•1H shift parameters are measured for H-bonded sites in L-ascorbic acid.•Via DFT calculations the NMR data can be compared with the X-ray structure.•This allows inaccurate H positions in the X-ray structure to be refined.•The resulting NMR structure is closer to the result obtained with neutrons.A 1H anisotropic-isotropic chemical shift correlation experiment which employs symmetry-based recoupling sequences to reintroduce the chemical shift anisotropy in ν1 and ultrafast MAS to resolve 1H sites in ν2 is described. This experiment is used to measure 1H shift parameters for L-ascorbic acid, a compound with a relatively complex hydrogen-bonding network in the solid. The 1H CSAs of hydrogen-bonded sites with resolved isotropic shifts can be extracted directly from the recoupled lineshapes. In combination with DFT calculations, hydrogen positions in crystal structures obtained from X-ray and neutron diffraction are refined by comparison with simulations of the full two-dimensional NMR spectrum. The improved resolution afforded by the second dimension allows even unresolved hydrogen-bonded sites 1H to be assigned and their shift parameters to be obtained.Download high-res image (203KB)Download full-size image

A combined NMR and neutron diffraction study has been carried out on three Li3−x−yCuxN materials with x = 0.17, x = 0.29 and x = 0.36. Neutron diffraction indicates that the samples retain the P6/mmm space group of the parent Li3N with Cu located only on Li(1) sites. The lattice parameters vary smoothly with x in a similar fashion to Li3−x−yNixN, but the Li(2) vacancy concentration for the Cu-substituted materials is negligible. This structural model is confirmed by wideline 7Li NMR spectra at 193 K which show three different local environments for the Li(1) site, resulting from the substitution of neighbouring Li atoms in the Li(1) layer by Cu. Since the Cu-substituted materials are only very weakly paramagnetic, variable temperature 7Li wideline NMR spectra can be used to measure diffusion coefficients and activation energies. These indicate anisotropic Li+ diffusion similar to the parent Li3N with transport confined to the [Li2N] plane at low temperature and exchange between Li(1) and Li(2) sites dominant at high temperature. For the intra-layer process the diffusion coefficients at room temperature are comparable to Li3N and Li3−x−y NixN, while Ea decreases as x increases in contrast to the opposite trend in Ni-substituted materials. For the inter-layer process Ea decreases only slightly as x increases, but the diffusion coefficients at room temperature increase rapidly with x.

Solid-state nuclear magnetic resonance (NMR) has been used to characterize the interface between the organic and inorganic components of “core−shell” colloidal nanocomposite particles synthesized by in situ aqueous (co)polymerization of styrene and/or n-butyl acrylate in the presence of a glycerol-functionalized silica sol. Polymer protons are in close proximity (<5 Å) to surface silanol sites in all the nanocomposites studied, indicating that either styrene or n-butyl side groups extend between the glycerol-functional silane molecules toward the surface of the silica particles. For the poly(styrene-co-n-butyl acrylate)/silica nanocomposite n-butyl acrylate residues are located closer to the surface of the silica particle than styrene residues, suggesting a specific interaction between the former and the glycerol-functionalized silica surface. The most likely explanation is a hydrogen bond between the ester carbonyl and the glycerol groups, although this cannot be observed directly. For the Bindzil CC40 glycerol-functionalized silica sol the relative intensities of 29Si NMR lines corresponding to T and Q3 environments imply that there are approximately twice as many unreacted silanol groups on the silica surface as attached silane molecules.

A new two-dimensional NMR experiment is described which is suitable for obtaining magic angle spinning (MAS) scalar correlation spectra in solids. The new experiment has several advantages, including increased cross peak intensities, coupled with good suppression of the diagonal. Its utility is demonstrated via assignments of the carbon-13 MAS spectra of progesterone at natural abundance and of the polymer phase of 50%-U-13C-CsC60.

Solid-state NMR experiments can be used to determine conformational parameters, such as interatomic distances and torsion angles. The latter can be obtained from measurements of the relative orientation of two chemical shift tensors, if the orientation of these with respect to the surrounding bonds is known. In this paper, a new rotor-synchronized magic angle spinning (MAS) dipolar correlation experiment is described which can be used in this way. Because the experiment requires slow MAS rates, a novel recoupling sequence, designed using symmetry principles, is incorporated into the mixing period. This recoupling sequence is based in turn on a new composite cyclic pulse referred to as COAST (for combined offset and anisotropy stabilization). The new COAST-C721 sequence is shown to give good theoretical and experimental recoupling efficiency, even when the CSA far exceeds the MAS rate. In this regime, previous recoupling sequences, such as POST-C721, exhibit poor recoupling performance. The effectiveness of the new method has been explored by a study of the dipeptideL-phenylalanyl-L-phenylalanine.

Lithium nitride has a unique layered structure and the highest reported Li+ ion conductivity for a crystalline material. The conductivity is highly anisotropic, with an intralayer contribution within the graphitic [Li2N] planes dominant at ambient temperature. In this paper transverse- and zero-field muon spin relaxation (μSR) studies on Li3N and two novel paramagnetic derivatives Li3−x−yNixN with x = 0.36 and 0.57 are reported. These new materials have potential as anodes in rechargeable lithium batteries. The decrease in the muon depolarization rate observed above 180 K for the three materials is shown to arise from motional narrowing due to intralayer Li+ diffusion. The increase in the measured activation energy with x for Li3−x−yNixN suggests that the reduction in the layer spacing that results at high substitution levels is responsible for raising the energy barrier to Li+ jumps, despite the concomitant expansion of the [Li2N] plane. In addition, the onset of interlayer diffusion appears at lower temperatures in Ni-substituted derivatives than in the parent Li3N. The muons themselves are quasi-static, most probably located in a 4h site between the [Li2N] plane and the Li(1)/Ni layer. This is similar to the Li+ interstitial position identified by molecular dynamics simulations as an intermediate for an exchange mechanism for interlayer diffusion. Finally, μSR gives no evidence for the formation of the muonium equivalent of the hydrogen defects thought to play an important role in intralayer diffusion in Li3N. These results demonstrate that μSR can be used to obtain diffusion coefficients and activation energies for Li+ transport even in paramagnetic materials where NMR studies are complicated by strong interactions with the electronic moments.

A new two-dimensional NMR experiment is described which is suitable for obtaining magic angle spinning (MAS) scalar correlation spectra in solids. The new experiment has several advantages, including increased cross peak intensities, coupled with good suppression of the diagonal. Its utility is demonstrated via assignments of the carbon-13 MAS spectra of progesterone at natural abundance and of the polymer phase of 50%-U-13C-CsC60.

A combined NMR and neutron diffraction study has been carried out on three Li3−x−yCuxN materials with x = 0.17, x = 0.29 and x = 0.36. Neutron diffraction indicates that the samples retain the P6/mmm space group of the parent Li3N with Cu located only on Li(1) sites. The lattice parameters vary smoothly with x in a similar fashion to Li3−x−yNixN, but the Li(2) vacancy concentration for the Cu-substituted materials is negligible. This structural model is confirmed by wideline 7Li NMR spectra at 193 K which show three different local environments for the Li(1) site, resulting from the substitution of neighbouring Li atoms in the Li(1) layer by Cu. Since the Cu-substituted materials are only very weakly paramagnetic, variable temperature 7Li wideline NMR spectra can be used to measure diffusion coefficients and activation energies. These indicate anisotropic Li+ diffusion similar to the parent Li3N with transport confined to the [Li2N] plane at low temperature and exchange between Li(1) and Li(2) sites dominant at high temperature. For the intra-layer process the diffusion coefficients at room temperature are comparable to Li3N and Li3−x−y NixN, while Ea decreases as x increases in contrast to the opposite trend in Ni-substituted materials. For the inter-layer process Ea decreases only slightly as x increases, but the diffusion coefficients at room temperature increase rapidly with x.

Solid-state NMR experiments can be used to determine conformational parameters, such as interatomic distances and torsion angles. The latter can be obtained from measurements of the relative orientation of two chemical shift tensors, if the orientation of these with respect to the surrounding bonds is known. In this paper, a new rotor-synchronized magic angle spinning (MAS) dipolar correlation experiment is described which can be used in this way. Because the experiment requires slow MAS rates, a novel recoupling sequence, designed using symmetry principles, is incorporated into the mixing period. This recoupling sequence is based in turn on a new composite cyclic pulse referred to as COAST (for combined offset and anisotropy stabilization). The new COAST-C721 sequence is shown to give good theoretical and experimental recoupling efficiency, even when the CSA far exceeds the MAS rate. In this regime, previous recoupling sequences, such as POST-C721, exhibit poor recoupling performance. The effectiveness of the new method has been explored by a study of the dipeptideL-phenylalanyl-L-phenylalanine.