Ray Dupree

The structure of soda-silica glasses: A mas NMR study

“Magic-angle spinning” NMR of sodium silicate glasses has shown that the introduction of Na2O into the SiO2 structure causes the progressive replacement of [SiO4] tetrahedra containing four bridging oxygens by [SiO4] tetrahedra with three bridging and one non-bridging oxygens. At 33.3 mol.% Na2O all [SiO4] are of the latter type and on further increase of Na2O content they are progressively replaced by [SiO4] with two non-bridging oxygens until complete replacement is achieved at 50 mol/% Na2O. The chemical shifts of both [29SiO4] types and also 23Na vary continuously with composition, i.e. there is no evidence for clustering of either species. The spectra of glasses of the disilicate and metasilicate compositions have been compared with those of the corresponding crystal phases and differences are observed between the sodium sites in both compositions and also for the silicon sites in the disilicate. This indicates that there are no “crystal-like” areas in the glass structure. In fact the increase in the 29Si resonance halfwidth on approaching a “crystal” composition suggests an increased range of SiOSi bond angles at these compositions.

A spectrometer designed for 6.7 and 14.1 T DNP-enhanced solid-state MAS NMR using quasi-optical microwave transmission

A Dynamic Nuclear Polarisation (DNP) enhanced solid-state Magic Angle Spinning (MAS) NMR spectrometer operating at 6.7 T is described and demonstrated. The 187 GHz TE(13) fundamental mode of the FU CW VII gyrotron is used as the microwave source for this magnetic field strength and 284 MHz (1)H DNP-NMR. The spectrometer is designed for use with microwave frequencies up to 395 GHz (the TE(16) second-harmonic mode of the gyrotron) for DNP at 14.1T (600 MHz (1)H NMR). The pulsed microwave output from the gyrotron is converted to a quasi-optical Gaussian beam using a Vlasov antenna and transmitted to the NMR probe via an optical bench, with beam splitters for monitoring and adjusting the microwave power, a ferrite rotator to isolate the gyrotron from the reflected power and a Martin-Puplett interferometer for adjusting the polarisation. The Gaussian beam is reflected by curved mirrors inside the DNP-MAS-NMR probe to be incident at the sample along the MAS rotation axis. The beam is focussed to a ~1 mm waist at the top of the rotor and then gradually diverges to give much more efficient coupling throughout the sample than designs using direct waveguide irradiation. The probe can be used in triple channel HXY mode for 600 MHz (1)H and double channel HX mode for 284 MHz (1)H, with MAS sample temperatures ≥85 K. Initial data at 6.7 T and ~1 W pulsed microwave power are presented with (13)C enhancements of 60 for a frozen urea solution ((1)H-(13)C CP), 16 for bacteriorhodopsin in purple membrane ((1)H-(13)C CP) and 22 for (15)N in a frozen glycine solution ((1)H-(15)N CP) being obtained. In comparison with designs which irradiate perpendicular to the rotation axis the approach used here provides a highly efficient use of the incident microwave beam and an NMR-optimised coil design.

Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (Cq) for 23Na. Off-resonance nutation experiments gave a mean Cq = 1.75 ± 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, δiso, and to provide an independent measurement of the values of Cq. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean Cq is essentially constant at 2.1-2.2 ± 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23Na NMR data of Kohn et al. 1989a and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H2O, but the other conclusions of Kohn et al. 1989a and the principal features of the dissolution mechanism developed by Kohn et al. 1989a, Kohn et al. 1992, Kohn et al. 1994 are essentially unchanged.

The structure of binary alkali silicate glasses

The role of alkali metal cations in determining the structure of binary silicate glasses has been studied by magic angle spinning NMR of 29Si and 133Cs in Rb2OSiO2 and Cs2OSiO2. Glasses with up to ≈ to mol% alkali metal oxide have been investigated and the results compared with those for Li2OSiO2 and Na2OSiO2. At high concentrations of Rb2O or Cs2O there is a deviation from the formation of solely Qn and Qn−1 species which was found for Na2OSiO2. We suggest that the structure of binary silicates is determined by a balance between the repulsion of non-bridging oxygens on different Q species and the mutual attraction or repulsion of the alkali metal cations.

Probing heteronuclear N-15-O-17 and C-13-O-17 connectivities and proximities by solid-state NMR spectroscopy

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing (15)N-(17)O dipolar and J couplings are presented for [(2)H(NH(3)),1-(13)C,(15)N,(17)O(2)]glycine.(2)HCl and [(15)N(2),(17)O(2)]uracil. Two-dimensional (15)N-(17)O correlation spectra are obtained using the R(3)-HMQC experiment; for glycine.(2)HCl, the intensity of the resolved peaks for the CO and C-O(2)H (17)O resonances corresponds to the relative magnitude of the respective (15)N-(17)O dipolar couplings. (17)O-(15)N REDOR curves are presented for glycine.(2)HCl; fits of the initial buildup (DeltaS/S < 0.2) yield effective dipolar couplings in agreement with (+/-20%) the root-sum-squared dipolar couplings determined from the crystal structure. Experimental (15)N-(17)O REAPDOR curves for the (15)N resonances in glycine.(2)HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear (13)C-(17)O and (15)N-(17)O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin-echo experiment, S(Q)(tau) = S(HET)(tau)/S(HOM)(tau). For glycine.(2)HCl, (1)J(CO) was determined as 24.7 +/- 0.2 and 25.3 +/- 0.3 Hz for the CO and C-O(2)H resonances, respectively, while for uracil, the average of the two NH...O hydrogen-bond-mediated J couplings was determined as 5.1 +/- 0.6 Hz. In addition, two-bond intramolecular J couplings, (2)J(OO) = 8.8 +/- 0.9 Hz and (2)J(N1,N3) = 2.7 +/- 0.1 Hz, were determined for glycine.(2)HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine.HCl [(1)J(CO)(CO) = 24.9 Hz, (1)J(CO)(COH) = 27.5 Hz, (2)J(OO) = 7.9 Hz] and uracil [(2h)J(N1,O4) = 6.1 Hz, (2h)J(N3,O4) = 4.6 Hz, (2)J(N1,N3) = 2.7 Hz].

Natural abundance 43Ca solid-state NMR characterisation of hydroxyapatite: identification of the two calcium sites

Natural abundance 43Ca solid‐state NMR of hydroxyapatite (Ca10(PO4)6(OH)2) was performed at three different fields (8.45, 14.1 and 18.8 T). The two crystallographically distinct calcium sites of the apatite structure were spectroscopically resolved at 18.8 T. The 43Ca NMR interaction parameters (δiso, CQ and etaQ) of each site were determined by multiple magnetic‐field simulations. The peaks with δiso = 11.2 ± 0.8 and − 1.8 ± 0.8 ppm, both with CQ = 2.6 ± 0.4 MHz, were assigned to the Ca(II) and Ca(I) sites, respectively, on the basis of their relative intensities. Copyright

Folding of xylan onto cellulose fibrils in plant cell walls revealed by solid-state NMR

Exploitation of plant lignocellulosic biomass is hampered by our ignorance of the molecular basis for its properties such as strength and digestibility. Xylan, the most prevalent non-cellulosic polysaccharide, binds to cellulose microfibrils. The nature of this interaction remains unclear, despite its importance. Here we show that the majority of xylan, which forms a threefold helical screw in solution, flattens into a twofold helical screw ribbon to bind intimately to cellulose microfibrils in the cell wall. 13C solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, supported by in silico predictions of chemical shifts, shows both two- and threefold screw xylan conformations are present in fresh Arabidopsis stems. The twofold screw xylan is spatially close to cellulose, and has similar rigidity to the cellulose microfibrils, but reverts to the threefold screw conformation in the cellulose-deficient irx3 mutant. The discovery that induced polysaccharide conformation underlies cell wall assembly provides new principles to understand biomass properties.

Structural influences on high-resolution yttrium-89 NMR spectra of solids

Structural influences on both the isotropic NMR chemical shift and resonance linewidth of 89Y are investigated by obtaining magic-angle spinning NMR spectra from a number of solid-state yttrium aluminate and silicate phases. These clearly show that the isotropic chemical shift for 89Y is dependent on the number of nearest neighbours and the nature of the next-nearest neighbours. However, unlike most other nuclei such as 29Si and 27Al, the shift ranges of different nearest-neighbour coordinations significantly overlap. In the silicates the 89Y spectrum becomes more shielded as the 29Si resonance becomes less shielded reflecting the reciprocity of the shifts.

A High-Resolution 43Ca Solid-State NMR Study of the Calcium Sites of Hydroxyapatite

High resolution Ca-43 solid-state NMR studies of hydroxyapatite (Ca-10(PO4)(6)(OH2)) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using H-1-Ca-43 rotational echo double resonance and H-1-Ca-43 cross polarization magic angle spinning experiments.

Modulation-aided signal enhancement in the magic angle spinning NMR of spin-5/2 nuclei

Abstract We report signal enhancement schemes using fast amplitude modulated pulses for the one-dimensional (1D) nuclear magnetic resonance (NMR) of spin-5/2 nuclei under magic-angle spinning. Signal enhancement by a factor of around 2.5 is observed when amplitude modulated pulses precede selective excitation of the central transition. This enhancement is a result of the redistribution of energy level populations through partial saturation of the satellite transitions. Results are shown for 27 Al and 17 O. The gain in signal intensity is very useful for the observation of weak signals from low abundance quadrupolar nuclei. The scheme works for wide ranges of quadrupole interactions and rf powers.