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

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.

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.