Density functional theory study of benzene adsorption on small pd and pt clusters

Restricted and unrestricted B3LYP/LANL2DZ and B3LYP/6-31G(d) calculations are employed to study the adsorption of benzene on small Pd-7, Pd(7+3), Pt-7, and Pt(7+3) clusters. Restricted calculations give the bridge-30 adsorption site as the most stable for adsorption of benzene on Pd-7 cluster, while on Pd(7+3) benzene is strongly tilted, leading to an adsorption mode where two benzene pi-bonds more strongly interact with the metal cluster. Unrestricted calculations (triplet spin state) strongly favor the tilted conformations so that on both Pd-7 and on Pd(7+3) the tilted di-pi-hcp adsorption mode is the most stable. On the Pt-7 and Pt(7+3) clusters even the restricted calculations lead to the tilted di-pi adsorption mode as the most stable, a result that is reinforced by the unrestricted formalism, which also favors the tilted adsorption mode. The differences between the restricted and the unrestricted calculations are much larger for platinum than for palladium, which is related to the fact that atomic palladium has closed shell electronic configuration while atomic platinum has open shell electronic configuration. The adsorption process is dominated by electron donation from the highest occupied molecular orbital of benzene to the lowest unoccupied molecular orbital of the metal clusters.