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A survey of the literature reporting solid-state 13C NMR spectra of native celluloses reveals inconsistencies in the reported 13C chemical shifts for cellulose Iα and Iβ allomorphs. With reported chemical shifts varying by up to 2 ppm, it is not clear what the correct chemical shifts actually are. Since reliable experimental data are important to future work, such as quantum chemical calculations of NMR parameters or identification of cellulose phases in complex cellulosic materials, it is important that the correct experimental chemical shifts be established with confidence. Through a process of digitization of previously reported spectra and careful consideration of how chemical shifts were referenced in the past, it has been possible to correct previously reported spectra of cellulose Iα and Iβ, putting them on the same chemical shift scale and establishing a definitive set of correctly referenced 13C chemical shifts for cellulose Iα and Iβ allomorphs. In addition, 1D and 2D 13C NMR experiments were carried out on a cellulose Iα-rich bacterial cellulose sample (with 25% 13C enrichment), providing additional evidence for these 13C chemical shifts and a new peak assignment of the 13C signals to the glucose units in cellulose Iα. This work resolves many of inconsistencies in the cellulose solid-state NMR literature and provides a definitive set of 13C chemical shifts that will be important for future work. [ABSTRACT FROM AUTHOR]

The [sup.13]C chemical shift tensors of two crystalline forms of glucose (α-glucose and α-glucose- [H.sub.2]O) were determined from one-dimensional (1D) and two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy experiments. The experimental values determined from 1D and 2D methods are in very good agreement. Quantum chemical calculations were also carried out using the gauge-including projector augmented wave (GIPAW) method for plane-wave density functional theory (DFT) as implemented in the CAmbridge Serial Total Energy Package (CASTEP). The calculated [sup.13]C chemical shifts were found to be in excellent agreement with experimental values for crystal structures that had their hydrogen atoms optimized and after an appropriate calibration was applied to convert calculated chemical shieldings into chemical shifts. The work presented here lays an important foundation for future solid-state NMR and quantum chemical calculation investigations of the various crystalline forms of cellulose. Key words: NMR crystallography, carbohydrates, cellulose, quantum chemical calculations, crystal structure. On a determine les tenseurs des deplacements chimiques du [sup.13]C de deux formes cristallines du glucose (α- glucose et α-glucose-[H.sub.2]O) a partir des valeurs obtenues par resonance magnetique nucleaire (RMN) monodimensionnelle (1D) et bidimensionnelle (2D), a l'etat solide. Les valeurs experimentales obtenues par les methodes 1D et 2D sont en tres bon accord. On a aussi effectue des calculs de chimie quantique en faisant appel a une onde amelioree par un operateur de projection comportant une jauge de la theorie de la fonctionnelle de la densite (<< GIPAW-DFT >>) avec une onde plane telle, qu'appliquee dans le << CAmbridge Serial Total Energy Package (CASTEP) >>. On a trouve que les valeurs calculees pour les deplacements chimiques du [sup.13]C sont en excellent accord avec les valeurs experimentales pour des structures cristallines pour lesquelles les atomes d'hydrogene avaient ete optimises et apres qu'une calibration appropriee a ete appliquee pour convertir les deplacements chimiques calcules en deplacements chimiques. Le present travail pose une fondation importante pour des etudes futures de RMN a l'etat solide et pour des calculs de chimie quantique de diverses formes cristallines de la cellulose. Mots-cles: cristallographie par RMN, hydrates de carbone, cellulose, calculs de chimie quantique, structure cristalline. [Traduit par la Redaction], Introduction The detailed characterization of the structure of cellulose, the most abundant biopolymer on earth, has presented numerous challenges due mainly to its fibrous nature and multiplicity of crystalline forms. [...]

The location of naphthalene in the zeolite ZSM-5 has been determined from solid-state ¹H/[sup.29]Si cross-polarization (CP) magic-angle-spinning (MAS) NMR data alone. With the peaks in the [sup.29]Si spectrum assigned to the inequivalent Si sites in the zeolite from a two-dimensional INADEQUATE spectrum, the rates of cross polarization between the ¹H nuclei of the guest sorbate molecules and the [sup.29]Si nuclei of the zeolite framework were used to determine the location of the naphthalene molecules by exploiting the proportional relationship between cross-polarization rate constants and ¹H/[sup.29]Si dipolar coupling second moments. The NMR structure determination was carried out on three different selectively deuterium-labeled naphthalene molecules (naphthalene-[d.sub.0], α-naphthalene-[d.sub.4], and β-naphthalene-[d.sub.4]). The average of the molecule locations in agreement with all three sets of NMR data was found to be in excellent agreement with an existing single crystal XRD structure of the naphthalene-ZSM-5 complex. Key words: solid-state NMR, cross polarization, zeolites, host-guest complexes, structure determination. On a determine la location du naphtalene dans le zeolite ZSM-5 en ne faisant appel qu'a des donnees de RMN a l'etat solide, a l'angle magique de rotation et une polarisation croisee ¹H/[sup.29]Si. Apres avoir attribuer les pics du spectre de [sup.29]Si aux sites de silicium inequivalents de la zeolite en se basant sur un spectre bidimensionnel << INADEQUATE >>, on a utilise les vitesses de polarisation croisee entre le noyau ¹H des molecules du sorbat invite et les noyaux de [sup.29]Si du squelette de zeolite pour determiner la location des molecules de naphtalene en exploitant la relation proportionnelle entre les constantes de vitesse de polarisation croisee et les deuxiemes moments du couplage dipolaire ¹H/[sup.29]Si. On a determiner la structure RMN sur trois molecules de naphtalenes marquees selectivement de trois facons differentes (naphtalene-[d.sub.0], α-naphtalene-[d.sub.4], β-naphtalene-[d.sub.4]). On a trouve que les locations moyennes de la molecule dans les trois ensembles de donnees RMN sont en excellent accord avec une structure XRD sur un cristal unique qui a ete determinee anterieurement pour le complexe naphtalene-ZSM-5. Mots cles : RMN a l'etat solide, polarisation croisee, zeolites, complexes hote-invite, determination d'une structure. [Traduit par la Redaction], Introduction A full understanding of the interactions between guest species and host zeolite frameworks, central to the synthesis and applications of zeolites, requires detailed structures for these complexes. For most [...]

A method for the structure determination of zeolite crystal structures that combines powder X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) is reported. The NMR data is combined with the unit cell parameters and space group to solve structural models that refine successfully against the powder XRD data.

The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlP[O.sub.4](F, OH). These compounds are ideal for studying F [left and right arrow] OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state [sup.6]Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F [left and right arrow] OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacement ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the [U.sub.3] eigenvalue, which lies between the c and c* directions. [U.sub.eq] values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder. At the montebrasite end of the series, there are five short Li-[phi] ([phi] = O, OH, F) distances and one very long Li-O4 bond. With increasing F content, the Li-O4 distance decreases and the Li-O3f distance increases, such that at the amblygonite end, the coordination is 4 + 2. The disorder in the Li site is obviously caused by the substitution of F for OH. The driving force is the loss of the hydrogen bond to 04, which causes the Li-O4 bond to strengthen and improves the bond valence to 04. The results show that the H atom position is imbedded within the distorted octahedral oxygen coordination of the Li atom. To represent the disorder better, we used a split Li site model. The results show that Li1 occupancy increases and the Li2 occupancy decreases with increasing F content, and that the Li1-Li2 distance is longer for intermediate compositions than for the end-members. The [sup.6]Li MAS NMR experiments provide important structural information complementary to the neutron diffraction results. The spectra of samples in the amblygonite-montebrasite series show two well-resolved peaks, confirming the presence of Li disorder over two distinct sites, and highly resolved [sup.6]Li MAS NMR spectra are obtained at the very high magnetic field strength of 18.8 T. The peaks at -0.3 ppm and -0.9 ppm were unambiguously assigned to the Li2(OH) and Li1(F) sites found in the neutron diffraction structures. The isotropic chemical shifts are consistent with the coordinations of these Li sites found in the neutron diffraction structures. The relative intensities of the two peaks across the series of samples reflect the varying F/(F + OH). In addition to confirming the assignments of the peaks, it is possible to measure H-Li2 and F-Li1 internuclear distances by [sup.6]Li{[sup.1]H} and [sup.6]Li {[sup.19]F} CP and REDOR NMR that are consistent with the corresponding distances from the neutron diffraction structures. The [sup.6]Li{[sup.1]H} and [sup.6]Li{[sup.19]F} CP and REDOR results indicate that the Li disorder is random throughout the crystals rather than over large domains, a conclusion that cannot be made from diffraction experiments. Variable temperature [sup.6]Li MAS NMR spectra confirm that the disorder is static and there is no dynamic exchange involving F, OH, or Li. Each Li ion has access to only one of the two observed sites as determined by the presence of either OH or F in its immediate environment and there is no possibility of a dynamic exchange.