Structure and NMR assignment in AlPO4-15: A combined study by diffraction,MAS NMR and first-principles calculations

A multi-technique investigation involving X-ray diffraction, solid-state NMR and first-principle calculations was carried out on the aluminophosphate material AlPO₄-15. A synchrotron X-ray single-crystal diffraction study was carried out on the same sample as that used in solid-state NMR studies. The model from the single crystal study, together with a model from a literature high resolution study of the same material, was used as starting points for the first-principles calculations of the NMR parameters. This enabled the ³¹P and ²⁷Al NMR spectra to be unambiguously assigned and all the NMR parameters calculated agreed well with the experimental spectra even without relaxing the X-ray derived structural models.     

Structural characterization of phosphorus-based networks and clusters: 31P MAS NMR spectroscopy and magnetic shielding calculations on Hittorf's phosphorus

The (31)P MAS NMR spectrum of Hittorf's phosphorus has been measured and assigned to the 21 crystallographically distinct phosphorus atoms based on two-dimensional dipolar correlation spectroscopies. Application of such 2D techniques to phosphorus-based networks is particularly challenging owing to the wide chemical shift dispersions, rapid irreversible decay of transverse magnetization, and extremely slow spin-lattice relaxation in these systems. Nevertheless, a complete assignment was possible by using the combination of correlated spectroscopy (COSY) and radiofrequency-driven dipolar recoupling (RFDR). The assignment is supported further by DFT-based ab initio chemical shift calculations using a cluster-model approach, which gives good agreement between experimental and calculated chemical shift values. The (31)P chemical shifts appear to be strongly correlated with the average P-P bond lengths within the P(P(1/3))(3) coordination environments, whereas no clear dependence on average P-P-P bond angles can be detected. Calculations of localized Kohn-Sham orbitals reveal that this bond-length dependence is reflected in energy variations in the corresponding localized p-p-σ orbitals influencing the paramagnetic deshielding contribution in Ramsey's equation. In contrast, the contributions of the lone pairs to shielding differences are small and/or do not vary in a systematic manner for the different crystallographically distinct phosphorus sites. The combined spectroscopic and quantum chemical approach applied here and the increased theoretical understanding of (31)P chemical shifts will facilitate the structural elucidation of other phosphorus-based clusters and networks.     

(119)Sn MAS NMR and first-principles calculations for the investigation of disorder in stannate pyrochlores

The local structure and cation disorder in Y(2)Ti(2-x)Sn(x)O(7) pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is studied using (119)Sn (I = 1/2) NMR spectroscopy. NMR provides an excellent probe of disorder, as it is sensitive to the atomic scale environment without the need for any long-range periodicity. However, the complex and overlapping spectral resonances that often result can be difficult to interpret. Here, we demonstrate how (119)Sn DFT calculations can be used to aid the spectral interpretation and assignment, confirming that Sn occupies only the six-coordinate pyrochlore B site, and that the Sn chemical shift is sensitive to the number of Sn/Ti on the neighbouring B sites. Although distinct resonances are resolved experimentally when the Ti content is low, there is significant spectral overlap for Ti-rich compositions. We establish that this is a result of two competing contributions to the Sn chemical shift; an upfield shift resulting from the incorporation of the more polarizing Ti(4+) cation onto the neighbouring B sites, and a concomitant downfield shift arising from the decrease in unit cell size. Despite the considerably easier spectral acquisition, the lower resolution in the (119)Sn spectra hinders the extraction of the detailed structural information previously obtained using (89)Y NMR. However, the spectra we obtain are consistent with a random distribution of Sn/Ti on the pyrochlore B sites. Finally, we consider whether an equilibrium structure has been achieved by investigating materials that have been annealed for different durations.     

Dynamics on the microsecond timescale in microporous aluminophosphate AlPO-14 as evidenced by 27Al MQMAS and STMAS NMR spectroscopy

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.     

Anhydrous proton-conducting properties of triazole-phosphonic acid copolymers: a combined study with MAS NMR

The synthesis, thermal and proton conducting properties of copolymers based on vinylphosphonic acid (VPA) and 1-vinyl-1,2,4-triazole (VTri) were investigated. The copolymers were synthesized by free-radical copolymerization of the corresponding monomers at several monomer feed ratios to obtain poly(VPA-co-VTri) copolymer electrolytes. The final structures of the copolymers were confirmed by spectroscopic methods. The composition of the low molecular weight copolymers was varied with the feed ratio of the monomers. The presence of triazole units in the copolymers suppresses the formation of phosphonic acid anhydrides up to 150 degrees C, as verified by both (31)P NMR and TGA. The observation of defined glass transition temperatures indicated that the ionic interactions do not prevent segmental relaxations of the polymer chains. In the absence of humidity, the copolymer electrolyte, poly(VPA-co-VTri), S2 (with 33% triazole content) showed proton conductivity of 10(-3) S cm(-1) at 120 degrees C, which is far higher than in imidazole based copolymers. Two different types of hydrogen-bonded protons were detected by (1)H MAS NMR in the solid copolymer systems, due to different arrangements of triazole and phosphonic acid units.     

An experimental study of decoupling sequences for multiple-quantum and high-resolution MAS experiments in solid-state NMR

Recently, a sequence for heteronuclear dipolar decoupling in solid-state NMR, namely SWf-TPPM, was introduced by us. Under magic-angle spinning (MAS), the decoupling efficiency of the sequence was unaffected over a range of values for various experimental parameters such as the pulse length, pulse phase, and 1H resonance offset. We here demonstrate its use in multiple-quantum (MQ) and high-resolution (HR) MAS experiments. This sequence further improves the MQMAS spectra compared to the earlier reported decoupling sequences with improved immunity to any missets of the pulse length, pulse phase and decoupler offset. In contrast, for HRMAS, the simple CW scheme is as efficient as any of the decoupling schemes that were studied.     

Concentration of hydroxyl groups in dental apatites: a solid-state 1H MAS NMR study using inverse 31P -->1H cross-polarization

The concentration of structural hydroxyl groups in the apatite mineral of enamel, dentin and cementum of healthy human teeth was estimated by reference to stoichiometric hydroxyapatite to be 73 +/- 3, 18 +/- 2 and 18 +/- 1%, respectively.     

Structure assignment in the solid state by the coupling of quantum chemical calculations with NMR experiments: a columnar hexabenzocoronene derivative.

We present a quantum chemical ab initio study which demonstrates a new combined experimental and theoretical approach, whereby a comparison of calculated and experimental (1)H NMR chemical shifts allows the elucidation of structural arrangements in solid-state molecular ensembles, taking advantage of the marked sensitivity of the (1)H chemical shift to intermolecular interactions. Recently, Brown et al. have shown that, under fast magic-angle spinning (MAS) at 35 kHz, the resolution in a (1)H NMR spectrum of the solid phase of an alkyl-substituted hexabenzocoronene (HBC) derivative is sufficient to observe the hitherto unexpected resolution of three distinct aromatic resonances ( J. Am. Chem. Soc. 1999, 121, 6712). Exploiting the additional information about proton proximities provided by (1)H double-quantum (DQ) MAS NMR spectroscopy, it was shown that the results are qualitatively consistent with the aromatic cores packing in a manner similar to that in unsubstituted HBC. Using the HBC-C(12) molecule as an example, we show here that the new combined experimental and theoretical approach allows the observed (1)H chemical shifts to be related in a quantitative manner to the intermolecular structure. In the quantum chemical calculations, a series of model systems of stacked HBC oligomers are used. On account of the marked dependence of the (1)H chemical shift to ring currents arising from nearby aromatic rings, the calculated (1)H chemical shifts are found to be very sensitive to the stacking arrangement of the HBC molecules. Moreover, the ring current effect is found to be particularly long range, with a considerable influence of the second neighbor, at a distance of 700 pm, being observed.     

A 13C CP/MAS and 31P NMR study of the interactions of dipalmitoylphosphatidylcholine with respirable silica and kaolin

The interaction of silica and kaolin with dipalmitoylphosphatidylcholine (DPPC) has been studied using 13C and 31P solid state nuclear magnetic resonance spectroscopy. These studies explore the molecular interactions of these respirable dusts with a model lung surfactant species to characterize silica toxicity in mixed systems. The choline head group of DPPC was found to remain mobile when adsorbed on kaolin, in contrast to an immobile head group on silica. Further, glycerol carbon intensities were greatly diminished relative to that of choline carbons, a result attributed to broadening effects. These preliminary findings suggest that silica toxicity may not be related to choline mobility as previously noted.     

A spectroscopic study of calcium surface sites and adsorbed iron species at aqueous fluorapatite by means of 1H and 31P MAS NMR

Assignments of the protolytic speciation at the calcium hydroxyl surface sites of synthetic fluorapatite and the chemical interactions between fluorapatite-maghemite and fluorapatite-Fe2+ ions have been studied by means of 1H and 31P single-pulse and 31P CP MAS NMR. Three possible forms of calcium hydroxyl surface sites have been suggested and assigned to [triple bond] CaOH, [triple bond]Ca(OH)2-, and [triple bond]CaOH2+, and their mutual ratios were found to vary as a function of pH. Due to their paramagnetic properties, iron species and Fe2+ ions adsorbed at the fluorapatite surface display a broad spinning sideband manifold in the single-pulse 31P MAS NMR spectra. The resonance lines in the 31P CP MAS NMR spectra originating from the bulk phosphate groups PO4(3-) and phosphorus surface sites [triple bond]POx and [triple bond]POxH decrease with increasing Fe2+ ion adsorption. When iron species originating from maghemite are adsorbed at the fluorapatite surface, no 31P NMR signal is detected, which supports the hypothesis that surface reactions occur between the phosphorus surface sites of fluorapatite and iron species.     

Temperature-dependent interconversion of phosphoramidite-Cu complexes detected by combined diffusion studies, 31P NMR, and low-temperature NMR spectroscopy

For copper-catalyzed enantioselective conjugate additions, knowledge about the precatalytic and catalytic complexes has not yet been sufficiently developed to understand the strong influence of different temperatures on these famous reactions. Therefore, NMR experiments with four Cu(I) salts and two phosphoramidite ligands have been performed to elucidate the temperature dependence and the low-temperature structures of these copper complexes. The existence of the precatalytic binuclear complex with a mixed trigonal/tetrahedral coordination on copper is for the first time proven with direct NMR spectroscopic methods. Below 200 K, intermolecular interactions between free ligands and [Cu2X2L3] complexes induce binuclear [Cu2X2L4] complexes similar to the crystal structures. By combining diffusion experiments and (31)P integrals at different temperatures, it is for the first time possible to follow the formation of stoichiometrically different complexes, even under experimental conditions in which the (31)P signals of the complexes are spectroscopically not resolved due to exchange processes. This allows a first correlation between the complex species observed and the synthetic conditions reported. Furthermore, different preferences to build homo- or heterochiral complexes are detected for binaphthol and biphenol phosphoramidite complexes.     

Straightforward detection of the secondary ionisation of the phosphate group and pK determinations by high-resolution solid-state 31P NMR

The suitability of high-resolution solid-state 31P NMR for a straightforward determination of the protonation state of phosphate groups as well as of their pK2 values extracted from solid state mono : dianionic ratios has been demonstrated.     

Nature and structure of aluminum surface sites grafted on silica from a combination of high-field aluminum-27 solid-state NMR spectroscopy and first-principles calculations

The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found.     

Revisiting H/Pt(111) by a combined experimental study of the H-D exchange reaction and first-principles calculations

We report a detailed investigation of the behavior of chemisorbed hydrogen atoms (Ha) on Pt(111) by a combination of an ex-perimental study of the Ha + Da reaction and first-principles calculations. The coverage-dependent adsorption and desorption behavior of Ha and Da on Pt(111) have been systematically established and can be well interpreted in terms of repulsive inter-actions between adsorbates. Ha adsorbs exclusively on the face-centered cubic (fcc) sites of Pt(111) at coverages not exceeding 1 monolaye...     

Structure and molecular dynamics of solid-state inclusion complexes of cyclodextrin and permethylated cyclodextrin with benzaldehyde studied by high-resolution CP/MAS13C NMR

The structure and molecular dynamics of the benzaldehyde inclusion-complexes with -and-cyclodextrins and permethylated -cyclodextrin in the solid state have been studied by high-resolution cross-polarization/magic angle sample-spinning¹³C NMR spectroscopy. It is shown that the guest benzaldehyde molecule undergoes motion in the host cyclodextrin cavity and the rate of motion depend on the cavity size. In the -cyclodextrin complex, compared to -and permethylated -cyclodextrin complexes, the benzaldehyde motion is severely restricted, but under high-vacuum benzaldehyde is released more easily from the cavity.     

New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by ¹⁷O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and ¹⁷O, ²⁷Al and ²⁹Si MAS NMR spectroscopy.     

Structure activity relationship by NMR and by computer: a comparative study

There has recently been considerable interest in using NMR spectroscopy to identify ligand binding sites of macromolecules. In particular, a modular approach has been put forward by Fesik et al. (Shuker, S. B.; Hajduk, P. J.; Meadows, R. P.; Fesik, S. W. Science 1996, 274, 1531-1534) in which small ligands that bind to a particular target are identified in a first round of screening and subsequently linked together to form ligands of higher affinity. Similar strategies have also been proposed for in silico drug design, where the binding sites of small chemical groups are identified, and complete ligands are subsequently assembled from different groups that have favorable interactions with the macromolecular target. In this paper, we compare experimental and computational results on a selected target (FKBP12). The binding sites of three small ligands ((2S)1-acetylprolinemethylester, 1-formylpiperidine, 1-piperidinecarboxamide) in FKBP12 were identified independently by NMR and by computational methods. The subsequent comparison of the experimental and computational data showed that the computational method identified and ranked favorably ligand positions that satisfy the experimental NOE constraints.