Anion effects in the scattering of CO2 from the room-temperature ionic liquids [bmim][BF4] and [bmim][Tf2N]: insights from quantum mechanics/molecular mechanics trajectories.

Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations have been carried out to model the scattering of hyperthermal (15 kcal/mol) CO(2) on the surfaces of two common imidazolium based room-temperature ionic liquids (RTILs) [bmim][BF4] and [bmim][Tf2N]. Good agreement was achieved in comparison with experiment. The [bmim][BF4] surface is found to be more absorptive of CO(2) than [bmim][Tf2N], which leads to greater loss in translational energy and less rotational excitation of CO(2)'s that scatter from [bmim][BF4]. These differences are found to result from a interplay of differences in the structure of the interface and the strength of interactions that depend on anion identity. Our results also suggest that CO(2) interacts strongly with ionic species on the RTIL surfaces due to the large induced dipole moments on CO(2) during the collisions. The inclusion of electronic polarization is critical in determining the final rotational excitation of CO(2) compared to results from an MM model with fixed charge.