H 2 O Nucleation around Au + J. Ulises Reveles, ² Patrizia Calaminici, ‡ Marcela R. Beltra ´ n, § Andres M. Ko ¨ ster, ‡ and Shiv N. Khanna* ,² Contribution from the Physics Department, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2000, Departamento de Quı ´mica, CinVestaV,AVenida Instituto Polite ´ cnico Nacional 2508, A.P. 14-740, C.P. 07000, Me ´ xico D.F., Me ´ xico, and Instituto de InVestigaciones en Materiales, UniVersidad Nacional Auto ´ noma de Me ´ xico, A.P. 70-360, C.P. 04510, Me ´ xico D.F., Me ´ xico Received June 29, 2007; E-mail: snkhanna@vcu.edu Abstract: First principles electronic structure calculations have been carried out to investigate the ground state geometry, electronic structure, and the binding energy of [Au(H2O) n] + clusters containing up to 10 H2O molecules. It is shown that the first coordination shell of Au + contains two H2O molecules forming a H2O-Au + -H2O structure with C2 symmetry. Subsequent H2O molecules bind to the previous H2O molecules forming stable and fairly rigid rings, each composed of 4 H2O molecules, and leading to a dumbbell structure at [Au(H2O)8] + . The 9th and the 10th H2O molecules occupy locations above the Au + cation mainly bonded to one H2O from each ring, leading to structures where the side rings are partially distorted and forming structures that resemble droplet formation around the Au + cation. The investigations highlight quantum effects in nucleation at small sizes and provide a microscopic understanding of the observed incremental binding energy deduced from collision induced dissociation that indicates that [Au(H2O) n] + clusters with 7-10 H2O molecules have comparable binding energy. The charge on the Au + is shown to migrate to the outside H2O molecules, suggesting an interesting screening phenomenon. 1. Introduction The condensation of water molecules around charged particles has been used, for a long time, to stimulate rain formation and was used, in a pioneering work by Wilson, to detect charged particles through condensate traces in cloud chambers. 1 Nucle- ation studies are also important in atmospheric chemistry as the formation of acid rain proceeds via condensation of water molecules around sulfate and other ions. 2 Although this phenomenon has been known for decades, it is only recently that the developments of the molecular beam experiments and the theoretical techniques have enabled accurate investigations into the condensation phenomenon. It is now well known that the presence of ions can help to overcome the nucleation barrier, leading to enhancement in nucleation rates. It is also suggested that the sign of the charge can affect the nucleation rates. Whereas cations form clusters with oxygen bonding to the ionic species and leading to fairly symmetric arrangements, the anions bind to H atoms leading to asymmetric arrangements. 3,4 One of the effects that has received attention, only recently, 5 is the quantum chemical nature of the ionic species. Studies by Nadykto et al. 5 indicate that the structure of the initial core of H 2 O molecules around the charged particles depends signifi- cantly on the charged state as well as the chemical nature of the charged particles. Their studies on M(H 2 O) n clusters containing up to 6 H 2 O molecules indicate that the water molecules can have different coordination to the ions of different chemical and charged state. In this work, we demonstrate a new feature to the H 2 O nucleation around a metal cation. Through studies on the nucleation of H 2 O around Au + , we show that the quantum chemical nature of the cation not only effects the initial attachment of H 2 O molecules but also leads to the formation of stable rigid H 2 O rings that are different from those in, for example, ice. The present investigations are partly motivated by the recent experiments by Poisson et al., 6 who investigated the binding of H 2 O molecules to Au + cations through collision induced dissociation (CID) of [Au(H 2 O) n ] + clusters. Their investigations revealed interesting trends in the incremental binding energy (B.E.) of a H 2 O molecule (energy required to remove the weakest bonded H 2 O molecule). Their results indicate that the first two water molecules bind strongly to the Au + (B.E. of 40.1 and 44.9 kcal/mol, respectively). The B.E. decreases from 23.0 to 16.1 kcal/mol from 3 to 6 H 2 O molecules, and for larger clusters [Au(H 2 O) n ] + with n ) 7, 8, 9, and 10, the B.E. remains at a constant value of around 10.4 kcal/mol, ² Virginia Commonwealth University. ‡ Cinvestav. § Universidad Nacional Auto ´noma de Me ´xico. (1) Wilson, C. T. R. Philos. Trans. R. Soc. London 1897, 189, 265; Das Gupta, N. N.; Ghosh, S. K. ReV. Mod. Phys. 1946, 18, 225. (2) Onianwa, P. C.; Odukoya, O. O.; Alabi, H. A. Bull. Chem. Soc. Ethiop. 2002, 16, 141; Aas, W, et al. Atmos. EnViron. 2007, 41, 1706. (3) Garret, B. C. Science 2004, 303, 1146. (4) Kathmann, S. M.; Schenter, G. K.; Garret, B. C. Phys. ReV. Lett. 2005, 94, 116104. (5) Nadykto, A. B.; Natsheh, A. A.; Yu, F.; Mikkelsen, K. V.; Ruuskanen, J. Phys. ReV. Lett. 2006, 96, 125701. (6) Poisson, L.; Lepetit, F.; Mestdagh, J. -M.; Visticot, J.-P. J. Phys. Chem. A 2002, 106, 5455. Published on Web 11/23/2007 10.1021/ja074336l CCC: $37.00 © 2007 American Chemical Society J. 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