Recent theoretical advances and molecular-dynamic estimates of the dielectric constant of water have extended the HKF model for aqueous solution chemistry up to 6.0 gigapascals (GPa), which are conditions well beyond the capabilities of conventional NMR spectroscopy (Pan and others, 2013; Sverjensky and others, 2014, see also Wasserman and others, 1995). These developments provide strong motivation to design a simple NMR probe that allows experiments on solutions, at high resolution, to pressures of a few GPa (Pautler and others, 2014; Ochoa and others, 2015, 2016). Here we describe the performance of a compact NMR probe that can reach several GPa pressures. The probe is made by placing a solenoid microcoil inside of a standard piston-cylinder device used in solid-state physics. High pressures are achieved in the sample by applying force to a coaxial piston. Early designs of the probe, although useful, were limited in sample size to ϳ10 to 15 L. Here we describe modifications that allow greatly improved resolution and sensitivity, including 1 H-1 H NMR correlation spectroscopy, on solutions at 2.8 GPa pressure. Sample sizes can be expanded if, instead of a standard NMR spectrometer that is built around a superconducting magnet, one employs a magnetic-resonance imaging (MRI) system that is built around a permanent magnet. The MRI systems can apply larger magnetic field gradients than conventional spectrometers, and thus have more robust shimming capability, which is needed because of the juxtaposition of different alloys in the pressure cell. More importantly, these systems can have magnetic fields that are oriented perpendicular to the magnet bore, which allows rotation of the coil within the axis of the pressure cell to increase sample volumes and pressure. Sensitivity is improved by replacing the traditional, but reduced-volume, closed sample container mounted in the center of the NMR detection coil with an open NMR coil that dangles freely in solution, thus making the sample solution itself the pressure-transmission fluid. The efficacy of these modifications are demonstrated by measuring 1 H NMR spectra for ethyl alcohol and ethyl alcohol/methyl alcohol mixtures at pressures up to 2.8 GPa. Further developments are discussed that will allow geochemists to acquire aqueous solution NMR spectra at higher 3 to 4 GPa pressures.

Aqueous geochemistry could be extended considerably if nuclear-magnetic resonance (NMR) methods could be adapted to study solutions at elevated temperatures and pressures. We therefore designed an NMR probe that can be used to study aqueous solutions at gigapascal pressures. Fluoride solutions were chosen for study because 19 F couples to other nuclei in the solutions (31 P and 11 B) in ways that make peak assignments unequivocal. Correspondingly, NMR spectra of 19 F-and 11 B were collected on aqueous HBF 4-NH 4 PF 6 solutions to pressures up to 2.0 GPa. At pressure, peaks in the 19 F spectra were clear and assignable to the BF 4 À (aq), F À (aq) and BF 3 OH À (aq) ions, and these aqueous complexes varied in signal intensity with pressure and time, for each solution. Peaks in the 11 B spectra at pressure could be assigned to the BF 4 À (aq) and BF 3 OH À (aq) species. Additionally, there is a single peak that is assignable to H 3 BO 3 o (aq) and B(OH) 4 À (aq) in rapid-exchange equilibria. These peaks broaden and move with pressure in ways that suggest reversible interconversion of borate and fluoroborate species. The PF 6 À ion was found to provide a suitable 19 F shift and intensity standard for high-pressure spectra because it was chemically inert. The positions and intensities of the doublet peak also remains constant as a function of pressure and pH. Addition of electrolytes considerably distorts the phase diagram of water such that the stability region of the aqueous solution expands to well beyond the 0.8 GPa freezing pressure of pure water; some fluoroborate solutions remain liquid until almost 2.0 GPa.

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 2 H compare well with those reported in the previous studies of Jonas et al. (1974), who examined lower pressures, and indicate that the probe functions properly. In some experiments, 133 Cs and 1 H 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 pressureenhanced rates of geochemical reactions, such as interconversion of aqueous complexes.

Nuclear spin relaxation rates of (2) H and (139) La in LaCl3 +(2) H2 O and La(ClO4 )3 +(2) H2 O solutions were determined as a function of pressure in order to demonstrate a new NMR probe designed for solution spectroscopy at geochemical pressures. The (2) H longitudinal relaxation rates (T1 ) vary linearly to 1.6 GPa, consistent with previous work at lower pressures. The (139) La T1 values vary both with solution chemistry and pressure, but converge with pressure, suggesting that the combined effects of increased viscosity and enhanced rates of ligand exchange control relaxation. This simple NMR probe design allows experiments on aqueous solutions to pressures corresponding roughly to those at the base of the Earth's continental crust.

In Abbildung 1dieser Zuschrift wurde als Einheit der vertikalen Achse fälschlicherweise Millisekunden anstelle korrekt Sekunden angegeben. Die korrigierte Abbildung 1i st daher hier gezeigt. Die Autoren bitten, dieses Versehen zu entschuldigen. Figure 1. a) T 1 values from 2 HNMR spectra as afunction of pressure of 4.5 m LaCl 3 + + 2 H 2 O solutions. The lines are linear regressions. The T 1 values as afunction of solution composition at 298 Kare shown in (b). The solid circles (*)are 4.5 m LaCl 3 and the red symbols (* *)i dentify data of Lee et al. (1974). [5] The LaCl 3 + + 2 H 2 Os olutionsa re: * = 1.0 m, ! = 0.5 m, ! = 0.1 m, and the La(ClO 4) 3 + + 2 H 2 Os olutionsa re: & = 1.0 m and & = 0.1 m.Uncertainties in pressure are AE 100 MPa at 400 MPa or less and 50 MPa at pressures higher than 400 MPa. They are shown only for the 0.1 m and 4.5 m LaCl 3 data to avoid clutter and are assigned as the 95 %p rediction interval from repeated external calibrations(see the SupportingInformation).