Thermal Activation of Methane by MgO+: Temperature Dependent Kinetics, Reactive Molecular Dynamics Simulations and Statistical Modeling
Abstract
The kinetics of MgO+ + CH4 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 1.5 10-10(T/300 K)-0.5 0.2 cm3 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 0.03 (T / 300 K)-0.8 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 [MgOCH4]+. 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-CH3OH+ complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH+ + CH3 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-CH3OH+ complex thereby allowing a more robust competition between the product channels.