Malte Döntgen

Automated Discovery of Reaction Pathways, Rate Constants, and Transition States Using Reactive Molecular Dynamics Simulations

We provide a methodology for deducing quantitative reaction models from reactive molecular dynamics simulations by identifying, quantifying, and evaluating elementary reactions of classical trajectories. Simulations of the inception stage of methane oxidation are used to demonstrate our methodology. The agreement of pathways and rates with available literature data reveals the potential of reactive molecular dynamics studies for developing quantitative reaction models.

Hydrogen Abstraction from n-Butyl Formate by H• and HO2•

The combustion chemistry of esters has been elucidated in the past through the study of smaller formates and acetates. Hydrogen abstraction from the fuel as an initiation step is mostly modeled based on estimations for similar abstractions from nonoxygenated hydrocarbons. This study reports computed ab initio rates for abstractions by H˙ and HO₂˙ radicals from the recently proposed biofuel candidate n-butyl formate. The energies are evaluated with a double hybrid density functional that performs especially well for barrier heights (B2KPLYP/aug-cc-pvtz). Hindered rotation of HO₂˙ with respect to n-butyl formate is treated using accurate eigenvalue summation and shows large influence on the rates. Transition states at the γ and δ positions are still influenced by the formate group. The abstraction from the γ carbon by HO₂˙ is slowest, although proceeding over the lowest barriers, due to the important influence of transition state entropies. A comparison with smaller esters and n-butanol shows that estimated rates deviate within 1 order of magnitude from the ab initio computations for similar groups in n-butyl formate.

Discussion of the Separation of Chemical and Relaxational Kinetics of Chemically Activated Intermediates in Master Equation Simulations

Chemical activation of intermediates, such as hydrogen abstraction products, is emerging as a basis for a fully new reaction type: hot β-scission. While for thermally equilibrated intermediates chemical kinetics are typically orders of magnitude slower than relaxational kinetics, chemically activated intermediates raise the issue of inseparable chemical and relaxational kinetics. Here, this separation problem is discussed in the framework of master equation simulations, proposing three cases often encountered in chemistry: insignificant chemical activation, predominant chemical activation, and the transition between these two limits. These three cases are illustrated via three example systems: methoxy (CH3Ȯ), diazenyl (ṄNH), and methyl formate radicals (CH3OĊO). For diazenyl, it is found that hot β-scission fully replaces the sequence of hydrogen abstraction and β-scission of thermally equilibrated diazenyl. Building on the example systems, a rule of thumb is proposed that can be used to intuitively judge the significance of hot β-scission: if the reverse hydrogen abstraction barrier height is comparable to or larger than the β-scission barrier height, hot β-scission should be considered in more detail.

Reactions of Chemically Activated Formic Acid Formed via HĊO + ȮH.

The chemistry of formyl radicals plays an important role in the kinetic modeling of oxygenated hydrocarbons. Here, the fate of rovibrationally excited formic acid produced via HĊO + ȮH is evaluated in a RRKM/Master Equation study. For that purpose, the HĊO + ȮH potential energy surface is studied theoretically using high-level quantum mechanics. Direct reaction from HĊO + ȮH to the bimolecular products is found to dominate for most relevant conditions due to formic acid well-skipping. The kinetics of these well-skipping reactions can only be evaluated when including the unimolecular intermediate, formic acid. Further, hydrogen abstraction from rovibrationally excited formic acid is found to be important at low-temperature conditions and for high radical concentrations.

Automated Chemical Kinetic Modeling via Hybrid Reactive Molecular Dynamics and Quantum Chemistry Simulations

An automated scheme for obtaining chemical kinetic models from scratch using reactive molecular dynamics and quantum chemistry simulations is presented. This methodology combines the phase space sampling of reactive molecular dynamics with the thermochemistry and kinetics prediction capabilities of quantum mechanics. This scheme provides the NASA polynomial and modified Arrhenius equation parameters for all species and reactions that are observed during the simulation and supplies them in the ChemKin format. The ab initio level of theory for predictions is easily exchangeable, and the presently used G3MP2 level of theory is found to reliably reproduce hydrogen and methane oxidation thermochemistry and kinetics data. Chemical kinetic models obtained with this approach are ready to use for, e.g., ignition delay time simulations, as shown for hydrogen combustion. The presented extension of the ChemTraYzer approach can be used as a basis for methodological advancement of chemical kinetic modeling schemes and as a black-box approach to generate chemical kinetic models.

Assessing Statistical Uncertainties of Rare Events in Reactive Molecular Dynamics Simulations

Reactive molecular dynamics (MD) simulations are a versatile tool which allow for studying reaction pathways and rates simultaneously. However, most reactions will be observed only a few times in such a simulation due to computational limitations or slow kinetics, and it is unclear how this will influence the obtained rate constants. Therefore, we propose a method based on the Poisson distribution to assess the statistical uncertainty of reaction rate constants obtained from reactive MD simulations.