Theoretical Study of the Pair-Correlated F + CHD3(v = 0,ν1 = 1) Reaction: Effect of CH Stretching Vibrational Excitation

Theoretical dynamics studies of the CH3 + HBr → CH4 + Br reaction: effects of isotope substitution and vibrational excitation of CH3

The rate coefficient for two deuterium substituted isotopologues of reaction CH3 + HBr → CH4 + Br has been determined using the quasiclassical trajectory (QCT) method. We used the analytical potential energy surface (PES) fitted to high-level ab initio points in earlier work. The PES exhibits a pre-reaction van der Waals complex and a submerged potential barrier. The rate coefficients of the deuterated isotopologue reactions, similarly to the pure-protium isotopologue, show significant deviation from the Arrhenius law, namely, the activation energy is negative below about 600 K and positive above it: k[CH3 + DBr] = 1.35 × 10-11 exp(- 2472/T) + 5.85 × 10-13 exp(335/T) and k[CD3 + HBr] = 2.73 × 10-11 exp(- 2739/T) + 1.46 × 10-12 exp(363/T). The CH3 + DBr reaction is slower by a factor of 1.8, whereas CD3 + HBr isotopologue is faster by a factor of 1.4 compared to the HBr + CH3 system across a wide temperature range. The isotope effects are interpreted in terms of the properties of various regions of the PES. Quantum state-resolved simulations revealed that the reaction of CH3 with HBr becomes slower when any of the vibrational modes of the methyl radical is excited. This contradicts the assumption that vibrational excitation of methyl radicals enhances its reactivity, which is of historical importance: this assumption was used as an argument against the existence of negative activation energy in a decade-long controversy in the 1980s and 1990s.

Global potential energy surface and product pair-correlated distributions for the F(2P) + SiH4 reaction - comparison with experiments

In this paper we study the gas-phase hydrogen abstraction reaction between fluorine atoms and silane in a three-step process: potential energy surface, kinetics and dynamics. Firstly, we developed for the first time an analytical full-dimensional surface, named PES-2021, using high-level explicitly-correlated ab initio data as the input. PES-2021 represents a continuous and smooth potential with analytical gradients and includes intuitive concepts (stretching and bending nuclear motions). Based on the PES-2021 quasi-classical trajectory (QCT) calculations were performed to analyse the kinetics and dynamics. Secondly, in the kinetics study at room temperature we observed a very fast reaction with a rate constant of 3.90 × 10^-10 cm³ molecule^-1 s^-1, reproducing the scarce experimental evidence. Finally, the third step is the dynamics study, which was performed under two different conditions, a temperature of 77 K and a collision energy of 2.5 kcal mol^-1, for direct comparison with experiments. In the first case, we found the largest fraction, 44%, deposited as HF(v) vibration, where the most populated states were HF(v = 2, 3), both results reproducing the experimental evidence. The largest discrepancy with the experiment was found in the HF(j) rotational distribution, where hotter distributions were found, this discrepancy being associated with limitations of the QCT method. The second case, E = 2.5 kcal mol^-1, was a state-to-state correlated study and, therefore, more difficult. The theory overestimates (again) and consequently underestimates, respectively, the rotational and vibrational fractions of the HF(v,j) product as compared with experiments. While experimentally the SiH₃ product appears excited only in the umbrella mode, ν₂ = 0-5, correlated with the HF(v) co-product vibrational excited in v = 3 and 4, theoretically a wider vibrational distribution is found in both products, and these distributions have, obviously, an influence on the product correlated speed distributions. However, the product correlated angular distribution is well reproduced. In general, these results allowed us to test the capacity of PES-2021 + QCT tools to simulate the experimental evidence, revealing that agreement is better when average properties are compared, making the comparison worse when state-to-state properties are compared. Different causes of the theory/experiment discrepancies were analysed, and it was found that they are due, mainly, to limitations of the QCT method.

Theoretical study of the Cl(2P) + SiH4 reaction: global potential energy surface and product pair-correlated distributions. Comparison with experiment

For the theoretical study of the title reaction, an analytical full-dimensional potential energy surface named PES-2021 was developed for the first time, by fitting high-level explicitly-correlated ab initio data. This reaction presented high exothermicity, (298 K) = -11.6 kcal mol^-1, reproducing the experimental evidence; it is a barrierless reaction and no intermediate complexes were found. PES-2021 is a continuous and smooth potential energy surface, it includes intuitive concepts in its development and fitting, such as stretching and bending nuclear motions, and it presents analytical first energy derivatives. Based on PES-2021, kinetics and dynamics studies were carried out using quasi-classical trajectory calculations. In the kinetics study, over the temperature range 300-450 K, we observed that rate constants were practically independent of temperature, with an almost zero activation energy, as compared to 0.0 and -0.48 kcal mol^-1 experimentally reported. In this kinetics study the role of the spin-orbit effect on reactivity was analysed. In the dynamics study, different product pair-correlated dynamics properties were compared with the only experimental evidence: product energy partition, product vibrational distribution, product angular distribution and product speed distribution. We observed two mechanisms of reaction, a stripping mechanism associated with large impact parameters and forward scattering, and an indirect mechanism associated with sideways-backward scattering related with "nearly-trapped" trajectories due to the product rotation. In general, theoretical results reasonably simulate the experimental measurements when they consider some rotational and vibrational constraints as well as binning techniques to mimic a quantum-mechanical behaviour. Although the agreement is not quantitative, the present results shed light on the mechanism of this difficult polyatomic reactive system.

Imaging pair-correlated reaction cross sections in F + CH3D(νb = 0, 1) → CH2D(ν4 = 1) + HF(ν)

The title reactions were studied in a crossed-beam experiment at collisional energies (Ec) from 0.5 to 4.7 kcal mol-1. The νb (ν4) vibrational mode denotes the bending (umbrella) motion of the CH3D reactant (CH2D product). Using a time-sliced, velocity-map imaging technique, we extracted the state-specific, pair-correlated integral and differential cross sections. As with other isotopically analogous ground-state reactions, an inverted vibrational population of the HF coproduct was observed. Both the step-like excitation function near the threshold and the oscillatory forward-backward peakings in the Ec-evolution of the two dominant pair-correlated angular distributions at lower Ec suggest a resonance-mediated, time-delay mechanism. As Ec increases, the angular distribution of the HF(ν = 2) product evolves into a smooth and broad swath in the backward hemisphere, indicative of a direct rebound mechanism. One quantum excitation of the bending modes of CH3D(νb = 1) promotes the reaction rate by two- to three-fold up to Ec = 2.1 kcal mol-1. Broadly speaking, all major findings are qualitatively in line with previous results in the reactions of the F atom with other isotopologues. However, the rainbow feature recently observed in the CH2D(00) + HF(ν = 3) product channel is entirely absent. A possible rationale is put forward, which reinforces the previous reactive rainbow conjecture and calls for future theoretical investigations.

Quasi-Classical Trajectory Dynamics Study of the Cl(2P) + C2H6 → HCl(v,j) + C2H5 Reaction. Comparison with Experiment

To understand and simulate the dynamics behavior of the title reaction, QCT calculations were performed on a recently developed global analytical potential energy surface, PES-2017. These calculations combine the classical description of the dynamics with pseudoquantization in the reactants and products to perform a theoretical/experimental comparison on the same footing. Thus, in the products a series of constraints are included to analyze the HCl(v = 0,j) product, which is experimentally detected. At collision energies of 5.5 and 6.7 kcal mol^-1 the largest fraction of available energy is deposited as translation, 67%, while the ethyl radical shows significant internal energy, 27%, and so it does not act as a spectator of the reaction, thus reproducing recent experimental evidence. The HCl(v=0, j) rotational distribution is cold, peaking at j = 2, only one unit hotter than experiment, which represents an error of 0.12 kcal mol^-1. At a collision energy of 5.5 kcal mol^-1 product translational distribution is slightly hotter than experiment, but at 6.7 kcal mol^-1 agreement with recent experiments is practically quantitative, suggesting that the first experiments should be revised. In addition, we observe that the HCl(v=0, j) scattering distribution shifts from isotropic at low values of j to backward at high values of j, which is in agreement with experimental data. Finally, no evidence was found for the "chattering" mechanism suggested to explain the low translational energy of the HCl product in the backward scattering region. In sum, agreement with experiments of a series of sensible dynamic properties permits us to be optimistic on the quality and accuracy of the theoretical tools used in the present work, QCT and PES-2017.

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.