Characterization of Hydrous Species in Minerals by High-speed ^1H MAS-NMR

Proton magic-angle spinning (MAS) NMR at 200 and 500 MHz and at high spinning speeds (ca. 8 kHz) has been used to characterize hydrous species, both stoichiometric and nonstoichiometric, in a variety of minerals. High-resolution ^1H MAS-NMR spectra of minerals containing stoichiometric hydroxyl groups as the only hydrous species are obtainable provided that the hydrogen density in the sample is less than about 15 atoms/nm^3. Structurally isolated water molecules in analcite, NaAlSi_2O_6•H_2O, and gypsum, CaSO_4•2H_2O, yield characteristic ^1H MAS-NMR spectra with numerous spinning sidebands extending over a range of about 100 kHz, reflecting the strong, largely inhomogeneous character of the homonuclear dipolar coupling. The field dependence of both line widths and spinning sideband patterns provides evidence about the nature of the broadening interactions. Lawsonite and hemimorphite, minerals containing stoichiometric amounts of both OH and H_2O groups, yield spectra with numerous intense spinning sidebands; strong dipolar interactions preclude discrimination of OH and H_2O. Nonstoichiometric hydrogen in nominally anhydrous minerals (feldspars, nepheline, quartz, and grossular garnet) is found to occur in a variety of forms: mobile H_2O in fluid inclusions, anisotropically constrained, isolated H_2O molecules, and clustered species consistent with H_4O_4^(4-) groups in a hydrogarnet substitution

Quantitative NMR studies of water in silicate glasses

The state of water in volcanic and synthetic silicate glasses is characterized by a number of different solid state NMR techniques. ^1H solid echo NMR is used to quantitate total water contents reliably down to levels of 0.1 wt%. High-speed ^1H magic-angle spinning NMR can be used to obtain relative species concentrations of OH and H_2O groups in glasses free from paramagnetic impurities. Similar results are obtained from deuterium quadrupolar echo NMR difference spectroscopy, utilizing the different spin-lattice relaxation times of OD and D_2O species. The results demonstrate that the percentage of water present in the form of molecular H_2O species increases with increasing total water content. The H_2O groups are anisotropically constrained, but undergo 180° flips about the bisector axis at rates greater than 10^5 s^(−1). The hydrous species are not clustered and are subjected to a distribution of hydrogen bonding strengths. A distribution function of O-D···O distances obtained from infrared results, shows good compatibility with the experimental ^2H NMR lineshapes. The relative merits of the different experimental approaches are discussed in detail

Spatially correlated distributions of local metallic properties in bulk and nanocrystalline GaN

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Solid-State Reaction of Poly(1,6-di-N-carbazolyl-2,4-hexadiyne) with Electrophiles

^(13)C cross-polarization/magic angle spinning (CP-MAS) NMR at 50.36 MHz reveals that the reaction of the crystalline polymer poly-N-dicarbazolyl-2,4-hexadiyne (poly-DCH) with the electrophilic reagents liquid bromine, gaseous chlorine, and nitric acid fumes results in the formation of covalent bonds in the polymer. Spectra have been obtained both of the products formed as well as of model compounds, and factors influencing the quantitative interpretation of peak intensities have been taken into consideration. The observed disappearance of peaks from carbon atoms either bonded to or adjacent to bromine atoms is due to a dipolar interaction not averaged to zero by magic-angle spinning. The NMR results show that the initial anisotropic reaction is a selective attack of the bromine at the 3,6 positions of the aromatic rings of the carbazole moiety, resulting in a polymer with four Br/repeat unit. At higher bromination levels electrophilic addition to the multiple bonds in the backbone occurs. Other evidence suggests that the triple bond may react preferentially, resulting in formation of a mixed polyacetylene for products of composition poly-DCH(Br_(6.0)). The reaction of poly-DCH with nitric acid fumes appears to result in substitution at the 3,6 positions of the carbazole ring. In contrast, exposure of poly-DCH to chlorine gas most likely effects both electrophilic substitution and addition in the carbazole sidechains

Solid-State Reaction of Poly(1,6-di-N-carbazolyl-2,4-hexadiyne) with Electrophiles

^(13)C cross-polarization/magic angle spinning (CP-MAS) NMR at 50.36 MHz reveals that the reaction of the crystalline polymer poly-N-dicarbazolyl-2,4-hexadiyne (poly-DCH) with the electrophilic reagents liquid bromine, gaseous chlorine, and nitric acid fumes results in the formation of covalent bonds in the polymer. Spectra have been obtained both of the products formed as well as of model compounds, and factors influencing the quantitative interpretation of peak intensities have been taken into consideration. The observed disappearance of peaks from carbon atoms either bonded to or adjacent to bromine atoms is due to a dipolar interaction not averaged to zero by magic-angle spinning. The NMR results show that the initial anisotropic reaction is a selective attack of the bromine at the 3,6 positions of the aromatic rings of the carbazole moiety, resulting in a polymer with four Br/repeat unit. At higher bromination levels electrophilic addition to the multiple bonds in the backbone occurs. Other evidence suggests that the triple bond may react preferentially, resulting in formation of a mixed polyacetylene for products of composition poly-DCH(Br_(6.0)). The reaction of poly-DCH with nitric acid fumes appears to result in substitution at the 3,6 positions of the carbazole ring. In contrast, exposure of poly-DCH to chlorine gas most likely effects both electrophilic substitution and addition in the carbazole sidechains

The state of water in rhyolitic glasses: A deuterium NMR study

The first application of ^2H NMR to the study of water in glasses is reported. Naturally occurring volcanic glass (Los Posos rhyolite) was autoclaved with D_2O to yield samples with total “D_2O” contents ranging from 1.6–4.8 wt%. Infrared results show that, in concordance with results from proton-containing glasses, the ratio of molecular D_2O to OD groups increases with increasing total “D_2O” content. Variable-temperature deuterium NMR spectra obtained at 11.7 T, using composite-pulse quadrupolar-echo pulse sequences, indicate the presence of two species with differing motional behavior. The species that is rigid over the entire temperature range investigated (173–393 K) is assigned to OD groups, and yields a powder pattern governed by a distribution of nuclear quadrupole coupling constants (NQCCs) and with a peak-to-peak splitting of ca. 160 kHz. The species that undergoes thermally activated motional processes in this temperature range is assigned to molecular D_2O. At room temperature, this species gives rise to a powder pattern characterized by a reduced quadrupole coupling constant and an asymmetry parameter (η) close to one, indicating that the D_2O molecules undergo twofold rotations about the bisector axis on a lime scale faster than ca. 10^(−5) s. Separation of both spectral components on the basis of their different spin-lattice relaxation times yields reasonable agreement with the D_2O/OD ratio determined from infrared spectroscopy. Lineshape changes at elevated temperatures suggest the activation of additional motional processes; however, even at 375 K only a minor fraction of isotropically mobile species is observed, indicating that “pools” of liquid-water do not exist in significant amounts in these glasses. Low-temperature studies indicate a gradual freezing-out of D_2O motion on the NMR time scale (ca. 10^(−5) s), resulting in a broad lineshape at 173 K governed by a distribution of NQCCs. The well-established correlation between the NQCC and the O-D ··· O distance, a measure of the hydrogen bonding strength, can be used to simulate the low-temperature ^2H NMR spectrum from an assumed distribution of O-D ··· O bond distances. The resultant spectrum, based on a distribution function obtained from independent IR data, is characterized by a mean NQCC of 223 kHz and is in good agreement with the experimental lineshape

Nature of Polyoxometalate Intramolecular Coordination to Quaternary Ammonium Salts from Paramagnetic Relaxation Enhancement

Polyoxometalates (POMs) exhibit catalytic activity toward a variety of harmful chemicals such as chemical warfare agents, qualifying them as promising candidates as additives to create self-decontaminating surfaces and materials. However, POMs exhibit poor solubility and dispersion behavior in organic matrices, including polymeric coatings. In an effort to improve compatibility with polymer coatings and impart surface segregating behavior, we describe the encapsulation of a Ni­(II)-containing POM, α₂-K₈P₂W₁₇O₆₁­(Ni²⁺·OH₂)·17H₂O (Ni-POM), with a series of amphiphilic alkyl ethoxy dimethyl quaternary ammonium salts (QASs) and elucidate their structural coordination. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were utilized to confirm that QASs are coordinated to the Ni-POM and that the average number of QAS coordinated to each Ni-POM increases with increasing alkyl moiety length. The ¹H NMR spectra of the QAS­(Ni-POM) complexes show marked site-specific broadening and reduced spin–lattice relaxation times <i>T</i>₁ compared to either a nonparamagnetic QAS­(POM) complex or the neat QAS ligand. These paramagnetic relaxation enhancement (PRE) effects were used to obtain structural and dynamical information about the binding of QASs to the Ni-POM. The single-exponential saturation recovery behavior observed in all cases indicated that all bound QAS molecules are rapidly moving about the entire (Ni-POM) surface on a time scale less than tens of milliseconds. Motionally averaged distances of the QAS protons to the paramagnetic Ni²⁺ center were estimated using a modified Solomon–Bloembergen equation. Comparisons of relative distances for protons at different sites on the QAS molecule provide key insights into the structural nature of the bonding. Surprisingly, the ethylene oxide moiety of the amphiphilic QAS was found to coordinate more closely with the surface of the Ni-POM than the quaternary ammonium nitrogen cation, and the alkyl moieties extended outward from the Ni-POM center. These results suggest that QAS­(Ni-POM) complexes should behave effectively as a hydrophobic nonpolar complexes in their desired roles as catalytic centers in coatings