FT-ICR mass spectrometric and density functional theory studies of solvated cerium chloride clusters

Abstract

The molecular-scale speciation of cerium in aqueous fluids (bulk liquid and vapor phase) is a primary field of interest with important implications for our understanding of the transport and deposition of REE’s in the Earth’s crust. Mass spectrometric and quantum chemical studies of cerium chloride solutions can provide important insight into the composition, structure and energetic properties of molecular cerium species, and as such, deliver new information with respect to the distribution and abundance of such materials in aqueous fluids. Here we present results from electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometric experiments of cerium chloride complexes and clusters in the gas-phase, in particular, clusters of the general form [CemXn]+ (X=Cl, OH, O) and corresponding microsolvated species. We also report equilibrium geometries and associated energetic properties using M06/cc-pVXZ-PP (X=D,T) level theory in combination with quasi-relativistic effective core potentials for Ce. Ion cluster experiments have been conducted on a custom-modified Bruker-Daltonics 7T FT-ICR mass spectrometer with temperature-control, ESI capability and pulsing valves facilitating ion-solvent reactions. Molecular species identified upon electrospray ionozation of aqueous CeCl3 include a) [CeCl2]+(H2O)n, [CeClOH]+(H2O)n and [Ce(OH)2]+(H2O)n with n=0-4; b) polynuclear [CemCl3m- 1]+(H2O)n with m=1-5 and n=0-4 c) mixed polynuclear [CemCl3m-2OH]+ and [CemCl3m-3O]+ with m=1-5 and d) larger solvated clusters of the general form [CemCl3m-4O]+2(H2O)n and [CemCl3m-4O]+2(H2O)n with m06 and up to six water molecules. We also conducted mass spectrometric experiments in which the concentration-dependence (1-20mM; pH 5.2-6.9) of the cerium cluster distribution was probed. Results from these concentration-dependent experiments demonstrate that cerium chloride clustering increases with CeCl3 concentration, in other words, at higher CeCl3 concentrations (lower pH) more cerium chloride is present as polynuclear chlorocerate. With respect to solution pH, it is likely that the emergence of clusters with bridging hydroxo groups, e.g. [CemCl3m-2OH]+ arises as a consequence of the partial hydrolysis of [CemCl3m- 1]+ clusters, in particular, in the acidic pH range. Last but not least, the observed correlation between ion mass spectra and solution content appears to demonstrate that dinuclear clusters are intermediates on the way from the [CeCl2]+ complex to the formation of larger “bulk-like” chlorocerate clusters. In conclusion, results from our ESI mass spectromentric and DFT study strongly point toward molecular clustering being an important factor in understanding Ce speciation in aqueous fluids.