Reactive scattering dynamics in atom+polyatomic systems: F+C2H6-->HF(v,J)+C2H5

Current Status of the X + C2H6 [X ≡ H, F(2P), Cl(2P), O(3P), OH] Hydrogen Abstraction Reactions: A Theoretical Review

This paper is a detailed review of the chemistry of medium-size reactive systems using the following hydrogen abstraction reactions with ethane, X + C₂H₆ → HX + C₂H₅; X ≡ H, F(²P), Cl(²P), O(³P) and OH, and focusing attention mainly on the theoretical developments. These bimolecular reactions range from exothermic to endothermic systems and from barrierless to high classical barriers of activation. Thus, the topography of the reactive systems changes from reaction to reaction with the presence or not of stabilized intermediate complexes in the entrance and exit channels. The review begins with some reflections on the inherent problems in the theory/experiment comparison. When one compares kinetics or dynamics theoretical results with experimental measures, one is testing both the potential energy surface describing the nuclei motion and the kinetics or dynamics method used. Discrepancies in the comparison may be due to inaccuracies of the surface, limitations of the kinetics or dynamics methods, and experimental uncertainties that also cannot be ruled out. The paper continues with a detailed review of some bimolecular reactions with ethane, beginning with the reactions with hydrogen atoms. The reactions with halogens present a challenge owing to the presence of stabilized intermediate complexes in the entrance and exit channels and the influence of the spin-orbit states on reactivity. Reactions with O(³P) atoms lead to three surfaces, which is an additional difficulty in the theoretical study. Finally, the reactions with the hydroxyl radical correspond to a reactive system with ten atoms and twenty-four degrees of freedom. Throughout this review, different strategies in the development of analytical potential energy surfaces describing these bimolecular reactions have been critically analyzed, showing their advantages and limitations. These surfaces are fitted to a large number of ab initio calculations, and we found that a huge number of calculations leads to accurate surfaces, but this information does not guarantee that the kinetics and dynamics results match the experimental measurements.

Time-dependent quantum dynamics study of the F + C2H6 → HF + C2H5 reaction

The reaction probabilities, integral cross sections, energy efficiency and rate constants are investigated for the F + C₂H₆ reaction using the quantum reaction dynamics, wave packet method. The ground-state integral cross section calculated using a six-degree-of-freedom approach is in very good agreement with the quasi-classical trajectory results. We find that the H-CH₂CH₃ stretching motion has the largest enhancement to reactivity, followed by the H-CH₂-CH₃ bending motion. However, the stretching motion between CH₂ and CH₃ slightly hinders the reactivity. The energy-form efficacy based on an equal amount of total energy shows that translational energy is more effective in enhancing the reactivity than vibrational energy of the H-CH₂CH₃ stretching motion at a relatively lower translational energy, while the reverse is true at a relatively high translational energy. An energy-shifting method is employed to calculate the full-dimensional rate constants. The quantum rate constants agree well with one of the two main experimental measurements, and the activation energy has an excellent agreement with the one calculated using canonical variational transition-state theory.

Vibrational mode-specific dynamics of the F(2P3/2) + C2H6 → HF + C2H5 reaction

We investigate the competing effect of vibrational and translational excitation and the validity of the Polanyi rules in the early- and negative-barrier F(²P_3/2) + C₂H₆ → HF + C₂H₅ reaction by performing quasi-classical dynamics simulations on a recently developed full-dimensional multi-reference analytical potential energy surface. The effect of five normal-mode excitations of ethane on the reactivity, the mechanism, and the post-reaction energy flow is followed through a wide range of collision energies. Promoting effects of vibrational excitations and interaction time, related to the slightly submerged barrier, are found to be suppressed by the early-barrier-induced translational enhancement, in contrast to the slightly late-barrier Cl + C₂H₆ reaction. The excess vibrational energy mostly converts into ethyl internal excitation while collision energy is transformed into product separation. The substantial reaction energy excites the HF vibration, which tends to show mode-specificity and translational energy dependence as well. With increasing collision energy, direct stripping becomes dominant over the direct rebound and indirect mechanisms, being basically independent of reactant excitation.

First-Principles Reaction Dynamics beyond Six-Atom Systems

Moving beyond the six-atomic benchmark systems, we discuss the new age and future of first-principles reaction dynamics, which investigates complex, multichannel chemical reactions. We describe the methodology starting from the benchmark ab initio characterization of the stationary points, followed by full-dimensional potential energy surface (PES) developments and reaction dynamics computations. We highlight our composite ab initio approach providing benchmark stationary-point properties with subchemical accuracy, the Robosurfer program system enabling automatic PES development, and applications for the Cl + C2H6, F + C2H6, and OH- + CH3I post-six-atom reactions focusing on ab initio issues and their solutions as well as showing the excellent agreement between theory and experiment.

Universal crossed beam imaging studies of polyatomic reaction dynamics

Crossed-beam imaging studies of polyatomic reactions show surprising dynamics not anticipated by extrapolation from smaller model systems.

Benchmark ab initio characterization of the abstraction and substitution pathways of the OH + CH4/C2H6 reactions

We report benchmark ab initio stationary-point properties for the hydrogen-abstraction, hydrogen-substitution, and methyl-substitution pathways of the OH + CH₄/C₂H₆ reactions.

Imaging H abstraction dynamics in crossed molecular beams: O(3P) + propanol isomers

Direct rebound dynamics are revealed for bimolecular reaction of the ground state O(³P) atom with propanol isomers, involving the post transition state long-range dipole–dipole interaction between the dipolar OH and hydroxypropyl radicals.

Detailed benchmark ab initio mapping of the potential energy surfaces of the X + C2H6 [X = F, Cl, Br, I] reactions

We provide benchmark relative energies for the stationary points of three different channels of the halogen atom + ethane reactions.

F(2P) + C2H6 → HF + C2H5 kinetics study based on a new analytical potential energy surface

An exhaustive kinetics study was performed for the title reaction using two theoretical approaches: variational transition-state theory and quasi-classical trajectory calculations, based on an original new analytical full-dimensional potential energy surface, named PES-2018, which has been fitted to high-level ab initio calculations.

Role of an ethyl radical and the problem of HF(v) bimodal vibrational distribution in the F(2P) + C2H6 → HF(v) + C2H5 reaction

A theoretical study of the dynamics of the F(²P) + C₂H₆ hydrogen abstraction reaction was presented using quasi-classical trajectories propagated on an ab initio fitted global potential energy surface, PES-2018.

Branching Ratios and Vibrational Distributions in Water-Forming Reactions of OH and OD Radicals with Methylamines

Reactions of OH and OD radicals with (CH3)3N, (CH3)2NH, and CH3NH2 were studied by Fourier transform infrared emission spectroscopy (FTIR) of the water product molecules from a fast-flow reactor at 298 K. The rate constants (4.4 ± 0.5) × 10(-11), (5.2 ± 0.8) × 10(-11), and (2.0 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) were determined for OD + (CH3)3N, (CH3)2NH, and CH3NH2, respectively, by comparing the HOD emission intensities to the HOD intensity from the OD reaction with H2S. Abstraction from the nitrogen site competes with abstraction from the methyl group, as obtained from an analysis of the HOD and D2O emission intensities from the OD reactions with the deuterated reactants, (CD3)2NH and CD3NH2. After adjustment for the hydrogen-deuterium kinetic isotope effect, the product branching fractions of the hydrogen abstraction from the nitrogen for di- and monomethylamine were found to be 0.34 ± 0.04 and 0.26 ± 0.05, respectively. Vibrational distributions of the H2O, HOD, and D2O molecules are typical for direct hydrogen atom abstraction from polar molecules, even though activation energies are negative because of the formation of pre-transition-state complexes. Comparison is made to the reactions of hydroxyl radicals with ammonia and with other compounds with primary C-H bonds to discuss specific features of disposal of energy to water product.

Crossed-beam DC slice imaging of fluorine atom reactions with linear alkanes

We report the reaction dynamics of F atom with selected alkanes studied by crossed beam scattering with DC slice ion imaging. The target alkanes are propane, n-butane, and n-pentane. The product alkyl radicals are probed by 157 nm single photon ionization following reaction at a collision energy of ∼10 kcal mol(-1). The analyzed data are compared with the corresponding theoretical studies. Reduced translational energy distributions for each system show similar trends with little of the reaction exoergicity appearing in translation. However, the pentane reaction shows a somewhat smaller fraction of available energy in translation than the other two, suggesting greater energy channeled into pentyl internal degrees of freedom. The center-of-mass angular distributions all show backscattering as well as sharp forward scattering that decreases in relative intensity with the size of the molecule. Possible reasons for these trends are discussed.

Dynamics of chlorine atom reactions with hydrocarbons: insights from imaging the radical product in crossed beams

We present a comprehensive overview of our ongoing studies applying dc slice imaging in crossed molecular beams to probe the dynamics of chlorine atom reactions with polyatomic hydrocarbons. Our approach consists in measuring the full velocity-flux contour maps of the radical products using vacuum ultraviolet "soft" photoionization at 157 nm. Our overall goal is to extend the range of chemical dynamics investigations from simple triatomic or tetraatomic molecules to systematic investigations of a sequence of isomers or a homologous series of reactants of intermediate size. These experimental investigations are augmented by high-level ab initio calculations which, taken together, reveal trends in product energy and angular momentum partitioning and offer deep insight into the reaction mechanisms as a function of structure, bonding patterns, and kinematics. We explore these issues in alkanes, for which only direct reactive encounters are found, and in unsaturated hydrocarbons, for which an addition-elimination mechanism competes with direct abstraction. The results for alkene addition-elimination in particular suggest a new view of these reactions: The only pathway to HCl elimination is accessed by means of roaming excursions of the Cl atom from the strongly bound adduct.

Dynamics of CN+alkane reactions by crossed-beam dc slice imaging

The hydrogen atom abstraction reactions of CN (X (2)Sigma(+)) with alkanes have been studied using the crossed molecular beam technique with dc slice ion imaging at collision energies of 7.5 and 10.8 kcalmol. The product alkyl radical images were obtained via single photon ionization at 157 nm for the reactions of CN (X (2)Sigma(+)) with n-butane, n-pentane, n-hexane, and cyclohexane. From analysis of the images, we obtained the center-of-mass frame product angular distributions and translational energy distributions directly. The results indicate that the products are largely backscattered and that most of the available energy ( approximately 80%-85%) goes to the internal energy of the products. The reaction dynamics is discussed in light of recent kinetics data, theoretical calculations, and results for related halogen and oxygen atom reactions.

Barriers of Hydrogen Abstraction from Primary, Secondary, and Tertiary Alkane Sites by O(3P)

We present an extensive study of the barriers of hydrogen abstraction from primary, secondary, and tertiary sites of acyclic alkanes by ground-state oxygen atoms. Our studies include the characterization of the lowest-energy transition states of the O(3P) reactions with methane, ethane, propane, isobutane, and isopentane using high-level ab initio methods. The order of the calculated barriers heights is primary > secondary > tertiary, in agreement with the trends gleaned from kinetic measurements. Analysis of the transition-state geometry reveals a shift toward more reagents-like structures in the primary --> secondary --> tertiary sequence, which concurs with the expectation from Hammond's postulate. Using the ab initio data, we calculate thermal rate constants via transition-state theory. Our highest-level calculations indicate that the room-temperature relative reactivities of primary, secondary, and tertiary alkane sites in hydrogen-abstraction reactions by ground-state oxygen atoms are 1, 29, and 422, respectively. These results are used to interpret recent experiments on the reactions of O(3P) with liquid alkanes.

Quantum state resolved scattering dynamics of F+HCl→HF(v,J)+Cl

State-to-state reaction dynamics of the reaction F+HCl-->HF(v,J)+Cl have been studied under single-collision conditions using an intense discharge F atom source in crossed supersonic molecular beams at Ecom=4.3(1.3) kcal/mol. Nascent HF product is monitored by shot-noise limited direct infrared laser absorption, providing quantum state distributions as well as additional information on kinetic energy release from high resolution Dopplerimetry. The vibrational distributions are highly inverted, with 34(4)%, 44(2)%, and 8(1)% of the total population in vHF=1, 2, and 3, respectively, consistent with predominant energy release into the newly formed bond. However, there is a small [14(1)%] but significant formation channel into the vHF=0 ground state, which is directly detectable for the first time via direct absorption methods. Of particular dynamical interest, both the HF(v=2,J) and HF(v=1,J) populations exhibit strongly bimodal J distributions. These results differ significantly from previous flow and arrested-relaxation studies and may signal the presence of microscopic branching in the reaction dynamics.

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.

The roaming atom pathway in formaldehyde decomposition

We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H(2) and CO following excitation to the 2(1)4(1) and 2(1)4(3) transitions in S(1). The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, j(CO)=5-45). These high-resolution measurements reveal the correlated internal state distribution in the H(2) cofragments. The results show that rotationally hot CO (j(CO) approximately 45) is produced in conjunction with vibrationally "cold" H(2) fragments (v=0-5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H(2)CO-->H+HCO) we also find formation of rotationally cold CO (j(CO)=5-28) correlated to highly vibrationally excited H(2) (v=6-8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this "roaming" mechanism are the focus of this paper. The correlations between the vibrational states of H(2) and rotational states of CO formed following excitation on the 2(1)4(3) transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel.

Quantum-state resolved reaction dynamics at the gas-liquid interface: Direct absorption detection of HF(v,J) product from F(P2)+Squalane

Exothermic reactive scattering of F atoms at the gas-liquid interface of a liquid hydrocarbon (squalane) surface has been studied under single collision conditions by shot noise limited high-resolution infrared absorption on the nascent HF(v,J) product. The nascent HF(v,J) vibrational distributions are inverted, indicating insufficient time for complete vibrational energy transfer into the surface liquid. The HF(v=2,J) rotational distributions are well fit with a two temperature Boltzmann analysis, with a near room temperature component (T(TD) approximately equal to 290 K) and a second much hotter scattering component (T(HDS) approximately equal to 1040 K). These data provide quantum state level support for microscopic branching in the atom abstraction dynamics corresponding to escape of nascent HF from the liquid surface on time scales both slow and fast with respect to rotational relaxation.

Direct evidence for nonadiabatic dynamics in atom+polyatom reactions: Crossed-jet laser studies of F+D2O→DF+OD

Quantum-state-resolved reactive-scattering dynamics of F+D(2)O-->DF+OD have been studied at E(c.m.)=5(1) kcal/mol in low-density crossed supersonic jets, exploiting pulsed discharge sources of F atom and laser-induced fluorescence to detect the nascent OD product under single-collision conditions. The product OD is formed exclusively in the v(OD)=0 state with only modest rotational excitation (<E(rot)> =0.50(1) kcal/mol), consistent with the relatively weak coupling of the 18.1(1) kcal/mol reaction exothermicity into "spectator" bond degrees of freedom. The majority of OD products [68(1)%] are found in the ground ((2)Pi(32) (+/-)) spin-orbit state, which adiabatically correlates with reaction over the lowest and only energetically accessible barrier (DeltaE( not equal) approximately 4 kcal/mol). However, 32(1)% of molecules are produced in the excited spin-orbit state ((2)Pi(12) (+/-)), although from a purely adiabatic perspective, this requires passage over a DeltaE( not equal) approximately 25 kcal/mol barrier energetically inaccessible at these collision energies. This provides unambiguous evidence for nonadiabatic surface hopping in F+D(2)O atom abstraction reactions, indicating that reactive-scattering dynamics even in simple atom+polyatom systems is not always isolated on the ground electronic surface. Additionally, the nascent OD rotational states are well fitted by a two-temperature Boltzmann distribution, suggesting correlated branching of the reaction products into the DF(v=2,3) vibrational manifold.

Stereodynamics of Chlorine Atom Reactions with Organic Molecules

A series of recent experimental and computational studies has explored how the dynamics of hydrogen abstraction from organic molecules are affected by the presence of functional groups in the molecule and by basic structural motifs such as strained ring systems. Comparisons drawn between reactions of Cl atoms with alkanes such as ethane, Cl + CH3CH3--> HCl + CH3CH2, which serve as benchmark systems, and with functionalized molecules such as alcohols, amines, and alkyl halides, Cl + CH3X --> HCl + CH2X (X = OH, NH2, halogen, etc.) expose a wealth of mechanistic detail. Although the scattering dynamics, as revealed from measured angular distributions of the velocities of the HCl with quantum-state resolution, show many similarities, much-enhanced rotational excitation of the HCl products is observed from reactions of the functionalized molecules. The degree of rotational excitation of the HCl correlates with the dipole moment of the CH2X radical and is thus attributed, at least in part, to post-transition-state dipole-dipole interactions between the separating, polar reaction products. This interpretation is supported by direct dynamics trajectories computed on-the-fly, and the HCl rotation is thus argued to serve as an in situ probe of the angular anisotropy of the reaction potential energy surface in the post-transition-state region. Comparisons between the dynamics of reactions of dimethyl ether and the three- and four-membered-ring compounds oxirane (c-C2H4O) and oxetane (c-C3H6O) raise questions about the role of reorientation of the reaction products on a time scale commensurate with their separation. The shapes and structures of polyatomic molecules are thus demonstrated to have important consequences for the stereodynamics of these direct abstraction reactions.