Differences in bonding, structures and vibrational force constants of SH and SCH 3 on Pd(111) and Pt(111) surfaces

Abstract

We have performed RECP Hartree-Fock + MBPT2 calculations of the equilibrium adsorbate structures and force constants of SH and SCH 3 chemisorbed on the Pd(111) and Pt(111) surfaces. The adsorbates are oriented perpendicular to the surface on the Pt(111) plane and are strongly bent (∼ 70° from the surface normal) on the Pd(111) surface. The bonding scheme between the sulfur atom and the platinum surface possesses σ and π character. A significant amount of π coordinate bonding is evident in the platinum systems through the mixing of the (fully occupied) sulfur π orbitals and the unoccupied π orbitals of the cluster model of the surface. This coordinate bonding in the platinum systems is due to the fact that the surface LUMO's composed primarily of 6s and 6p atomic orbitais are much closer to the Fermi level than are the palladium analogs and are therefore much more accessible to the adsorbates for bonding. We attribute the straightening of the surface-S-X angle in the platinum systems to this interaction. The sulfur atom in the Pt(111) systems is similar to what one expects for sp hybridization. The force field parameters and structures we determine are necessary for molecular dynamics modeling of thio-alkyl systems condensed on these metals.