Theoretical adsorption studies of HCN and HNC on Ni(111)

The interaction of hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) with Ni(111) is studied by using ab initio embedding theory. The Ni(111) surface is modeled as a three-layer, 28-atom cluster with the Ni atoms fixed at bulk lattice sites. The present calculations show that both HCN and HNC bind to the surface in an end-on geometry with the molecular axis perpendicular to the surface. Tilting either the H atom or the H-C bond or the H-N bond away from the surface normal destabilizes the system. The side-on bonded HCN and HNC with the C-N bond parallel to the surface are energetically less stable than the corresponding end-on bonded species. The calculated adsorption energy of the end-on HCN is 18 kcal/mol with the fee 3-fold site favored over other sites by about 1-3 kcal/mol. The end-on HNC binds to the surface at the atop site with adsorption energy of 11 kcal/mol. The calculated adsorption energy for the side-on bonded HCN is only 7 kcal/mol. The side-on bonded HNC species is; found to be unbound by 12 kcal/mol on Ni(lll). Calculated C-N stretching frequencies are 2200 cm(-1) and 2100 cm(-1) for end-oh bonded HCN and HNC and 2105 cm(-1) and 1530 cm(-1) for the side-on bonded HCN and HNC, respectively. Dipole moment calculations show that the bonding of HCN to the surface is relatively more ionic than HNC.