Modeling O(3P) and Ar Scattering from the Ionic Liquid [emim][NO3] at 5 eV with Hybrid QM/MM Molecular Dynamics

In this paper, we develop a hybrid QM/MM-MD direct dynamics method for the study of Ar and O scattering from room-temperature ionic liquids (RTIL) at hyperthermal energies, with the goal of providing an understanding of the reactivity of ionic liquids as potential hypergolic fuels. The RTIL is chosen to be 1-ethyl-3-methylimidazolium nitrate ([emim][NO3]) as the bulk properties of this have been analyzed experimentally and theoretically, and a force field has been developed. Within our simulation time of 7.3 ps, Ar collisions with the liquid surface cause 2−3 proton transfers to occur from the imidazolium ring to the anion, and a small fraction of these events leads to the desorption of neutral "emim" or HNO3 species. These charge-transfer processes also occur for O(3P) collisions, but in addition, there is a rich amount of chemical change at the surface leading to four main collision types, reaction with NO3 to form species like [NO2] and O2, O addition to the imidazolium ring causing ring scission, H abstractions from the hydrocarbon chains (ethyl or methyl) or imidazolium ring generating OH, and O atom inelastic scattering. Within the MSINDO model, the major scattered products (all are neutral) from the O collisions, with their probabilities in parentheses, are O (0.13), emim (0.12), HNO2 (0.05), HNO3 (0.04), and OH (0.04). Of these products, only OH is unique to O atom collisions; the HNO2 and HNO3 products also arise when the incident atom is Ar or O.