The oxygen-isotope fractionations between brucite and water, portlandite and water, and brucite and portlandite have been calculated over the temperature range of 0 to 450 °C using quantum-chemical methods and several basis sets and functionals. The calculations also employ embedded clusters that are chosen using the Pauling-bond-strength-conserving termination method that maintains a neutral cluster with fractional charges assigned to terminal atoms. These calculations improve upon the previous semiempirical methods for predicting mineral-mineral fractionations. These semiempirical methods fail to accurately predict the relative enrichment and depletion of oxygen isotopes for the brucite-portlandite pair. The quantum calculations presented here also fail to predict at the absolute values for enrichment of oxygen isotopes between minerals and water, and a simple correction must be employed to achieve agreement with experiments if water is in the reaction. No such correction is needed to predict fractionation between minerals. The trends derived from the calculations are robust to changes in basis sets and functionals.

When a metal oxide surface is immersed in aqueous solution, it has the ability to bind, orient, and order interfacial water, affecting both chemical and physical interactions with the surface. Structured interfacial water thus possesses time-averaged, spatially varying polarization charge and potential that are comparable to those arising due to ion accumulation. It is well established that interfacial water structure propagates from the surface into bulk solution. Here, we show that interfacial water structure also propagates laterally, with important consequences. The constant pH molecular dynamics was used to impose a pH difference between opposite faces of a model goethite (α-FeOOH) nanoparticle and quantify water polarization charge on intervening faces. We find that the structure of water on one face is strongly affected by the structure on nearby surfaces, revealing the importance of long-range lateral hydrogen bonding networks with implications for particle aggregation, oriented attachment, and processes such as dissolution and growth.

The fractionation factor in the magnesium-isotope fractionation between aqueous solutions of magnesium and brucite changes sign with increasing temperature, as uncovered by recent experiments. To understand this behavior, the Reduced Partition Function Ratios and isotopic fractionation factors (Δ26/24Mgbrucite-Mg(aq)) are calculated using molecular models of aqueous [Mg(OH2)6]2+ and the mineral brucite at increasing levels of density functional theory. The calculations were carried out on the [Mg(OH2)6]2+·12H2O cluster, along with different Pauling-bond-strength-conserving models of the mineral lattice of brucite. Three conclusions were reached: (i) all levels of theory overestimate 〈Mg–O〉 bond distances in the aqua ion complex relative to Tutton's salts; (ii) the calculations predict that brucite at 298.15 K is always enriched in the heavy isotope, in contrast with experimental observations; (iii) the temperature dependencies of Wimpenny et al. (2014) and Li et al. (2014) could only be achieved by fixing the 〈Mg–O〉 bond distances in the [Mg(OH2)6]2+·12H2O cluster to values close to those observed in crystals that trap the hydrated ion.

Soluble metal-hydroxide clusters of Group 13 metals are popular in both materials chemistry and geochemistry. In geochemistry these clusters are used as experimental models for clay-like minerals. In materials chemistry these clusters are the precursors for useful thin films. However, little is known about the formation mechanisms and solution dynamics of these species or how they compare to monomer ions. In this study we use 1H-NMR spectroscopy to estimate the exchange rates of protons on sites in the Ga13(μ3-OH)6(μ2-OH)18(H2O)24(NO3)15 (Ga13) and Ga7In6(μ3-OH)6(μ2-OH)18(H2O)24(NO3)15 (Ga7In6) cluster species. To date, these clusters provide the best molecular-scale models for the surface chemistry of thin films and layered minerals. Trivalent metal-hydroxide minerals are environmentally important, because of the rich acid-base chemistry that exists in the pH regions of most soils. Our data suggest millisecond timescales for the exchange of protons on the μ2-OH bridges with bulk water in Group 13 metal-hydroxide minerals, such as tsumgallite and diaspore.

Nuclear-magnetic resonance (NMR) spectra of CsCl and LaCl3 in D2O/H2O solutions were collected up to pressures of 1.9 GPa using a new NMR probe design that considerably extends the pressure range available for geochemical experiments. The longitudinal-relaxation times (T1) for 2H compare well with those reported in the previous studies of Lee et al. (1974), who examined lower pressures, and indicate that the probe functions properly. In some experiments, 133Cs and 1H NMR spectra could be taken on solutions to pressures well beyond the nominal freezing pressure of D2O or H2O to form Ice VI (near 0.9 GPa). Freezing to form the high-pressure ice is kinetically slow on an experimental time scale (minutes to hours). The data indicate that the electrolyte concentrations increase the freezing pressure of the solution. This result means that solution NMR spectra can be collected at pressures that are nearly twice the nominal freezing pressure of pure D2O or H2O. Pulsed-magnetic-field-gradient NMR methods are used to independently measure the self-diffusion coefficient of H2O in these solutions, which yields estimates of solution viscosity via the Stokes–Einstein relation. The increased viscosity accounts for the pressure variation of T1 values as rates of molecular tumbling are affected. Accounting for such changes is essential if NMR spectral line widths are used to infer pressure-enhanced rates of geochemical reactions, such as interconversion of aqueous complexes.

The uranyl cage clusters (UO2)22(O2)15(PHO3)20(H2O)1026- (U22) and (UO2)28(O2)20(PHO3)24(H2O)1232- (U28) can be probed in aqueous solutions by using a combination of 1H Diffusion-Ordered Spectroscopy (DOSY) and 1H-31P Heteronuclear-Single Quantum Coherence (HSQC) spectroscopy. This class of clusters is ideal for 1H NMR analysis in D2O because of the covalent character of the H-P bond in the phosphonic bridges. 1H DOSY indicates that the clusters are stable in solution and provides hydrodynamic radii of 9.8 ± 0.4 Å for the U22 and 12.3 ± 0.5 Å for the U28 clusters. Furthermore, 1H-31P HSQC delivers unequivocal signal assignment for both nuclei, which enables solution dynamics to be monitored by variable-temperature experiments, and reveals the presence of phosphonic-bridge conformers. The results provide some of the first dynamic information about steady conformational changes in these enormous actinide macroions.

The intercalation of lithium from solution into the six-membered μ2-oxo rings on the basal planes of gibbsite is well-constrained chemically. The product is a lithiated layered-double hydroxide solid that forms via in situ phase change. The reaction has well established kinetics and is associated with a distinct swelling of the gibbsite as counter ions enter the interlayer to balance the charge of lithiation. Lithium reacts to fill a fixed and well identifiable crystallographic site and has no solvation waters. Our lithium-isotope data shows that ⁶Li is favored during this intercalation and that the solid-solution fractionation depends on temperature, electrolyte concentration and counter ion identity (whether Cl^-, NO3^- or ClO4^-). We find that the amount of isotopic fractionation between solid and solution (δLisolid-solution) varies with the amount of lithium taken up into the gibbsite structure, which itself depends upon the extent of conversion and also varies with electrolyte concentration and in the counter ion in the order: ClO4^-3^--. Higher electrolyte concentrations cause more rapid expansion of the gibbsite interlayer and some counter ions, such as Cl^-, are more easily taken up than others, probably because they ease diffusion. The relationship between lithium loading and δLisolid-solution indicates two stages: (1) uptake into the crystallographic sites that favors light lithium, in parallel with adsorption of solvated cations, and (2) continued uptake of solvated cations after all available octahedral vacancies are filled; this second stage has no isotopic preference. The two-step reaction progress is supported by solid-state NMR spectra that clearly resolve a second reservoir of lithium in addition to the expected layered double-hydroxide phase.

Geochemists have models to predict solute speciation and mineral equilibria in aqueous solutions up to 1200 °C and 6 GPa. These models are useful to uncover reaction pathways deep in the Earth, though experimental confirmation is extremely difficult. Here we show speciation changes among aqueous silicate complexes to pressures of 1.8 GPa through use of a high-pressure solution-state NMR probe. The radiofrequency circuit uses a microcoil geometry that is coupled with a piston-cylinder pressure cell to generate and maintain these high pressures. The 1.8 GPa pressure corresponds to pressures reached at the lower crust or upper mantle. Although these experiments are limited to ambient temperature, we show that the increased pressure affects complexation and oligomerization reactions by eliminating bulk waters and that the pressure effects are completely reversible.

Invited for the cover of this issue are Anna Oliveri from Southern Oregon University, USA, and co-workers. The cover image shows the outer surface of a nanoscale U28 cage cluster.

The conformational dynamics of nanometer-sized actinide ions are exceptionally sensitive to the choice of counter-cations. A new means of following these dynamics in solution is presented that follows 1H NMR signals. The direct bond between hydrogen and phosphorus atoms in the bridging phosphonic groups of the [(UO2)28(O2)20(PHO3)24(H2O)12]32– (U28) cluster allows unparalleled recording of the orientations of these bridges in situ. The µ3-PHO3 bridges are organized into two supersets of conformers (facing inward vs. outward from the cluster), but each of these supersets additionally have four subsets that can be identified based upon orientations of the lone pairs of electrons in the associated oxygen atoms. The ensemble of these subsets changes over days or weeks depending upon bulk solution chemistry, temperature, and pressure. They also reflect cations trapped within the molecule. The 1H NMR spectra at room temperature indicate that the molecular orientation of this enormous ion tunes itself in response to solution composition, which suggests a strategy for selecting host–guest combinations.