Chemical Dynamics Simulations of CO₂Scattering off a Fluorinated Self-Assembled Monolayer Surface

Abstract

Chemical dynamics simulations at collision energies of 3.0, 10.6, and 20.0 kcal/mol were performed to study energy transfer in perpendicular (θi = 0°) collisions of carbon dioxide with a perfluorinated octanethiol self-assembled monolayer (F-SAM) surface. An accurate carbon dioxide + F-SAM intermolecular potential was developed from ab initio calculations extrapolated to the complete basis set limit. Three types of collision events are observed in the trajectories: direct scattering, physisorption on the top of the surface, and penetration of the surface. Energy transfer to carbon dioxide rotation and the final translational energy of carbon dioxide are both analyzed for each of these trajectory types. Penetration leads to near complete thermal accommodation with the F-SAM, whereas accommodation is not reached for either the direct or physisorption trajectories. The distributions of the translational energy and rotational angular momentum, P(Ef) and P(J), of the scattered CO2 molecules and for the different trajectory types were analyzed to study their relationship to the atomic-level dynamics observed in the simulations. For each of the collision energies, these two distributions give similar thermal accommodation dynamics for the penetrating trajectories. For the physisorption trajectories, P(Ef) and P(J) are only similar at the lowest collision energy. These distributions are different at all collision energies for the direct trajectories, reflecting the complex dynamics for this trajectory type. The results found from these simulations are in excellent agreement with experiments for carbon dioxide scattering off perfluoropolyether (PFPE).