Ab initioand direct quasiclassical-trajectory study of the F+CH4→HF+CH3 reaction

Feshbach resonances in the F + CHD3 → HF + CD3 reaction

The signature of dynamics resonances was observed in the benchmark polyatomic F + CH₄/CHD₃ reactions more than a decade ago; however, the dynamical origin of the resonances is still not clear due to the lack of reliable quantum dynamics studies on accurate potential energy surfaces. Here, we report a six-dimensional state-to-state quantum dynamics study on the F + CHD₃ → HF + CD₃ reaction on a highly accurate potential energy surface. Pronounced oscillatory structures are observed in the total and product rovibrational-state-resolved reaction probabilities. Detailed analysis reveals that these oscillating features originate from the Feshbach resonance states trapped in the peculiar well on the HF(v' = 3)-CD₃ vibrationally adiabatic potential caused by HF chemical bond softening. Most of the resonance structures on the reaction probabilities are washed out in the well converged integral cross sections (ICS), leaving only one distinct peak at low collision energy. The calculated HF vibrational state-resolved ICS for CD₃(v = 0) agrees quantitatively with the experimental results, especially the branching ratio, but the theoretical CD₃ umbrella vibration state distribution is found to be much hotter than the experiment.

Multichannel dynamics in the OH+ n-butane reaction revealed by crossed-beam slice imaging and quasiclassical trajectory calculations

Multidimensional reactions present various channels that can exhibit very different dynamics and give products of varying subsequent reactivity. Here, we present a combination of experiment and theory to reveal the dynamics of hydrogen abstraction by OH radical at primary and secondary sites in n-butane at a collision energy of 8 kcal/mol. Crossed molecular beam slice imaging experiments unequivocally probe the secondary abstraction channel showing backward angular distributions with mild energy release to product translation, which are accurately captured by trajectory calculations using a specific-reaction-parameter Hamiltonian. Experiments containing both reaction channels indicate a less marked backward character in the angular distribution, whose origin is shown by trajectory calculations to appear as an evolution toward more sideways scattering from the secondary to primary channel. While the two channels have markedly different angular distributions, their energy release is largely comparable, showing ample energy release into the water product. The synergistic combination of crossed-beam imaging and trajectories opens the door to detailed reaction-dynamics studies of chemical reactions with ever-increasing complexity.

Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF): Automatic Generation of Sum-of-Products Form Potential Energy Surfaces for Quantum Dynamical Calculations

We present Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF), an automated methodology for the generation of global potential energy surfaces (PES), molecular properties surfaces, e.g., dipole, polarizabilities, etc. using a single random geometry as input. The SRP-MGPF workflow integrates: (i) a fully automated procedure for the global topographical characterization of a (intermolecular) PES based on the Transition State Search Using Chemical Dynamical Simulations (TSSCDS) family of methods;i (ii) the global optimization of the parameters of a semiempirical Hamiltonian in order to reproduce a given level of electronic structure theory; and (iii) a tensor decomposition algorithm which turns the resulting SRP-PES into sum of products (Tucker) form with the Multigrid POTFIT algorithm. The latter is necessary for quantum dynamical studies within the Multiconfiguration Time-Dependent Hartree (MCTDH) quantum dynamics method. To demonstrate our approach, we have applied our methodology to the cis-trans isomerization reaction in HONO in full dimensionality (6D). The resulting SRP-PES has been validated through the computation of classical on-the-fly dynamical calculations as well as calculations of the lowest vibrational eigenstates of HONO as well as high-energy wavepacket propagations.

Ab Initio and Quasiclassical Trajectory Study of the O(3P) + 2-Propanol Hydrogen Abstraction Reaction

We present a theoretical study of the hydrogen abstraction reaction from 2-propanol by ground-state oxygen atoms. First, ab initio calculations are used to characterize the stationary points of the potential energy surface. Rotation around the C-C-O-H dihedral affords two conformers in 2-propanol, which gives rise to 13 hydrogen abstraction reaction pathways grouped into three channels, Cα, Cβ, and O, depending on the abstraction site. Reaction at Cα exhibits the lowest barrier and largest exothermicity, followed by reaction at Cβ, and at 2-propanol's oxygen atom. Additional ab initio calculations beyond the stationary points are employed to obtain a grid of energies with which a specific-reaction-parameters (SRP) PM6 semiemipirical Hamiltonian is derived for the title reaction. The SRP-PM6 model captures the energetics of the reaction with higher accuracy than some conventional first-principles methods but is efficient enough to allow for extensive reaction dynamics calculations. Quasiclassical trajectories are subsequently propagated with the SRP-PM6 Hamiltonian to obtain reaction dynamics properties that are compared to experiments. Product translational energy and angular distributions for reaction at Cα with the two conformers of 2-propanol are in good agreement with recent molecular-beam measurements, and they exhibit largely backward scattering with modest energy release to relative translation. Most of the energy is deposited into the organic product, substantiating a reaction mechanism dominated by rebound dynamics.

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.

A neural network potential energy surface for the F + CH4reaction including multiple channels based on coupled cluster theory

We report here a new global and full dimensional potential energy surface (PES) for the F + CH₄reaction.

Reduced-dimensional quantum computations for the rotational-vibrational dynamics of F(-)-CH4 and F(-)-CH2D2

Variational rotational-vibrational quantum chemical computations are performed for the F(-)-CH4 and F(-)-CH2D2 anion complexes using several reduced-dimensional models in a curvilinear polyspherical coordinate system and utilizing an accurate ab initio potential energy surface (PES). The implementation of the models is made practical by using the general rovibrational code GENIUSH, which constructs the complicated form of the exact rovibrational kinetic energy operator in reduced and full dimensions in any user-specified coordinates and body-fixed frames. A one-dimensional CF stretch, 1D(RCF), a two-dimensional intermolecular bend, 2D(θ,φ), and a three-dimensional intermolecular, 3D(RCF,θ,φ), rigid methane model provide vibrational energies for the low-frequency, large-amplitude modes in good agreement with full-dimensional MCTDH results for F(-)-CH4. The 2D(θ,φ) and 3D(RCF,θ,φ) four-well computations, describing equally the four possible CH-F(-) bonds, show that the ground-state tunneling splitting is less than 0.01 cm(-1). For the hydrogen-bonded CH stretching fundamental a local-mode model is found to have almost spectroscopic accuracy, whereas a harmonic frequency analysis performs poorly. The 2D(θ,φ) and 3D(RCF,θ,φ) rotational-vibrational computations on the Td-symmetric four-well PES reveal that in most cases F(-)-CH4 behaves as a semirigid C3v symmetric top. For the degenerate intermolecular bending vibrational states substantial splittings of the rigid rotor levels are observed. For F(-)-CH2D2 the rotational levels guide the assignment of the vibrational states to either F(-)-H or F(-)-D connectivity.

Quasiclassical trajectory calculations of correlated product distributions for the F + CHD3(v1 = 0, 1) reactions using an ab initio potential energy surface

We report quasiclassical trajectory (QCT) calculations of the correlated product distributions and branching ratios of the reactions F+CHD(3)(v(1)=0,1)-->HF(v)+CD(3)(v) and DF(v)+CHD(2)(v) using a recently published ab initio-based full-dimensional global potential energy surface [G. Czako et al., J. Chem. Phys. 130, 084301 (2009)]. Harmonic normal mode analysis is done for the methyl products to determine the classical actions of each normal mode and then standard histogram binning and Gaussian binning (GB) methods are employed to obtain quantum state-resolved probabilities of the products. QCT calculations have been performed for both the vibrationally ground state and the CH stretching excited F+CHD(3)(v(1)=0,1) reactions at eight different collision energies in the 0.5-7.0 kcal/mol range. HF and DF vibrationally state-resolved rotational and angular distributions, CD(3) and CHD(2) mode-specific vibrational distributions, and correlated vibrationally state-specific distributions for the product pairs have been obtained and the correlated results were compared to the experiment. We find that the use of GB can be advantageous especially in the threshold regions. The CH stretching excitation in the reactant does not change the CD(3) vibrational distributions significantly, whereas the HF molecules become vibrationally and rotationally hotter. On the other hand in the case of the DF+CHD(2) channel the initially excited CH stretch appears mainly "intact" in the CHD(2) product and the DF distributions are virtually the same as formed from the ground state CHD(3) reaction. The computed results qualitatively agree with recent crossed molecular beam experiment [W. Zhang et al., Science 325, 303 (2009)] that (a) CHD(2)(v(1)=1) is the most populated product state of the F+CHD(3)(v(1)=1) reaction and this reaction produces much less CHD(2)(v=0) compared to the reaction F+CHD(3)(v=0); (b) the cross section ratio of CHD(2)(v(1)=1):CHD(2)(v=0) formed from the reactions F+CHD(3)(v(1)=1):F+CHD(3)(v=0) is less than 1 and shows little collision energy dependency; (c) the reactant CH stretch excitation increases the DF:HF ratio at low collision energies; (d) the correlated vibrational and angular distributions for DF(v)+CHD(2)(v(1)=0,1) from the ground state and stretch-excited reactions, respectively, are almost identical.

Role of the C-H stretch mode excitation in the dynamics of the Cl + CHD3 reaction: a quasi-classical trajectory calculation

To analyze the effect of the C-H stretch mode excitation on the dynamics of the Cl + CHD3 gas-phase abstraction reaction, an exhaustive state-to-state dynamics study was performed. This reaction can evolve along two channels: H-abstraction, CD3 + ClH, and D-abstraction, CHD2 + ClD. On an analytical potential energy surface constructed previously by our group, named PES-2005, quasi-classical trajectory calculations were performed at a collision energy of 0.18 eV, including corrections to avoid zero-point energy leakage along the trajectories. First, strong coupling between different vibrational modes in the entry valley was observed; i.e., the reaction is vibrationally nonadiabatic. Second, for the ground-state CHD3(nu=0) reaction, the diatomic fragments appeared in their ground states, and the H- and D-abstraction reactions showed similar reactivities. However, when the reactivity per atom is considered, the H is three times more reactive than the D atom. Third, when the C-H stretch mode is excited by one quantum, CHD3(nu1=1), the H-abstraction is strongly favored, and the C-H stretch excitation is maintained in the product CHD2(nu1=1) + ClD channel; i.e., the reaction shows mode selectivity, reproducing the experimental evidence, and also the reactivity of the vibrational ground state is increased, in agreement with experiment. Fourth, the state-to-state angular distributions of the CD3 and CHD2 products showed the products to be practically sideways for the reactant ground state, while the C-H excitation yielded a more forward scattering, reproducing the experimental data. The role of the zero-point energy correction was also analyzed, and we find that the dynamics results are very sensitive on how the ZPE issue is treated. Finally, a comparison is made with the similar H + CHD3(nu1=0,1) and Cl + CH4(nu1=0,1) reactions.

Quasi-classical trajectory calculations analyzing the dynamics of the C-H stretch mode excitation in the H+CHD3 reaction

A state-to-state dynamics study was performed at a collision energy of 1.53 eV to analyze the effect of the C-H stretch mode excitation on the dynamics of the gas-phase H+CHD3 reaction, which can evolve along two channels, H-abstraction, CD3+H2, and D-abstraction, CHD2+HD. Quasi-classical trajectory calculations were performed on an analytical potential energy surface constructed previously by our group. First, strong coupling between different vibrational modes in the entry channel was observed; i.e., the reaction is non-adiabatic. Second, we found that the C-H stretch mode excitation has little influence on the product rotational distributions for both channels, and on the vibrational distribution for the CD3+H2 channel. However, it has significant influence on the product vibrational distribution for the CHD2+HD channel, where the C-H stretch excitation is maintained in the products, i.e., the reaction shows mode selectivity, reproducing the experimental evidence. Third, the C-H stretch excitation by one quantum increases the reactivity of the vibrational ground-state, in agreement with experiment. Fourth, the state-to-state angular distributions of the CD3 and CHD2 products are reported, finding that for the reactant ground-state the products are practically sideways, whereas the C-H excitation yields a more forward scattering.

New analytical potential energy surface for the F(2P)+CH4 hydrogen abstraction reaction: kinetics and dynamics

A new potential energy surface for the gas-phase F(2P)+CH4 reaction and its deuterated analogues is reported, and its kinetics and dynamics are studied exhaustively. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and it is calibrated to reproduce the topology of the reaction and the experimental thermal rate constants. For the kinetics, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. The theoretical results reproduce the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. Two sets of kinetic isotope effects were calculated, and they show good agreement with the sparse experimental data. The coupling between the reaction coordinate and the vibrational modes shows qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. The dynamics study was performed using quasi-classical trajectory calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found that the FH(nu',j') rovibrational distributions agree with experiment. Second, the excitation function presents an oscillatory pattern, reminiscent of a reactive resonance. Third, the state specific scattering distributions present reasonable agreement with experiment, and as the FH(nu') vibrational state increases the scattering angle becomes more forward. These kinetics and dynamics results seem to indicate that a single, adiabatic potential energy surface is adequate to describe this reaction, and the reasonable agreement with experiment (always qualitative and sometimes quantitative) lends confidence to the new surface.

Product pair correlation in bimolecular reactions

The vast majority of chemical reactions involve polyatomic species as reactants and/or products. The added degree of complexity offers opportunities to address dynamical questions other than those already encountered in a typical atom + diatom reaction. Product pair correlation is one of them. This article introduces the basic concept, outlines the experimental approach we developed and then highlights some of the applications to bimolecular reaction dynamics. Particular emphasis is placed on the information contents and unique insights gained from this type of measurements, which otherwise would have been lost by the conventional approach.

Ab initio study of the potential energy surface for the OH+CO→H+CO2 reaction

Potential energy surface for the reaction OH+CO-->H+CO2 has been calculated using the complete active space self-consistent-field and multireference configuration interaction methods with the correlation consistent triple-, quadruple-, and quintuple-zeta basis sets. A specific- reaction-parameters density functional theory has been suggested, in which the B3LYP functional is reoptimized to give the highly accurate potential energy surface with less computational efforts.

Ab initio and direct quasiclassical-trajectory study of the Cl+CH4→HCl+CH3 reaction

We present an electronic structure and dynamics study of the Cl + CH(4)--> HCl + CH(3) reaction. We have characterized the stationary points of the ground-state potential-energy surface using various electronic structure methods and basis sets. Our best calculations, CCSD(T) extrapolated to the complete basis-set limit based on geometries and harmonic frequencies obtained at the CCSD(T)/aug-cc-pvtz level, are in agreement with the experimental reaction energy and indirect measurements of the barrier height. Using ab initio information, we have reparametrized a semiempirical Hamiltonian so that the predictions of the improved Hamiltonian agree with the higher-level calculations in various regions of the potential-energy surface. This improved semiempirical Hamiltonian is then used to propagate quasiclassical trajectories and characterize the reaction dynamics. The good agreement of the calculated HCl rotational and angular distributions with the experiment indicates that reparametrizing semiempirical Hamiltonians is a promising approach to derive accurate potential-energy surfaces for polyatomic reactions. However, excessive energy leakage from the initial vibrational energy of the CH(4) molecule to the reaction coordinate in the trajectory calculations calls into question the suitability of the standard quasiclassical-trajectory method to describe energy partitioning in polyatomic reactions.