The relativistic effects on the methane activation by gold(I) cations

The reactivity of gold has been investigated for a long time. Here, we performed an in-depth analysis of relativistic effects over the chemical kinetic properties of elementary reactions associated with methane activation by gold(I) cations, CH₄ + Au⁺ ↔ AuCH₂ ⁺ + H₂. The global reaction is modeled as a two-step process, CH₄ + Au⁺ ↔ HAuCH₃ ⁺ ↔ AuCH₂ ⁺ + H₂. Moreover, the barrierless dissociation of the initial adduct between reactants, AuCH₄ ⁺, is discussed as well. Higher-order relativistic treatments are used to provide corrections beyond the commonly considered scalar effects of relativistic effective core potentials (RECPs). Although the scalar relativistic contributions predominate, lowering the forward barrier heights by 48.4 and 36.1 kcal mol^-1, the spin-orbit coupling effect can still provide additional reductions of these forward barrier heights by as much as 9% (1.0 and 2.2 kcal mol^-1). The global reaction proceeds rapidly at low temperatures to the intermediate attained after the first hydrogen transfer, HAuCH₃ ⁺. The relativistic corrections beyond the ones from RECPs are still able to double the rate constant of the CH₄ + Au⁺ → HAuCH₃ ⁺ process at 300 K, while the reverse reaction becomes five times slower. The formation of global products from this intermediate only becomes significant at much higher temperatures (∼1500 K upward). The scalar relativistic contributions decrease the dissociation energy of the initial adduct, AuCH₄ ⁺, into the global products by 105.8 kcal mol^-1, while the spin-orbit effect provides an extra lowering of 1.8 kcal mol^-1.