A theoretical study of methyl chemisorption on Ni(111)

Calculations including electron correlation have been performed for methyl adsorbed on nickel clusters mimicking the Ni(111) surface. The chemisorption energies are evaluated following a recently developed scheme, in which the cluster is prepared for bonding. In this way cluster dependent oscillations of the chemisorption energies are largely eliminated. By also using calculated vibrational frequencies of the adsorbed methyl an almost certain assignment of the preferred chemisorption site is obtained. Methyl is found to adsorb in the threefold hollow site with a chemisorption energy in the range 50–55 kcal/mol. The origin of the soft C–H frequencies observed experimentally is a charge transfer from the metal into the C–H antibonding orbitals. The only weak sign of a direct metal–carbon–hydrogen interaction in the calculations is that the C–H frequency is slightly lower for an eclipsed compared to a staggered orientation of methyl in the threefold hollow. The present results are compared to previous experimental and theoretical results for methyl adsorbed on metal surfaces.