Theoretical study of the O(3P) + SiH4 reaction: global potential energy surface, kinetics and dynamics study

Abstract

In order to understand the gas-phase hydrogen abstraction reaction between O(³P) and silane we began by developing the first full-dimensional analytical potential energy surface, named PES-2022. It is basically a valence bond function augmented with molecular mechanic terms describing in an intuitive way stretching and bending nuclei motions, and it is fitted to high level ab initio calculations. The surface presents continuous and smooth behaviour, with analytical first energy derivatives, on which the hydrogen atoms in silane are permutationally symmetric. Based on PES-2022, a kinetics study was performed using the variational transition-state theory with multidimensional tunnelling corrections in the temperature range of 300–1000 K. We observed that experimental and theoretical results show widely spread results, both in absolute value and temperature dependence, possibly because they include the reactivity from both O(³P) and O(¹D) electronic states, which present different mechanisms and multiple channels. When the comparison is performed on the same footing, O(³P) + SiH₄ → HO + SiH₃, the present results agree with Ding and Marshall's experiments and with Zhang et al.'s theoretical rate constants. The kinetic isotope effects (KIEs) reproduced the only experimental value, improving previous theoretical results. Finally, a dynamics study was performed on PES-2022 using quasi-classical trajectory calculations under two different initial conditions, at fixed room temperature and at a fixed collision energy of 8.0 kcal mol⁻¹. In the first case, the available energy deposited as HO(v) vibration was 47%, with population inversion, P(v = 0)/P(v = 1) = 11/89%, reproducing the experimental evidence. In the second case, the experimental product translational distribution was reasonably simulated, while the angular product distribution presented opposite behaviour, backward versus forward. On analysing this discrepancy, we found that while in the present work the O(³P) + SiH₄ reaction was reported, in the experiment both O(³P) and O(¹D) electronic states are reported. So, the comparison was not performed on the same footing. In sum, agreement of the present results with experiments permits us to be reasonably optimistic about the quality and accuracy of the new PES, and at the same time to highlight the fact that theory/experiment comparisons must be performed on the same footing.