MASS SPECTROMETRIC AND QUANTUM CHEMICAL STUDIES OF HYDRATED GOLD-COPPER CLUSTERS: IMPLICATIONS FOR AU AND CU TRANSPORT IN LOW DENSITY CRUSTAL FLUIDS

The mobility and deposition of Au and Cu in crustal fluids is strongly controlled by molecular association and solvation processes. Characterization of individual Au and Cu complex stoichiometries and structures however poses a special challenge to experimentalists, and this task is in many cases either difficult or impossible to achieve solely by solution-based techniques. Electrospray ionization (ESI) mass spectrometry, on the other hand, is emerging as a useful tool for the study of the vapor-phase chemistry of metal clusters, and in combination with results from vapor phase studies of Au and Cu (Hurtig and Williams-Jones, GCA, 2014), can provide valuable insight into the speciation of metal clusters in low-density crustal fluids. Here we report results from a combined ESI mass spectrometric and theoretical study of aqueous AuCl₃ and AuCl₃/CuCl₂ binary. Theoretical calculations at the MP2 theory level have been applied to probe the structures and energies of Au and Cu chloride clusters, both water-free and microsolvated, and by doing so, provide insight into the stability of metal clusters in water vapor. Briefly, ESI-MS experiments conducted on the AuCl₃ and AuCl₃/CuCl₂ system demonstrate that a range of solvated mononuclear clusters, i.e. [AuCl₂]⁺(H₂O)_n and [AuCl₂]⁺(HCl)₂(H₂O)_n, dominate Au speciation in the lower concentration range (0.1-1.0 mM, pH≈2.8), with strong ion signals due to polynuclear [Au₂Cl_5-xOH_x]⁺(H₂O)_n present at higher concentrations (50mM, pH≈2.3). Results from theoretical calculations show that ion-neutral complexes Au⁺(Cl₂)(H₂O)_n, with a formal Au(I) center, are the dominant form of mononuclear gold with n=0-2, whereas higher hydrates are covalently bound complexes of the type [AuCl₂]⁺(H₂O)_n, in which gold exists as Au(III). Results from concentration-dependent ESI-MS experiments demonstrate an increasing trend toward Au cluster growth with increasing solution concentration, in other words, at higher AuCl₃ concentrations more gold chloride is present as [Au₂Cl_5-xOH_x]⁺(H₂O)_n. In general, results from ESI mass spectrometry indicate that polynuclear Au clusters are important vapor phase species and therefore of particular interest for understanding the speciation and transport of Au and Au-Cu chloride complexes in hydrothermal ore fluids with gas-like densities.