Chemical kinetic properties of HNgF → HF plus Ng (Ng = Ar, Kr, Xe, and Rn) reactions: An example of fortuitous cancelling of relevant relativistic effects

The relativistic effects on energetic quantities of the HNgF -> HF + Ng (Ng = Ar, Kr, Xe, and Rn) reactions were evaluated by means of advanced four-component calculations with high-level electron correlation treatment and adequate relativistic basis sets. The results indicate that the scalar relativistic corrections can provide classical barrier height increments of as much as 4.8% (1.9 kcal mol(-1)) for the reaction with the heaviest noble gas, radon, while the spin-orbit coupling is almost as important in this case, lowering the value by 3.8% (-1.5 kcal mol(-1)). Therefore, these two relevant relativistic corrections cancel almost completely. A similar picture is seen for the energy variation from the reactant (HRnF) to the products (HF + Rn), but the contribution signs are reversed now (-4.0 and 3.4 kcal mol(- 1)). A partial cancelling is also noticed for the HXeF -> HF + Xe reaction. Such pattern is surprising since scalar relativistic effects usually predominate by magnitude orders over the spin-orbit coupling. These conclusions are reflected in the changes observed in rate constant values. Finally, an alternative decomposition route catalyzed by HF is suggested for these HNgF compounds, which can shed some light on the controversies regarding their thermal stability. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study evaluated the relativistic effects on energetic quantities of the HNgF -> HF + Ng (Ng = Ar, Kr, Xe, and Rn) reactions, finding that scalar relativistic corrections and spin-orbit coupling almost completely cancel out when reacting with the heaviest noble gas, radon. The contribution signs of energy variation during the reaction are reversed, impacting the rate constant values.