Reaction path identification and validation from molecular dynamics simulations of hydrocarbon pyrolysis

Abstract

Creation of complex chemical mechanisms for hydrocarbon pyrolysis and combustion is challenging due to the large number of species and reactions involved. Reactive molecular dynamics (RMD) enables the simulation of thousands of reactions and the discovery of previously unknown components of the reaction network. However, due to the inherent imprecision of reactive force fields, it is necessary to verify RMD-obtained reaction paths using more accurate methods such as Density Functional Theory (DFT). We demonstrate a method for identification and confirmation of reaction pathways from RMD that supplement an established mechanism, using the example of benzene formation from n-heptane and iso-octane pyrolysis. We establish a validation workflow to extract reaction geometries from RMD and optimize transition states using the Nudged-Elastic-Band method on semi-empirical and quantum mechanical levels of theory. Our findings demonstrate that the widely recognized ReaxFF parameterization, CHO2016, can identify known pathways from a established soot formation mechanism while also indicating new ones. We also show that CHO2016 underestimates hydrogen migration barriers by up to $40{\mathrm{kcal}\mathrm{mol}}^{-1}$ as compared to DFT and can lower activation barriers significantly for spin-forbidden reactions. This highlights the necessity for validation or potentially even reparametrization of CHO2016.