Theoretical study of the dynamics of F+alkanethiol self-assembled monolayer hydrogen-abstraction reactions

The dynamics of the reactions of F atoms with octanethiol self-assembled monolayers (SAMs) has been studied using theoretical methods. F+SAM classical trajectories have been propagated directly using a quantum-mechanics (QM)/molecular-mechanics scheme in which the QM portion is described using a specific-reaction-parameters (SRP) semiempirical Hamiltonian. This SRP Hamiltonian has been derived using ab initio information of model gas-phase F+alkane reactions and its accuracy has been calibrated via comparison of the result of direct-dynamics calculations with available experiments on the F+CH(4)-->HF+CH(3) and F+C(2)H(6)-->HF+C(2)H(5) reactions. The F+SAM calculations are used to analyze HF product-energy distributions at collision energies ranging from 0.80 to 11.53 kcal mol(-1) and 0 degrees, 30 degrees, and 60 degrees incident angles with respect to the surface normal. The calculations show that while the HF product is vibrationally excited, it desorbs translationally and rotationally cold at all collision energies and incident angles explored. The calculated results shed light into recent experiments of F-atom reactions with liquid alkane surfaces by providing mechanistic understanding of the factors that govern the amount of energy deposited into the various degrees of freedom of the HF product. Specifically, examination of the dynamics of postreaction HF collisions with the surface shows the role that secondary collisions play in quenching rotational and translational excitation of HF before desorption from the surface.