ReaxFF reactive force field development and applications for molecular dynamics simulations of ammonia borane dehydrogenation and combustion

Ammonia borane (AB) has attracted significant attention due to its high hydrogen content (19.6% by mass). To investigate the reaction mechanism associated with the combustion of AB, a reactive force field (ReaxFF) has been developed for use in molecular dynamics (MD) simulations. The ReaxFF parameters have been derived directly from quantum mechanical data (QM). NVT-MD simulations of single- and polymolecular AB thermolysis were conducted in order to validate the force field. The release of the first equivalent H₂ is a unimolecular reaction, and MD simulations show an activation energy of 26.36 kcal mol^'1, which is in good agreement with experimental results. The release of the second H₂ is also a unimolecular reaction; however, the release of a third H₂ requires the formation of a B'N polymer. Similar simulations were conducted with a boron and oxygen system, since the oxidation of boron will be an integral step in AB combustion, and show good agreement with the established mechanism for this system. At low temperatures, boron atoms agglomerate into a cluster, which is oxidized at higher temperatures, eventually forming condensed and gas phase boron'oxide-species. These MD results provide confidence that ReaxFF can properly model the oxidation of AB and provide mechanistic insight into the AB dehydrogation and combustion reactions.