of freedom of the system makes accurate quantum dynamics calculations prohibitively expensive. Nevertheless, studies of the chemical dynamics of such systems can reveal novel aspects of reactivity not anticipated based upon the behavior of smaller model systems. This Perspective will highlight applications of soft vacuum ultraviolet photoionization at 157 nm as a universal probe in combination with crossed beams and DC slice velocity map ion imaging to study bimolecular reaction dynamics of molecules of intermediate or large size, illuminated with support of high-level ab initio calculations. Here, we report on the chemical dynamics of atomic oxygen or chlorine reactions with organic compounds: propanol isomers, alkylamines (N(CH3)3 and NH(CH3)2), and isobutene ((CH3)2CCH2) studied using this approach. The polyatomic radical products from the hydrogen abstraction process have been detected by 157 nm photoionization and their slice ion images embody translational energy and angular information that directly reflect the underlying collision dynamics. Various reaction mechanisms (such as direct abstraction and addition-elimination) along with the involvement of roaming dynamics and novel intersystem crossing pathways are presented. These demonstrate the power of this technique to reveal fundamentally new aspects of reaction dynamics that arise in larger reaction systems.

Benchmarking the polyatomic reaction dynamics of X+methane

With recent developments of sophisticated experimental techniques and advanced theoretical methods/computations, the field of chemical dynamics has reached the point that theory-experiment comparisons can be made at a quantitative level in very fine details for a prototypical A+BC system. As the system becomes larger, more degrees of freedom are involved and the complexity increases exponentially. At the same time, the multifaceted nature of polyatomic systems also opens up the possibilities for observing many new chemistry and novel phenomena—a land of opportunities. For the past 15 years or so my laboratory has delved into the reaction dynamics of methane+X (X: F, Cl, O(3P), and OH). This effort shifts the paradigm in the field of reaction dynamics by making the title reaction a benchmark polyatomic system. In this account, I shall disclose my thinking behind some of the key concepts and methods we introduced and how the unexpectedly discovered phenomena led to other uncharted territories. Those findings not only enrich our understanding of the specific reactions we studied at the most fundamental level and inspire the theoretical developments, but also shape our thinking and lay the foundation for future explorations of different aspects of the multifaceted nature of polyatomic reactivity.

Ab Initio Potential Energy Surfaces and Quantum Dynamics for Polyatomic Bimolecular Reactions.

There has been great progress in the development of potential energy surfaces (PESs) and quantum dynamics calculations in the gas phase. The establishment of a fitting procedure for highly accurate PESs and new developments in quantum reactive scattering on reliable PESs allow accurate characterization of reaction dynamics beyond triatomic systems. This review will give the recent development in our group in constructing ab initio PESs based on neural networks and the time-dependent wave packet calculations for bimolecular reactions beyond three atoms. Bimolecular reactions of current interest to the community, namely, OH + H2, H + H2O, OH + CO, H + CH4, and Cl + CH4, are focused on. Quantum mechanical characterization of these reactions uncovers interesting dynamical phenomena with an unprecedented level of sophistication and has greatly advanced our understanding of polyatomic reaction dynamics.

Crossed beam polyatomic reaction dynamics: recent advances and new insights.

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(3P), 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 CH4 (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.

Vibrational Control of Bimolecular Reactions with Methane by Mode, Bond, and Stereo Selectivity.

Vibrational motions of a polyatomic molecule are multifold and can be as simple as stretches or bends or as complex as concerted motions of many atoms. Different modes of excitation often possess different capacities in driving a bimolecular chemical reaction, with distinct dynamic outcomes. Reactions with vibrationally excited methane and its isotopologs serve as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. Here, some recent progress in this area is briefly reviewed. Particular emphasis is placed on the key concepts developed from those studies. The interconnections among mode and bond selectivity, Polanyi's rules, and newly introduced vibrational-induced steric phenomena are highlighted.

Semiclassical Wigner theory of photodissociation in three dimensions: Shedding light on its basis

The semiclassical Wigner theory (SCWT) of photodissociation dynamics, initially proposed by Brown and Heller [J. Chem. Phys. 75, 186 (1981)] in order to describe state distributions in the products of direct collinear photodissociations, was recently extended to realistic three-dimensional triatomic processes of the same type [Arbelo-González et al., Phys. Chem. Chem. Phys. 15, 9994 (2013)]. The resulting approach, which takes into account rotational motions in addition to vibrational and translational ones, was applied to a triatomic-like model of methyl iodide photodissociation and its predictions were found to be in nearly quantitative agreement with rigorous quantum results, but at a much lower computational cost, making thereby SCWT a potential tool for the study of polyatomic reaction dynamics. Here, we analyse the main reasons for this agreement by means of an elementary model of fragmentation explicitly dealing with the rotational motion only. We show that our formulation of SCWT makes it a semiclassical approximation to an approximate planar quantum treatment of the dynamics, both of sufficient quality for the whole treatment to be satisfying.

Quantum approaches to polyatomic reaction dynamics

In this review we discuss some quantum dynamical approaches to studying chemical reactions. We begin with a brief introductory section on how thermal rate constants can be calculated. This is followed by a section on time-dependent wave-packet calculations, where a brief description of the Multi-Configurational Time-Dependent Hartree (MCTDH) approach is included with emphasis on aspects relevant to thermal rate constant calculations. The next section is the main focus and treats time-independent quantum scattering theory biased to areas of our own interest. This includes a description of the guided spectral transform method to obtain eigenstates and the two-layer Lanczos method to treat presently up to twelve degrees of freedom. We end with concluding remarks and summary.

Quasi-Classical Trajectory–Gaussian Binning Study of the OH + D2 → HOD(v1′,v2′,v3′) + D Angle–Velocity and Vibrational Distributions at a Collision Energy of 0.28 eV

The angle-velocity and product vibrational state distributions for the OH + D(2) reaction at a collision energy of 0.28 eV have been calculated using the quasi-classical trajectory-gaussian binning (QCT-GB) method and the Wu-Schatz-Lendvay-Fang-Harding (WSLFH) analytical potential energy surface. Comparison with high resolution molecular beam experiments shows that, differing from what happens when using the standard QCT method (i.e., histogram binning), very good results are obtained for both distributions. Hence, the strong differences previously observed between QCT and experimental results mainly come from an inadequate pseudoquantization of HOD rather than from other quantum effects. This is probably the first time that such a high level of agreement between theory and high resolution experimental data has been found in polyatomic reaction dynamics.

Classical description in a quantum spirit of the prototype four-atom reaction OH + D2

Classical mechanics is the only practical way to simulate internuclear motion in many complex systems that are important for biology, materials science, and technology. It is therefore important to test classical dynamics on simpler systems. The OH + H(2) or OH + D(2) system is the prototype four-body chemical reaction for fundamental studies of this nature, and high-resolution experiments have been reported by Davis and co-workers. Here we use those experiments to test state-resolved quasi-classical trajectory calculations in a quantum spirit, and find good agreement with experiment. Our simulations correctly predict that the newly formed oxygen-deuterium bond in the HOD product is preferentially excited to the vibrational state v = 2, and they yield a state-resolved product translational energy distribution. Of special interest is that the triple (angle-velocity) differential cross section shows the same state-resolved structure associated with the product vibrational states that were analyzed. This level of accuracy has previously been achieved only for triatomic systems, and these new simulations demonstrate that the quasi-classical method with quantum corrections may represent a reasonable alternative to quantum scattering approaches for the description of polyatomic reaction dynamics.

The intrinsic reaction coordinate approach in polyatomic reaction dynamics

The intrinsic reaction coordinate (mc) approach in polyatomic reaction dynamics is investigated based on “vibrational frequency (VF) correlation diagrams” which demonstrate the frequency changes of the vibrational modes normal to the IRC. Ab initio calculations of the correlation diagrams for the chemically reacting systems, CH3 + H2 CH4 + H and the decomposition of formic acid, are performed and the qualitative features of the reaction dynamics are discussed.

Probing the dynamics of polyatomic multichannel elementary reactions by crossed molecular beam experiments with soft electron-ionization mass spectrometric detection

In this Perspective we highlight developments in the field of chemical reaction dynamics. Focus is on the advances recently made in the investigation of the dynamics of elementary multichannel radical-molecule and radical-radical reactions, as they have become possible using an improved crossed molecular beam scattering apparatus with universal electron-ionization mass spectrometric detection and time-of-flight analysis. These improvements consist in the implementation of (a) soft ionization detection by tunable low-energy electrons which has permitted us to reduce interfering signals originating from dissociative ionization processes, usually representing a major complication, (b) different beam crossing-angle set-ups which have permitted us to extend the range of collision energies over which a reaction can be studied, from very low (a few kJ mol(-1), as of interest in astrochemistry or planetary atmospheric chemistry) to quite high energies (several tens of kJ mol(-1), as of interest in high temperature combustion systems), and (c) continuous supersonic sources for producing a wide variety of atomic and molecular radical reactant beams. Exploiting these new features it has become possible to tackle the dynamics of a variety of polyatomic multichannel reactions, such as those occurring in many environments ranging from combustion and plasmas to terrestrial/planetary atmospheres and interstellar clouds. By measuring product angular and velocity distributions, after having suppressed or mitigated, when needed, the problem of dissociative ionization of interfering species (reactants, products, background gases) by soft ionization detection, essentially all primary reaction products can be identified, the dynamics of each reaction channel characterized, and the branching ratios determined as a function of collision energy. In general this information, besides being of fundamental relevance, is required for a predictive description of the chemistry of these environments via computer models. Examples are taken from recent on-going work (partly published) on the reactions of atomic oxygen with acetylene, ethylene and allyl radical, of great importance in combustion. A reaction of relevance in interstellar chemistry, as that of atomic carbon with acetylene, is also discussed briefly. Comparison with theoretical results is made wherever possible, both at the level of electronic structure calculations of the potential energy surfaces and dynamical computations. Recent complementary CMB work as well as kinetic work exploiting soft photo-ionization with synchrotron radiation are noted. The examples illustrated in this article demonstrate that the type of dynamical results now obtainable on polyatomic multichannel radical-molecule and radical-radical reactions might well complement reaction kinetics experiments and hence contribute to bridging the gap between microscopic reaction dynamics and thermal reaction kinetics, enhancing significantly our basic knowledge of chemical reactivity and understanding of the elementary reactions which occur in real-world environments.

Tracking the energy flow along the reaction path

We report a comprehensive study of the quantum-state correlation property of product pairs from reactions of chlorine atoms with both the ground-state and the CH stretch-excited CHD(3). In light of available ab initio theoretical results, this set of experimental data provides a conceptual framework to visualize the energy-flow pattern along the reaction path, to classify the activity of different vibrational modes in a reactive encounter, to gain deeper insight into the concept of vibrational adiabaticity, and to elucidate the intermode coupling in the transition-state region. This exploratory approach not only opens up an avenue to understand polyatomic reaction dynamics, even for motions at the molecular level in the fleeting transition-state region, but it also leads to a generalization of Polanyi's rules to reactions involving a polyatomic molecule.

A new crossed molecular beam apparatus using time-sliced ion velocity imaging technique

A new crossed molecular beam apparatus has been constructed for investigating polyatomic chemical reactions using the time-sliced ion velocity map imaging technique. A unique design is adopted for one of the two beam sources and allows us to set up the molecular beam source either horizontally or vertically. This can be conveniently used to produce versatile atomic or radical beams from photodissociation and as well as electric discharge. Intensive H-atom beam source with high speed ratio was produced by photodissociation of the HI molecule and was reacted with the CD(4) molecule. Vibrational-state resolved HD product distribution was measured by detecting the CD(3) product. Preliminary results were also reported on the F+SiH(4) reaction using the discharged F atom beam. These results demonstrate that this new instrument is a powerful tool for investigating chemical dynamics of polyatomic reactions.

Computational Chemistry of Polyatomic Reaction Kinetics and Dynamics: The Quest for an Accurate CH5Potential Energy Surface

Computational Chemistry of Polyatomic Reaction Kinetics and Dynamics: The Quest for an Accurate CH₅Potential Energy Surface

Rotationally resolved reactive scattering: Imaging detailed Cl+C2H6 reaction dynamics

The hydrogen atom abstraction reaction of Cl (2P3/2) with ethane has been studied using the crossed molecular beam technique with dc slice imaging at collision energies from 3.2 to 10.4 kcal/mol. The products HCl (v,J) (v = 0, J = 0-5) were state-selectively detected using 2+1 resonance enhanced multiphoton ionization. The images were used to obtain the center-of-mass frame product angular distributions and translational energy release distributions. Two general features were found in all probed HCl quantum states at 6.7 kcal/mol collision energy, and these features have distinct translational energy release and angular distributions, as described for HCl (v = 0, J = 2) in a recent preliminary report [Li et al., J. Chem. Phys. 124, 011102 (2006)]. The results for HCl (v = 0, J = 2) at four collision energies were also compared to investigate the energy-dependent dynamics. We discuss the reaction in terms of a variety of models of polyatomic reaction dynamics. The dynamics of this well studied system are more complicated than can be accounted for by a single mechanism, and the results call for further theoretical and experimental investigations.

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

In our laboratory a recent series of experiments by means of the crossed molecular beam (CMB) scattering technique with mass-spectrometric detection and time-of-flight analysis has been instrumental in fostering progress in the understanding of the dynamics of both simple triatomic insertion reactions and complex polyatomic addition–elimination reactions exhibiting competing channels. In the first part of this review we survey the advances made in the comprehension of the dynamics of the insertion reactions involving excited carbon, nitrogen and oxygen atoms – C(1D), N(2D), O(1D) – with H2(D2), as made possible by synergistic comparisons of experimental reactive differential cross-sections with the results of exact quantum, quasiclassical trajectory and statistical calculations on reliable ab initio potential energy surfaces. Related experimental and theoretical work from other laboratories is noted throughout. In the second part, we review the progress made in the understanding of the dynamics of polyatomic multichannel reactions, such as those of ground state oxygen and carbon atoms, O(3P) and C(3P), with the simplest alkyne, acetylene, and alkene, ethylene, as made possible by the gained capability of identifying virtually all primary reaction channels, characterising their dynamics, and determining their branching ratios. Such a capability is based on an improved crossed molecular beam instrument which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power, and variable beam crossing angle for a larger collision energy range and increased angular and velocity resolution. The scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces. These detailed studies on polyatomic multichannel reactions provide an important bridge between crossed beam dynamics and thermal kinetics research. Contents PAGE 1. Introduction 110 2. Experimental: improved crossed molecular beams apparatus 113 2.1. Soft electron-ionisation detection 114 2.2. Product angular and velocity distributions 116 2.3. Crossed molecular beam experiments with variable beam crossing angle 118 2.4. Supersonic sources of radical beams 120 3. Triatomic insertion reactions 120 3.1. Reaction C(1D) + H2 122 3.2. Reaction N(2D) + H2 126 3.3. Reaction O(1D) + H2 129 4. Polyatomic multichannel reactions 133 4.1. Reaction O(3P) + C2H2 134 4.1.1. Product angular and TOF distributions 135 4.1.2. Determination of branching ratios 138 4.1.3. Determination of product ionisation energies 138 4.2. Reaction C(3P) + C2H2 138 4.2.1. H and H2 elimination channels, and branching ratios 140 4.2.2. Ionisation energy of c/l-C3H radicals 145 4.3. Reaction O(3P) + C2H4 145 4.3.1. Observation of all product channels and branching ratios 146 4.4. Reaction C(3P) + C2H4 151 4.4.1. H-elimination channels 151 4.4.2. C–C bond fission channels 154 5. Summary and outlook 154 Acknowledgements 157 References 158

Development of Accurate Quantum Dynamical Methods for Tetraatomic Reactions

The time-dependent quantum wavepacket approach has proven to be a powerful computational approach for studying large scale quantum reactive scattering problems involving three or more atoms. This article presents an account of some recent development of time-dependent wavepacket methods for accurate quantum dynamics calculation of tetraatomic reactions in full dimensional space. The salient features of the time-dependent approach and important computational strategies that have been employed to successfully calculate state-specific reaction dynamics for realistic four-atom reactions are discussed. Some results from the application of the time-dependent methods to several specific reactions, in particular the benchmark H2 + OH reaction, are presented. The article is then highlighted with the presentation of a general reactant−product decoupling method for state-to-state reactive scattering study. Finally, the future outlook of the theoretical study of polyatomic reaction dynamics is discussed.

Molecular graphics and the classical adiabatic and nonadiabatic dynamics of small polyatomic reactions

A molecular graphics system for displaying the classical trajectories of small polyatomic molecule dynamics has been developed. The system can be used in adiabatic and nonadiabatic dynamic studies. Applications to the photodissociation of water at 10e V illustrate the usefulness of computer graphics in molecular reaction dynamics.