31P Magic Angle Spinning Nuclear Magnetic Resonance Research Articles

Local structure and connectivity in lithium phosphate glasses: a solid-state 31P MAS NMR and 2D exchange investigation

High resolution solid-state 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the local structure and connectivity for three xLi 2O·(1 - x)P 2O 5 glasses. The principal components of the 31P chemical shift anisotropy (CSA) tensors for the different phosphate tetrahedron sites (Q n ) and the relative populations of each Q n species were determined from simulation of the MAS spectra. The medium range structure and connectivity between similar or different Q n units in these lithium phosphate glasses were probed using novel two-dimensional (2D) MAS exchange experiments. Radio-frequency dipolar recoupling (RFDR) techniques allowed the reintroduction of the through space 31P- 31P dipolar-dipolar interaction even in the presence of fast MAS. For these binary phosphate glasses the connectivity and medium range structure for different Q n species is described well by a statistical distribution, and is consistent with a model of random depolymerization.

Application of 31P and 27Al MAS NMR for phosphate speciation studies in soil and aluminium hydroxides: promises and constraints

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) was used to investigate the chemical environment of P in soil and soil components. 31P and 27Al MAS NMR spectra are presented of synthetic aluminium hydroxides (amorphous aluminium hydroxide and gibbsite), reacted with P under different conditions of P concentration, temperature and pH. The reaction product is amorphous octahedral aluminium phosphate, which transforms (partly reversibly) to tetrahedral aluminium phosphate upon drying. Results of several experiments on excessively fertilized sandy soil material are discussed. The soil particle fraction smaller than 50 μm was used for NMR analysis. 31P and 27Al MAS NMR spectra show a CaP pool and an AlP pool. A six-fold water extraction removes part of both P pools. Oxalate extraction removes all CaP and AlP from the sample. Removal of the labile P pool by the HFO-DMT long-term P desorption technique, does not drastically change the 31P MAS NMR spectrum. The formerly mentioned CaP and AlP are thus stable P pools. The 1H 31P cross-polarization (CP) spectrum of the original soil sample revealed a third chemical environment, which was identified as labile CaP: this P pool does not appear in the 1H 31P CP spectrum of the soil sample from which the fast P pool had been removed. The combination of MAS and CP MAS NMR can thus reveal at least three different P species in soil, of which two pools were identified as stable, and one as labile.

The short-range structure of zinc polyphosphate glass

31P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and Raman spectroscopy have been used to examine the polyhedral arrangements in xZnO · (1 − x)P 2O 5 (0.50 ≤ x ≤ 0.71) glasses. The depolymerization of P metaphosphate chains by the addition of ZnO is quantitatively described by the increase in the concentration of Q 1-phosphate sites, determined from the 31P MAS-NMR spectra. When x > 0.60, the NMR and Raman spectra exhibit peaks due to Q 0 and Q 2 tetrahedra, indicating that structures disproportionate in glass melts near the pyrophosphate composition. The splitting of the Raman peak due to the Q 1 terminal oxygen stretching mode indicates that a variety of P-O-Zn bonds participate in the polyphosphate glass structure. The complex mixture of P and Zn polyhedra contributes to the glass-forming tendency of the high ZnO (> 60 mol%) compositions.

Multifield magic-angle spinning and double-rotation nuclear magnetic resonance studies of a hydrated aluminophosphate molecular sieve: AlPO 4-H2

AlPO 4-H2 is a microporous hydrated aluminophosphate, structurally related to VPI-5, whose framework has highly elliptical 10-ring channels (2.9 × 7.6 Å) parallel to the c crystallographic axis. To resolve a previously reported discrepancy between nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) data a highly crystalline AlPO 4-H2 has been further characterized with 27Al and 31P magic-angle spinning (MAS) NMR at 11.7 T and 27Al double-rotation (DOR) NMR at 4.7 T. These present NMR data definitively show that the true space group symmetry of the AlPO 4-H2 framework structure is triclinic rather than the higher orthorhombic symmetry proposed earlier from XRD studies.

Chemical bond distribution in chalcogenide glasses. A 31P MAS and spin-echo NMR study of glasses in the phosphorus-germanium-selenium system

Glasses in the phosphorus-germanium-selenium system have been prepared at compositions within the entire glass-forming region and their properties have been measured by differential scanning calorimetry and 31P nuclear magnetic resonance (NMR) spectroscopy, using spin-echo and magic-angle spinning (MAS) techniques. The glass transition temperature, T g, is a monotonic function of the average coordination number, 〈 r〉, with a distinct change in slope, d T g/d〈 r〉, near 〈 r〉 = 2.4, the theoretically expected percolation threshold. The NMR results are consistent with a structure in which phosphorus and germanium atoms compete for bonding to the selenium atoms but are otherwise randomly distributed. After taking into account the bonding requirements of the germanium atoms, which form tetrahedral GeSe 4 2 units, the presence of germanium in these glasses is found to have a secondary effect on the phosphorus environment. Based on the compositional dependence of the 31P- 31P dipolar second moments, it is suggested that the formation of P-bonded phosphorus atoms is more favored than in binary PSe glasses. 31P MAS-NMR data show that the presence of germanium tends to reduce the concentration of four-coordinated Se= PSe 3 2 units. This effect is attributed to the decrease in available selenium and to the fact that the coordinatively saturated GeSe 4 2 units are unable to provide intermolecular stabilization for the four-coordinated species.

Magic-angle spinning nuclear magnetic resonance spectra of second-order two-spin systems in the solid state

Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of second-order two-spin (AB) systems are investigated. Using average Hamiltonian theory (AHT), general expressions for the positions and relative intensities of the four allowed transitions are derived. In principle, correction terms to any order of the average Hamiltonian may be applied; however, terms up to and including third order appear to be adequate in reproducing the most important experimental features. In general, both the positions and relative intensities of the peaks are sensitive to the sample spinning frequency. Only at the high MAS frequency extreme do the MAS NMR spectra of two-spin (AB) systems in solids correspond to those predicted using formulas derived for solution samples. Under slow MAS conditions, MAS NMR spectra of AB spin systems deviate considerably from the corresponding AB spectra in solution NMR studies. Three general types of MAS NMR spectra are identified and their characteristic features are discussed. The theoretical expressions derived here are applied to reproduce the observed 31P MAS NMR spectra of a phosphole tetramer and cis-1,2-bis(diphenylphosphino)ethylene. It is shown that correction terms higher than first order must be considered in order to reproduce the anomalous spinning-frequency dependencies in MAS NMR spectra. The importance of carrying out measurements at two different applied fields is also demonstrated in the case of the phosphole tetramer.

J-recoupling patterns arising from two chemically equivalent nuclear spins in magic-angle spinning spectra of solids

A general description of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra arising from a pair of chemically equivalent nuclear spins is presented in terms of average Hamiltonian theory (AHT). In general, the MAS NMR spectra of such a spin-pair exhibit a spinning frequency dependent four-line pattern from which it is possible to extract the indirect spin–spin coupling constant, J, involving the ‘‘equivalent’’ spin pair. Explicit expressions for the spinning frequency dependence of the four-line pattern have been derived using AHT. In principle, correction terms to any order can be included; however, consideration of correction terms up to and including third order appear to be sufficient to interpret the most important features characteristic of J-recoupled spectra involving chemically equivalent spin pairs. The average Hamiltonian theory predicts three different general types of recoupling patterns. The type of recoupling pattern observed for a particular chemically equivalent spin pair is predicted to depend on the relative magnitudes of the indirect homonuclear coupling constant, J, the direct homonuclear dipolar coupling constant, R, the magnitude of the instantaneous chemical shift difference between the ‘‘equivalent’’ spins in frequency units, and the MAS spinning frequency. All reported examples of spinning frequency dependent MAS NMR spectra arising from a pair of chemically equivalent spins can be understood using the theoretical expressions derived here. As an example, we interpret the unusual J-recoupling pattern observed in 31P MAS NMR spectra of Hg(PPh3)2(NO3)2. The recoupling pattern is unusual in that 2J(P,P) is given by the separation of alternate lines in the four-line pattern. Similar unusual J-recoupling patterns were first reported by Eichele, Wu, and Wasylishen.

Solid-State Phosphorus-31 Nuclear Magnetic Resonance Differentiation of Bone Mineral and Synthetic Apatite Used to Fill Bone Defects

Bioabsorption of synthetic apatite compounds used to promote bone healing and remodeling has been difficult to evaluate. In this study, solid-state phosphorus-31 nuclear magnetic resonance (NMR) has been used to characterize and quantitate bone mineral and a synthetic apatite in order to establish a model for bioabsorption studies. Pulverized solid samples of cortical rabbit bone and a synthetic fluoridated apatite were examined in vitro at variable degrees of hydration. A 9.4 T superconducting spectrometer was used to obtain 31P magic angle spinning NMR spectra and T1 relaxation times. Quantitation was attempted in mixed samples using T1 recovery data. Bone mineral and synthetic apatite could be distinguished by chemical shift and T1 relaxation time in variable hydration states, and were readily differentiated in mixtures by their T1 relaxation time. NMR estimates of relative proportions of components in mixed samples were accurate within 2% of evaluations based on weight. Solid-state 31P NMR therefore provides a suitable method for monitoring the bioabsorption of synthetic apatites.

Modification of zeolite Y using phosphorus pentachloride

NaY and ultra-stable HY have been allowed to react with PCl5 in a closed cell under varying reaction conditions. NaY was extremely reactive, and its crystallinity was easily destroyed unless the quantity of PCl5 in contact with the zeolite was significantly curtailed. Infrared spectra indicated the removal of framework Al after the reaction. At higher reaction temperatures realumination appeared to occur. In contrast, the framework of USY showed little reactivity towards PCl5. 31P MASNMR (magic-angle spinning nuclear magnetic resonance) spectra gave peaks near –30 and –18 ppm. It is proposed that the former arises from phosphorus substituted isomorphously into the zeolite framework, and the latter from phosphorus grafted onto the surface of zeolite Y.

Paramagnetic shift probes in high-resolution solid-state NMR

Small additive shifts due to changes in the local environment of the resonating nucleus have been observed in magic-angle spinning nuclear magnetic resonance (MASNMR) studies of several classes of solid. The 29.Si resonances in zeolites, for example, shift by approximately 5 p.p.m. per aluminium next-nearest-neighbour1,2, and similar effects have been observed in 31P MASNMR spectra of Zn3−xMgx (PO4)2 solid solutions3. The use of paramagnetic ions to induce large chemical shifts is well established in solution NMR4 and has also been observed for molecular structures in the solid state5. Here we describe a 119Sn study of the pyrochlore solid solution, Y2−xSmx,Sn2O7, and show for the first time the use of paramagnetic shifts in MASNMR to probe the structures of continuous solids.