Solid-State 27Al NMR Spectroscopy of the γ-Al13 Keggin Containing Al Coordinated by a Terminal Hydroxyl Ligand.

We report solid-state 27Al NMR spectroscopic results for the sulfate salt of the γ-Al13 Keggin cluster, γ-[AlO4Al12(OH)25(OH2)11][SO4]3·[H2O]14, that provide a spectroscopic signature for partial hydrolysis of this Keggin-type cluster. In 27Al multiple-quantum magic-angle spinning NMR spectra, all 13 Al positions of the cluster are at least partially resolved and assigned with the aid of density functional theory (DFT) calculations of the 27Al electric field gradients. The isotropic chemical shift of the single tetrahedral site, 75.7 ppm, is nearly identical to that reported for solutions from which the cluster crystallizes. Reflecting broadly similar coordination environments, the octahedral Al show mostly small variations in isotropic chemical shift (+7 to +11 ppm) and quadrupolar coupling constant (CQ; 6-7.5 MHz), except for one resonance that exhibits a much smaller CQ and another site with a larger value. DFT calculations show that deprotonation of a terminal water ligand, to form an η-OH group, causes a large reduction in the 27Al CQ, allowing assignment of a distinct, narrow peak for octahedral Al to this hydroxyl-terminated site. This result suggests a relationship between octahedral 27Al NMR line width and hydrolysis for solids prepared from Keggin-type clusters.

Synthesis of the Stable Ordered Conjugated Polymer Poly(dibromodiacetylene) from an Explosive Monomer.

Dibromobutadiyne is an extremely unstable compound that explodes at room temperature, even under inert atmosphere. This instability has limited the studies of dibromobutadiyne almost entirely to spectroscopic characterization. Here we report an approach to control the reactivity of dibromobutadiyne, via topochemical reaction in cocrystals, leading to the ordered polymer poly(dibromodiacetylene), PBDA. At low temperatures (-15 to -18 °C), dibromobutadiyne can form cocrystals with oxalamide host molecules containing either pyridyl or nitrile side groups, in which halogen bonds align the dibromobutadiyne monomers for topochemical polymerization. The cocrystals with the bis(nitrile) oxalamide host undergo complete ordered polymerization to PBDA, demonstrated by solid-state MAS-NMR, Raman, and optical absorption spectroscopy. Once formed, the polymer can be separated from the host; unlike the monomer, PBDA is stable at room temperature.

Solid-state NMR spectroscopic study of phosphate sorption mechanisms on aluminum (Hydr)oxides.

Sorption reactions occurring at mineral/water interfaces are of fundamental importance in controlling the sequestration and bioavailability of nutrients and pollutants in aqueous environments. To advance the understanding of sorption reactions, development of new methodology is required. In this study, we applied novel (31)P solid-state nuclear magnetic resonance (NMR) spectroscopy to investigate the mechanism of phosphate sorption on aluminum hydroxides under different environmental conditions, including pH (4-10), concentration (0.1-10 mM), ionic strength (0.001-0.5 M), and reaction time (15 min-22 days). Under these conditions, the NMR results suggest formation of bidentate binuclear inner-sphere surface complexes was the dominant mechanism. However, it was found that surface wetting caused a small difference. A small amount (<3%) of monodentate mononuclear inner-sphere surface complexes was observed in addition to the majority of bidentate binuclear surface complexes on a wet paste sample prepared at pH 5, which was analyzed in situ by a double-resonance NMR technique, namely, (31)P{(27)Al} rotational echo adiabatic passage double resonance (REAPDOR). Additionally, we found that adsorbents can substantially impact phosphate sorption not only on the macroscopic sorption capacity but also on their (31)P NMR spectra. Very similar NMR peaks were observed for phosphate sorbed to gibbsite and bayerite, whereas the spectra for phosphate adsorbed to boehmite, corundum, and γ-alumina were significantly different. All of these measurements reveal that NMR spectroscopy is a useful analytical tool for studying phosphorus chemistry at environmental interfaces.

Inhibition mechanisms of Zn precipitation on aluminum oxide by glyphosate: a 31P NMR and Zn EXAFS study.

In this research, the effects of glyphosate (GPS) on Zn sorption/precipitation on γ-alumina were investigated using a batch technique, Zn K-edge EXAFS, and (31)P NMR spectroscopy. The EXAFS analysis revealed that, in the absence of glyphosate, Zn adsorbed on the aluminum oxide surface mainly as bidentate mononuclear surface complexes at pH 5.5, whereas Zn-Al layered double hydroxide (LDH) precipitates formed at pH 8.0. In the presence of glyphosate, the EXAFS spectra of Zn sorption samples at pH 5.5 and 8.0 were very similar, both of which demonstrated that Zn did not directly bind to the mineral surface but bonded with the carboxyl group of glyphosate. Formation of γ-alumina-GPS-Zn ternary surface complexes was further suggested by (31)P solid state NMR data which indicated the glyphosate binds to γ-alumina via a phosphonate group, bridging the mineral surface and Zn. Additionally, we showed the sequence of additional glyphosate and Zn can influence the sorption mechanism. At pH 8, Zn-Al LDH precipitates formed if Zn was added first, and no precipitates formed if glyphosate was added first or simultaneously with Zn. In contrast, at pH 5.5, only γ-alumina-GPS-Zn ternary surface complexes formed regardless of whether glyphosate or Zn was added first or both were added simultaneously.

Formation of crystalline Zn-Al layered double hydroxide precipitates on γ-alumina: the role of mineral dissolution.

To better understand the sequestration of toxic metals such as nickel (Ni), zinc (Zn), and cobalt (Co) as layered double hydroxide (LDH) phases in soils, we systematically examined the presence of Al and the role of mineral dissolution during Zn sorption/precipitation on γ-Al(2)O(3) (γ-alumina) at pH 7.5 using extended X-ray absorption fine structure spectroscopy (EXAFS), high-resolution transmission electron microscopy (HR-TEM), synchrotron-radiation powder X-ray diffraction (SR-XRD), and (27)Al solid-state NMR. The EXAFS analysis indicates the formation of Zn-Al LDH precipitates at Zn concentration ≥0.4 mM, and both HR-TEM and SR-XRD reveal that these precipitates are crystalline. These precipitates yield a small shoulder at δ(Al-27) = +12.5 ppm in the (27)Al solid-state NMR spectra, consistent with the mixed octahedral Al/Zn chemical environment in typical Zn-Al LDHs. The NMR analysis provides direct evidence for the existence of Al in the precipitates and the migration from the dissolution of γ-alumina substrate. To further address this issue, we compared the Zn sorption mechanism on a series of Al (hydr)oxides with similar chemical composition but differing dissolubility using EXAFS and TEM. These results suggest that, under the same experimental conditions, Zn-Al LDH precipitates formed on γ-alumina and corundum but not on less soluble minerals such as bayerite, boehmite, and gibbsite, which point outs that substrate mineral surface dissolution plays an important role in the formation of Zn-Al LDH precipitates.

Differential pair distribution function study of the structure of arsenate adsorbed on nanocrystalline γ-alumina.

Structural information is important for understanding surface adsorption mechanisms of contaminants on metal (hydr)oxides. In this work, a novel technique was employed to study the interfacial structure of arsenate oxyanions adsorbed on γ-alumina nanoparticles, namely, differential pair distribution function (d-PDF) analysis of synchrotron X-ray total scattering. The d-PDF is the difference of properly normalized PDFs obtained for samples with and without arsenate adsorbed, otherwise identically prepared. The real space pattern contains information on atomic pair correlations between adsorbed arsenate and the atoms on γ-alumina surface (Al, O, etc.). PDF results on the arsenate adsorption sample on γ-alumina prepared at 1 mM As concentration and pH 5 revealed two peaks at 1.66 Å and 3.09 Å, corresponding to As-O and As-Al atomic pair correlations. This observation is consistent with those measured by extended X-ray absorption fine structure (EXAFS) spectroscopy, which suggests a first shell of As-O at 1.69 ± 0.01 Å with a coordination number of ~4 and a second shell of As-Al at ~3.13 ± 0.04 Å with a coordination number of ~2. These results are in agreement with a bidentate binuclear coordination environment to the octahedral Al of γ-alumina as predicted by density functional theory (DFT) calculation.

Surface speciation of phosphate on boehmite (gamma-AlOOH) determined from NMR spectroscopy.

Interaction of phosphate with the surfaces of clays and metal oxyhydroxides is important for nutrient cycling in natural and agricultural systems. We examined the specific adsorption of phosphate to boehmite (gamma-AlOOH) by solid-state (31)P NMR spectroscopy, which yields evidence for the presence of two bridging bidentate surface complexes differing in protonation. For samples prepared along the sorption isotherm at pH 5, distinct phosphate environments are observed as two major peaks in (31)P NMR spectra (chemical shifts of 0 and -6 ppm) that show little change in relative intensity with adsorbate loading. Both peaks correspond to rigid phosphate in close proximity to H, as indicated by (31)P{(1)H} cross-polarization magic-angle-spinning (CP/MAS) data, and yield nearly identical (31)P{(27)Al} dephasing curves in rotational echo adiabatic passage double resonance (REAPDOR) experiments. The REAPDOR results indicate that both phosphate environments have similar coordination to Al and are best fit by dephasing curves simulated for bridging bidentate configurations. The two resolved phosphate species exhibit distinct (31)P chemical shift anisotropy (CSA) and intensity variations with pH, the peak near 0 ppm being dominant at pH > 7. (31)P CSA's from quantum chemical calculations of hydrated bidentate cluster models with varying protonation state show that the CSA for monoprotonated phosphate is unique and closely matches that for the peak at -6 ppm. The CSA for the peak at 0 ppm is consistent with both di- and nonprotonated phosphate, but assignment to the latter is suggested based on the dominance of this peak in samples prepared at high pH and with trends in (31)P NMR chemical shifts.

Solid-state NMR spectroscopy of Pb-rich apatite.

Pb-containing hydroxylapatite phases synthesized under aqueous conditions were investigated by X-ray diffraction and solid-state nuclear magnetic resonance (NMR) techniques to determine the Pb, Ca distribution. 31P and 1H magic-angle spinning (MAS) NMR results indicate slight shifts of the isotropic chemical shift with increased Ca content and complex lineshapes at compositions with near equal amounts of Ca and Pb. 31P{207Pb} and 1H{207Pb} rotational-echo double resonance (REDOR) results for intermediate compositions show that resolved spectral features cannot be assigned simply in terms of local Ca, Pb configurations or coexisting phases. 207Pb MAS NMR spectra are easily obtained for these materials and contain well-resolved resonances for crystallographically unique A1 and A2 Pb sites. Splitting of the A1 and A2 207Pb resonances for pure hydroxyl-pyromorphite (Pb10(PO4)6(OH)2) compared to natural pyromorphite (Pb5(PO4)3Cl) suggests symmetry reduced from hexagonal. We find that 207Pb{1H} CP/MAS NMR is impractical in Pb-rich hydroxylapatites due to fast 207Pb relaxation.

Molecular-scale investigation of fluoride sorption mechanism by nanosized hydroxyapatite using 19F solid-state NMR spectroscopy.

Hydroxyapatite (Hap) has been shown to be an excellent sorbent for F- removal of elevated levels of fluoride in groundwater worldwide; however, the molecular mechanisms of this process have not been clearly addressed. Herein, we used 19F solid-state NMR spectroscopy to investigate F- sorption mechanisms by nanosized Hap combined with 1H NMR and 1H{19F} Rotational Echo DOble Resonance (REDOR) technology in addition to other characterization methods such as Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and Nanoscale Secondary Ion Mass Spectroscopy (NanoSIMS). Our experimental results showed that F- sorption mechanisms depend on solution pH and fluoride concentration ([F-]). At pH 7 and [F-] ≤ 50 mM, a single 19F NMR peak at -103 ppm was observed, which could be assigned to fluorapatite [Ca5(PO4)3F] (Fap) or fluoro-hydroxyapatite solid solution [Ca5(PO4)3Fx(OH)1-x; x = 0-1] (F-Hap). A simultaneous formation of fluorite (CaF2) precipitates (δF-19 = -108 ppm) was observed at higher [F-] (e.g., 100 mM), which was further confirmed by TEM and XRD analysis. The NanoSIMS and 1H{19F} REDOR analyses indicated that a dissolution-precipitation process was involved in the F- sorption on Hap. Our results strongly support the efficacy of Hap for F- removal even after several instances of regeneration, making it a cost-effective strategy for fluoride treatment.

NMR spectroscopy of citrate in solids: cross-polarization kinetics in weakly coupled systems.

Solid-state NMR spectroscopy is a potentially powerful method for obtaining molecular level structural information crucial for understanding the specific relationship between calcite crystals and occluded organic molecules that are important in biomineralization and biomimetic materials. In this work, a method is developed based on cross-polarization/magic angle spinning (CP/MAS) NMR to measure the heteronuclear distances and obtain structural information for large intracrystalline citrate defects in a synthetic calcite/citrate composite. Using compounds with well-characterized crystal structures, Mg(II) citrate and Sr(II) citrate, a correlation is established between T(IS), the CP time, and M(2) (IS), the van Vleck heteronuclear dipolar second moment, which contains distance and structural information. This correlation is supported by peak assignments obtained from calculations of the (13)C chemical shifts for crystalline Mg(II) citrate. On the basis of T(IS) (-1) versus M(2) (IS) correlation, measurement of T(IS) for carbonate ions associated with citrate defects in a calcite((13)C-enriched)/citrate coprecipitate yields an estimate for the distance between citrate and the nearest carbonate carbon that indicates close spatial proximity and provides useful constraints for future computational study. The applicability of T(IS) (-1) versus M(2) (IS) correlations to other weakly coupled spin-1/2 systems is discussed in terms of the effects of (1)H homonuclear dipolar coupling, using the CP kinetics of Zn(II) dihydroxybenzoate and kaolinite for comparison. The results suggest a limited range of correlation constants and indicate that quantitative information can be obtained from CP/MAS kinetics obtained under similar experimental conditions.

Characterization of Al30 in commercial poly-aluminum chlorohydrate by solid-state (27)Al NMR spectroscopy.

Investigation of commercially produced hydrolysis salts of aluminum by solid-state (27)Al NMR spectroscopy and size-exclusion chromatography (SEC) reveals well-defined and distinct Al environments that can be related to physicochemical properties. (27)Al MAS and MQ-MAS NMR spectroscopic data show that the local structure of the solids is dominated by moieties that closely resemble the Al30 polyoxocation (Al30O8(OH)56(H2O)26(18+)), accounting for 72-85% of the total Al. These Al30-like clusters elute as several size fractions by SEC. Comparison of the SEC and NMR results indicates that the Al30-like clusters includes intact isolated clusters, moieties of larger polymers or aggregates, and possibly fragments resembling δ-Al13 Keggin clusters. The coagulation efficacy of the solids appears to correlate best with the abundance of intact Al30-like clusters and of smaller species available to promote condensation reactions.

An (17)O-NMR Study of the Exchange of Water on AlOH(H(2)O)(5)(2+)(aq).

The pH-dependence of the rate of water exchange between hydrated aluminum complexes and bulk aqueous solution is reported from variable-temperature dynamic (17)O-NMR measurements. Over the temperature range 298-348 K, the pseudo-first-order rate coefficient for exchange at any given temperature, is described by = + [H(+)](-1), where is the rate coefficient for exchange of inner-sphere water molecules with bulk solution for the Al(H(2)O)(6)(3+)(aq) complex at that temperature, and [H(+)](-1) is the pH-dependent contribution to from the first hydrolysis product: AlOH(H(2)O)(5)(2+)(aq). The rate parameters are = 1.3 s(-1), DeltaH(1)() = 84.7 kJ.mol(-1), DeltaS(1)() = 41.6 J.mol(-1).K(-1) (from 6); and = 7.2 x 10(-2) M.s(-1), DeltaH(2)() = 90.4 kJ.mol(-1), DeltaS(2)() = 36.6 J.mol(-1).K(-1) (this work). From this result we obtained the activation parameters for water exchange on the AlOH(H(2)O)(5)(2+)(aq) complex: = 3.1 x 10(4) s(-1), DeltaH() = 36.4 kJ.mol(-1), DeltaS() = -36.4 J.mol(-1).K(-1). Thus, deprotonation of an inner-sphere water in Al(H(2)O)(6)(3+)(aq) enhances by a factor of approximately 10(4) the lability of other inner-sphere oxygens. This labilizing effect of hydroxyl is much larger than for other geochemically important ligands, including fluoride.

Phosphate uptake by TiO2: batch studies and NMR spectroscopic evidence for multisite adsorption.

Systematic studies, combining batch experiments with NMR spectroscopic methods, are carried out for phosphate sorption on titanium dioxide (TiO(2)). It is found that phosphate sorption on TiO(2) decreases with increasing pH, whereas the phosphate uptake by TiO(2) increases with increasing ionic strength of the solution. In I ≤ 0.1 M, the sorption sharply increases and reaches a near maximum and then followed by little changes showing Langmuir-type behavior, whereas in I = 0.7 M, non-Langmuirian uptake becomes evident as equilibrium phosphate concentrations increase in solution. The sorption of phosphate on TiO(2) is rapid and mostly irreversible at pH 4.5 and 7.0. At pH 9.0, however, the phosphate sorption is initially reversible and followed by resorption of phosphate on TiO(2) at the system re-equilibration. (31)P{(1)H} cross-polarization and magic angle spinning (CP/MAS) NMR spectra contain at least four main peaks which appear similar in position and width under all adsorption conditions, but vary in intensity with surface loading. The spectral characteristics of these peaks, including cross-polarization dynamics and chemical shift anisotropy obtained from spinning sideband analysis, suggest that they arise from distinct inner-sphere adsorption complexes, most of which are protonated. These results indicate that uptake of phosphate by TiO(2) occurs by formation of several types of surface complexes.

Quantum mechanical calculation of aqueuous uranium complexes: carbonate, phosphate, organic and biomolecular species.

BACKGROUND: Quantum mechanical calculations were performed on a variety of uranium species representing U(VI), U(V), U(IV), U-carbonates, U-phosphates, U-oxalates, U-catecholates, U-phosphodiesters, U-phosphorylated N-acetyl-glucosamine (NAG), and U-2-Keto-3-doxyoctanoate (KDO) with explicit solvation by H2O molecules. These models represent major U species in natural waters and complexes on bacterial surfaces. The model results are compared to observed EXAFS, IR, Raman and NMR spectra. RESULTS: Agreement between experiment and theory is acceptable in most cases, and the reasons for discrepancies are discussed. Calculated Gibbs free energies are used to constrain which configurations are most likely to be stable under circumneutral pH conditions. Reduction of U(VI) to U(IV) is examined for the U-carbonate and U-catechol complexes. CONCLUSION: Results on the potential energy differences between U(V)- and U(IV)-carbonate complexes suggest that the cause of slower disproportionation in this system is electrostatic repulsion between UO2 [CO3]3(5-) ions that must approach one another to form U(VI) and U(IV) rather than a change in thermodynamic stability. Calculations on U-catechol species are consistent with the observation that UO2(2+) can oxidize catechol and form quinone-like species. In addition, outer-sphere complexation is predicted to be the most stable for U-catechol interactions based on calculated energies and comparison to 13C NMR spectra. Outer-sphere complexes (i.e., ion pairs bridged by water molecules) are predicted to be comparable in Gibbs free energy to inner-sphere complexes for a model carboxylic acid. Complexation of uranyl to phosphorus-containing groups in extracellular polymeric substances is predicted to favor phosphonate groups, such as that found in phosphorylated NAG, rather than phosphodiesters, such as those in nucleic acids.

The structure of ferrihydrite, a nanocrystalline material.

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

Observation of solid-state 103Rh NMR by cross-polarization.

Using 103Rh[1H] cross-polarization (CP) methods, we have obtained solid-state 103Rh NMR spectra for diamagnetic Rh(III) compounds. The isotropic chemical shift and chemical shift anisotropy (CSA) are reported for a crystalline form of the dihydroxy-bridged Rh(III) dimer and for a salt of the oxo-centered acetate-bridged Rh(III) trimer, from analysis of conventional CP/MAS spectra. Comparison of the CP kinetics of the dimer with new crystal structure data suggests a strategy for predicting 103Rh CP/MAS properties in solids.

Organic coprecipitates with calcite: NMR spectroscopic evidence.

Dissolved organic ligands are well known to interact strongly with the calcite surface, altering precipitation and dissolution rates, crystal morphology, and possibly the ability of calcite to sequester metal contaminants. We show, using NMR spectroscopic techniques, that some of the citrate molecules present in a solution of precipitating calcite are incorporated structurally into the calcite crystal. Calcite grown by a seeded constant-addition method contains approximately 1 wt % coprecipitated citrate and yields 13C{1H} cross-polarization magic-angle spinning NMR spectra that contain broad peaks for citrate plus a signal from carbonate. Results from 13C{1H} heteronuclear correlation NMR experiments show that citrate is located in close spatial proximity to carbonate groups. In addition, calcite/citrate coprecipitates contain about 0.4 wt % excess water, which is not present as fluid inclusions, and some of which occurs as rigid structural water. These results suggestthat water and hydrogen-bonding interactions play a role in the interface between included organic molecules and the calcite host. Additional NMR data obtained for calcite coprecipitates of aspartic and glutamic acids suggest they are also incorporated structurally but at concentrations much lower than for citrate, whereas no evidence was found for phthalate incorporation.

A new aluminum hydroxide octamer, [Al8(OH)14(H2O)18](SO4)5 x 16H2O.

A new aluminum hydroxide cluster with eight Al(III) ions linked together by a series of hydroxyl bridges as a sulfate salt has been synthesized from aqueous solution and characterized by X-ray crystallography and 27Al NMR spectroscopy.