Paul Guerry

31P MAS refocused INADEQUATE spin-echo (REINE) NMR spectroscopy: revealing J coupling and chemical shift two-dimensional correlations in disordered solids.

Two-dimensional (2D) variations in (2)J(P(1),P(1)), (2)J(P(1),P(2)), and (2)J(P(2),P(2)) are obtained--using the REINE (REfocused INADEQUATE spin-Echo) pulse sequence presented by Cadars et al. (Phys. Chem. Chem. Phys. 2007, 9, 92-103)--from pixel-by-pixel fittings of the spin-echo modulation for the 2D correlation peaks due to linked phosphate tetrahedra (P(1)-P(1), P(1)-P(2), P(2)-P(1), and P(2)-P(2)) in a (31)P refocused INADEQUATE solid-state MAS NMR spectrum of a cadmium phosphate glass, 0.575CdO-0.425P(2)O(5). In particular, separate variations for each 2D (31)P REINE peak are obtained which reveal correlations between the J couplings and the (31)P chemical shifts of the coupled nuclei that are much clearer than those evident in previously presented 2D z-filtered (31)P spin-echo spectra. Notably, such correlations between the J couplings and the (31)P chemical shifts are observed even though the conditional probability distributions extracted using the protocol of Cadars et al. (J. Am. Chem. Soc. 2005, 127, 4466-4476) indicate that there is no marked correlation between the (31)P chemical shifts of neighboring phosphate tetrahedra. For 2D peaks at the P(2) (31)P chemical shift in the direct dimension, there can be contributions from chains of three units (P(1)-P(2)-P(1)), chains of four units (P(1)-P(2)-P(2)-P(1)), or longer chains or rings (-P(2)-P(2)-P(2)-): for the representative glass considered here, best fits are obtained assuming a glass comprised predominantly of chains of four units. The following variations are found: (2)J(P(1),P(1)) = 13.4 +/- 0.3 to 14.8 +/- 0.5 Hz, (2)J(P(1),P(2)) = 15.0 +/- 0.3 to 18.2 +/- 0.3 Hz, and (2)J(P(2),P(2)) = 5.9 +/- 0.6 to 9.1 +/- 0.9 Hz from the fits to the P(1)-P(1), P(1)-P(2), and P(2)-P(2) peaks, respectively. The correlation of a particular J coupling with the (31)P chemical shifts of the considered nucleus and the coupled nucleus is quantified by the coefficients C(F(2)) and C(F(1)) that correspond to the average pixel-by-pixel change in the J coupling with respect to the chemical shift of the observed (F(2)) and neighboring (F(1)) (31)P nuclei, respectively.

Sol–gel synthesis and structural characterisation of P2O5–B2O3–Na2O glasses for biomedical applications

Glasses in the system 40(P2O5)-x(B2O3)-(60 - x)(Na2O) (10 <= x <= 25 mol%) were prepared by the sol gel technique. A mixture of mono- and diethylphosphates was used as precursor for P2O5, boric acid and sodium methoxide were used as source compounds for B2O3 and Na2O, respectively. The dried gels obtained were heat treated at 200, 300 and 400 degrees C. Structural development occurring during heat treatment and changes with composition were investigated using X-ray diffraction, thermal analysis, infrared spectroscopy, B-11 and P-31 solid state NMR. Systems with x = 20 and x = 25 mol% are amorphous up to 400 degrees C, whereas systems with lower B2O3 content are partially crystalline. This work extends sol-gel preparation of amorphous borophosphate systems having P2O5 as the main component.

The structure of phosphate glass biomaterials from neutron diffraction and P-31 nuclear magnetic resonance data

Neutron diffraction and 31P nuclear magnetic resonance spectroscopy were used to probe the structure of phosphate glass biomaterials of general composition (CaO)0.5-x(Na2O)x(P2O5)0.5 (x = 0, 0.1 and 0.5). The results suggest that all three glasses have structures based on chains of Q2 phosphate groups. Clear structural differences are observed between the glasses containing Na2O and CaO. The P-O bonds to bridging and non-bridging oxygens are less well resolved in the neutron data from the samples containing CaO, suggesting a change in the nature of the bonding as the field strength of the cation increases [Formula: see text]. In the (CaO)0.5(P2O5)0.5 glass most of the Ca2+ ions are present in isolated CaOx polyhedra whereas in the (Na2O)0.5(P2O5)0.5 glass the NaOx polyhedra share edges leading to a Na-Na correlation. The results of the structural study are related to the properties of the (CaO)0.4(Na2O)0.1(P2O5)0.5 biomaterial.

Improving the sensitivity of J coupling measurements in solids with application to disordered materials

It has been shown previously that for magic angle spinning (MAS) solid state NMR the refocused INADEQUATE spin-echo (REINE) experiment can usefully quantify scalar (J) couplings in disordered solids. This paper focuses on the two z filter components in the original REINE pulse sequence, and investigates by means of a product operator analysis and fits to density matrix simulations the effects that their removal has on the sensitivity of the experiment and on the accuracy of the extracted J couplings. The first z filter proves unnecessary in all the cases investigated here and removing it increases the sensitivity of the experiment by a factor ∼1.1–2.0. Furthermore, for systems with broad isotropic chemical shift distributions (namely whose full widths at half maximum are greater than 30 times the mean J coupling strength), the second z filter can also be removed, thus allowing whole-echo acquisition and providing an additional √2 gain in sensitivity. Considering both random and systematic errors in the values obtained, J couplings determined by fitting the intensity modulations of REINE experiments carry an uncertainty of 0.2–1.0 Hz (∼1−10 %).

Solid State NMR as a probe of Inorganic Materials: Examples from Glasses and Sol-gels

To understand amorphous and structurally disordered materials requires the application of a wide-range of advanced physical probe techniques and herein a combined methodology is outlined. The relatively short-range structural sensitivity of solid state NMR means that it is a core probe technique for characterizing such materials. The aspects of the solid state NMR contribution are emphasized here with examples given from a number of systems, with especial emphasis on the information available from 17 O NMR in oxygen-containing materials. 17 O NMR data for crystallization of pure sol-gel prepared oxides is compared, with new data presented from In 2 O 3 and Sc 2 O 3 . Sol-gel formed oxide mixtures containing silica have been widely studied, but again the role and effect of the other added oxide varies widely. In a ternary ZrO 2 -TiO 2 -SiO 2 silicate sol-gel the level of Q 4 formation is dependent not only on the composition, as expected, but also the nature of the second added oxide. Sol-gel formed phosphates have been much less widely studied than silicates and some 31 P NMR data from xerogel, sonogel and melt-quench glasses of the same composition are compared. The effect of small amounts of added antibacterial copper on phosphate glass networks is also explored.

Strong-coupling induced damping of spin-echo modulations in magic-angle-spinning NMR: Implications for J coupling measurements in disordered solids

In the context of improving J coupling measurements in disordered solids, strong coupling effects have been investigated in the spin-echo and refocused INADEQUATE spin-echo (REINE) modulations of three- and four-spin systems under magic-angle-spinning (MAS), using density matrix simulations and solid-state NMR experiments on a cadmium phosphate glass. Analytical models are developed for the different modulation regimes, which are shown to be distinguishable in practice using Akaikes information criterion. REINE modulations are shown to be free of the damping that occurs for spin-echo modulations when the observed spin has the same isotropic chemical shift as its neighbour. Damping also occurs when the observed spin is bonded to a strongly-coupled pair. For mid-chain units, the presence of both direct and relayed damping makes both REINE and spin-echo modulations impossible to interpret quantitatively. We nonetheless outline how a qualitative comparison of the modulation curves can provide valuable information on disordered networks, possibly also pertaining to dynamic effects therein.