Structure, Bonding, and Dynamics of Alkanethylhiolates on Copper and Gold Clusters and Surfaces

The interaction of alkanethiolates with small coinage metal clusters and (111) surfaces of copper and gold was studied based on density functional theory with a focus on the metal-thiolate junction. Calculation of fragmentation energies indicate that for Cu cluster-thiolate (n=1,3,5,7, and 9) there is a progressive lowering in energy for the fragmentation of the S-C bond in the thiolate from a value of 2.9 eV for n=1 to 1.4 eV for n=9. The detailed electronic origins of this specific weakening are attributed to a polarization of electron density in the S-C bond as induced by bonding with the Cu cluster. For the gold analogues this effect is not observed and fragmentation at the S-C bond experiences only a slight 10\% destabilization as n increases from 3 to 9 On the Cu(111) surface the metal to thiolate charge transfer which leads to a non-direction partially ionic bonding with a concurrent flat adsorption energy landscape, As a result, occupation of fcc-hollow, hcp-hollow and fcc-bridge sites is observed during the coarse of a short finite temperature ab-initio molecular dynamics simulation as opposed to a static model where only the hollow sites are stable minima. Comparison of our results with the available experimental evidence and consequences of the electrostatic profile of the metal-molecule interface are presented. The difference between Cu and Au are discussed in the context of relativistic effects.