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

Gold cation–water complexes with attached argon atoms are produced via a laser vaporization supersonic cluster source. The [Au(H2O) n Ar x ]+ (n = 1–8; x = 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) n + 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 n = 6. For the complexes with n ≤ 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 n = 8 complex, indicating that the identification of a single global minimum becomes less important upon increasing the number of solvating water molecules.