Thermal Activation of Methane by MgO${}^{+}$: Temperature Dependent Kinetics, Reactive Molecular Dynamics Simulations and Statistical Modeling

Abstract

The kinetics of MgO${}^{+}$ + CH${}_4$ was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 $-$ 600 K and computationally by running and analyzing reactive atomistic simulations. Rates and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of $k=5.9\pm 1.510^{-10}(T/300$ K${)}^{-0.5\pm 0.2}$ cm${}^3$ s${}^{-1}$. MgOH${}^{+}$ was the dominant product at all temperatures, but Mg${}^{+}$, the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of $0.08\pm 0.03(T/300$ K${)}^{-0.8\pm 0.7}$. Reactive molecular dynamics simulations using a reactive force field, as well as a neural network yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH${}_4$]${}^{+}$. Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate-limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH${}_3$OH${}^{+}$ complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH${}^{+}$ + CH${}_3$ product channel. An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH${}_3$OH${}^{+}$ complex thereby allowing a more robust competition between the product channels.