The structures of pure gold and silver clusters (Au k , Ag k , k ) 1-13) and neutral and anionic gold-silver binary clusters (Au m Ag n , 2 e k ) m + n e 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Au k clusters favor 2-D planar configurations, while pure Ag k clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Au k clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Ag k tends to be large in s-like spherical 3-D coordination in contrast to that of Au in Au k which tends to be small in 1-D or 2-D coordination. This trend is critical in determining the cluster structures. The calculated electronic properties and dissociation energy of both pure and binary clusters are in good agreement with the available experimental data. Since the Ag-5s orbital is much higher in energy than the Au-6s orbital energy, the partial charge transfer from Au to Ag takes place in gold-silver binary clusters. Au atoms tend to be negatively charged, while Ag atoms tend to be positively charged. Combined with the trend that Au atoms favor the surface, edges, or vertices with smaller Nc, the outer part of the cluster tends to be negatively charged, while Ag atoms favor the inside with larger Nc, and so the inner part tends to be positively charged. The partial charge transfer in the binary system results in electrostatic energy gain for the binary Au m Ag n cluster over pure Au k and Ag k clusters, which is responsible for the formation of alloys. In a neutral alloy, the equivalent mixing is favored, and the even numbered k tends to be more stable due to the electron spin pairing, whereas in an anionic alloy the odd numbered k tends to be more stable.

Structure, electronic properties, and vibrational spectra of the water octamer with an extra electron: Ab initio studyAlthough various low-lying energy structures of electron-water clusters, e Ϫ -(H 2 O) 2 -6 , have been reported, some of the global minimum energy structures ͑in particular, for the tetramer and pentamer͒ are still not clearly characterized yet. Therefore, using high-level ab initio calculations, we have investigated several new low-lying energy conformers in addition to previously reported ones. The lowest energy conformer for the pentamer is found to have a wedge-like structure which has never been studied before. Based on the experimental vertical electron-detachment energies and OH vibrational spectra of the electron-water clusters, we report the most probable structures and their nearly isoenergetic structures. The OH vibrational frequencies of e Ϫ (H 2 O) 2 -6 and e Ϫ HOD(D 2 O) 1 -5 are investigated, and are found to be in excellent agreement with the available experimental data. Their O-H stretch frequency shifts are classified in terms of the types of water molecules.

Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: An ab initio mixed quantum-classical study J. Chem. Phys. 126, 074304 (2007) To understand the hydration phenomena of noble transition metals, we investigated the structures, hydration energies, electronic properties, and spectra of the Cu + ͑H 3 O͒ 1-6 and Au + ͑H 2 O͒ 1-6 clusters using ab initio calculations. The coordination numbers of these clusters are found to be only two, which is highly contrasted to those of Ag + ͑H 2 O͒ n ͑which have the coordination numbers of 3-4͒ as well as the hydrated alkali metal ions ͑which have the coordination numbers of ϳ6͒. For the possible identification of their interesting hydration structures, we predict their IR spectra for the OH stretch modes.

Using molecular-orbital analysis, we have elucidated the quantum-chemical origin of the intriguing phenomena in sequential hydration energies of the gold cation, which is known to be the most conspicuous among all transition metals. The hydration energy of Au+ with the second water molecule is found to be much larger than that with the first water molecule. Owing to the large relativistic effect of gold (i.e., significant lowering of the 6s orbital energy and significant raising of the 5d orbital energy), the highest occupied molecular orbital of the hydrated gold cation has a large portion of the 6s orbital. As the electron density of the 6s orbital populates in a large outer spherical shell far off the gold nucleus, the p orbitals (or sp hybridized lone-pair orbitals) of the water molecules are able to overlap with the outer part of the 6s orbital in the dihydrated gold cation, resulting in the unusual skewed overlap of p-6s-p orbitals (not the atom-to-atom bond overlap). No previous molecular-orbital analysis has reported this peculiar skewed orbitals overlap. Since this skewed orbitals overlap is saturated with two water molecules, this property is responsible for the low coordination number of the gold ion.

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