Coordination and Solvation of the Au⁺ Cation: Infrared Photodissociation Spectroscopy of Mass-Selected Au(H₂O)_<i>n</i>⁺ (<i>n</i> = 1–8) Complexes

Gold cation–water complexes with attached argon atoms are produced via a laser vaporization supersonic cluster source. The [Au­(H₂O)_<i>n</i>Ar_<i>x</i>]⁺ (<i>n</i> = 1–8; <i>x</i> = 1 or 2) complexes are each mass selected and studied by infrared photodissociation spectroscopy in the OH stretching frequency region to explore the coordination and solvation structures of the Au⁺ cation. Density functional calculations have been performed, and the calculated vibrational spectra are compared to the experimental spectra to identify the gas-phase structures of the Au­(H₂O)_<i>n</i>⁺ complexes. Confirming previous theoretical predications, the first coordination shell of the Au⁺ cation contains two water molecules forming a linear O–Au⁺–O arrangement; subsequent water molecules bind to the two H₂O ligands of the Au­(H₂O)₂⁺ core ion via hydrogen bond forming of the second hydration shell, which is complete at <i>n</i> = 6. For the complexes with <i>n</i> ≤ 7, the experimental spectrum can in general be assigned to the predicted global minimum structure. However, the spectrum suggests that two or more conformers coexist for the <i>n</i> = 8 complex, indicating that the identification of a single global minimum becomes less important upon increasing the number of solvating water molecules