AB-INITIO STUDIES OF CN ADSORBED ON NI(111)

The adsorption of cyanide (CN) on Ni(111) is treated using an ab initi o embedding theory. The Ni(111) surface is modeled as a three-layer, 2 8-atom cluster with the Ni atoms fixed at bulk lattice sites. The pres ent calculations show that CN is able to bind to the surface either vi a the carbon, or nitrogen, or in a side-on geometry with very small di fferences in total energy (approximate to 0.1 eV). Adsorption energies at threefold, bridge and atop sites are comparable, with the fee thre efold site more favorable over other adsorption sites by approximate t o 0.1 eV. At the fee threefold site, adsorption energies and C-N stret ching frequencies are 5.0 eV and 2150 cm(-1) for the eta(1)-cyanide-N, 4.9 eV and 1970 cm(-1) for the eta(1)-cyanide-C, and 4.9 eV and 1840 cm(-1) for the eta(2)-cyanide-C,N, respectively. Dipole moment calcula tions show that the bonding of CN to the Ni surface is largely ionic, while eta(2)-cyanide-C,N has more covalent character. Calculated energ y barriers in going from eta(1)-cyanide-C to eta(2)-cyanide-C,N, and f rom eta(2)-cyanide-C,N to eta 1-cyanide-N are around 0.1 eV. Thus alth ough CN is strongly bound to the surface (at approximate to 5 eV), wit hin an energy range of approximate to 0.2 eV, the molecule is free to rotate to other geometries. During this rotation there are large chang es in the dipole moment. (C) 1995 American Institute of Physics.