Interaction of short-chain alkane thiols and thiolates with small gold clusters:: Adsorption structures and energetics

Gold-sulfur bonding is investigated theoretically using a variety of electronic structure methods, including the Becke-Perdew semilocal density functional, the B3LYP hybrid approach, the Hartree-Fock method, and the post Hartree-Fock approaches MP2 and QCISD(T). Particular emphasis is given to adsorption structure and energetics in the case of weak and strong interactions of this general type, using up to five gold atoms and up to three carbon atoms in the aliphatic chain. It is found that all methods which take into account electron correlation, including the density functional methods, lead to quite similar structures. Concerning the energetics, the Becke-Perdew functional is found to overbind typically by about 5-15%. Quasiglobal structural relaxation based on ab initio simulated annealing clearly shows that the adsorption of thiolates onto gold clusters results in a dramatic distortion of the cluster framework. From a structural point of view the thiolate sulfur-gold bond has a distinctive directional (covalent) character which results in a clear preference for Au-S-C bond angles in the range of 103.5 degrees and 108.7 degrees. In general, dissociation into open-shell species is preferred against the formation of the ionic closed-shell counterparts if the sulfur-gold bond is forced to break. However, neutral closed-shell products can be favored if fragmentation of the gold cluster is allowed for as a dissociation channel. Finally, it is demonstrated that using ethyl or n-propyl chains instead of the methyl group leads to only small changes of the binding energies. (C) 2001 American Institute of Physics.