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Thermal Activation of Methane by MgO$^+$: Temperature Dependent Kinetics, Reactive Molecular Dynamics Simulations and Statistical Modeling
- Publication Year :
- 2020
- Publisher :
- arXiv, 2020.
-
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 simula- tions. Rate coefficients 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 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 ± 0 . 03( T/ 300 K) − 0 . 8 ± 0 . 7 . Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures 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 .
- Subjects :
- Chemical Physics (physics.chem-ph)
Materials science
010304 chemical physics
Branching fraction
Kinetics
General Physics and Astronomy
Thermodynamics
FOS: Physical sciences
010402 general chemistry
Rate-determining step
Branching (polymer chemistry)
01 natural sciences
Dissociation (chemistry)
0104 chemical sciences
Molecular dynamics
Physics - Chemical Physics
0103 physical sciences
Thermal
Physical and Theoretical Chemistry
Order of magnitude
Subjects
Details
- Database :
- OpenAIRE
- Accession number :
- edsair.doi.dedup.....2227439787e0124b6166666ec5797133
- Full Text :
- https://doi.org/10.48550/arxiv.2002.02151