Sylvian Cadars

NMR crystallography of oxybuprocaine hydrochloride, Modification II°

The (13)C CPMAS spectrum is presented for the polymorph of oxybuprocaine hydrochloride which is stable at room temperature, i.e. Mod. II degrees . It shows crystallographic splittings arising from the fact that there are two molecules, with substantially different conformations, in the asymmetric unit. An INADEQUATE two-dimensional experiment was used to link signals for the same independent molecule. The chemical shifts are discussed in relation to the crystal structure. Of the four ethyl groups attached to NH(+) nitrogens, one gives rise to unusually low chemical shifts, very different from those of the other three ethyl groups. This is attributed empirically to gamma-gauche conformational effects, as is confirmed by shielding computations. These considerations allow (13)C signals to be assigned to specific carbons in the two crystallographically inequivalent molecules in the crystal structure. Indeed, information about the conformations is inherent in the NMR spectrum, which thus provides data of crystallographic significance. A (13)C/(1)H HETCOR experiment enabled resolution to be obtained in the (1)H dimension and allowed (1)H and (13)C signals for the same independent molecule to be linked.

Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR

Unlike the long-range order of ideal crystalline structures, local order is an intrinsic characteristic of real materials and often serves as the key to the tuning of their properties and their final applications. Although researchers can easily assess local ordering using two-dimensional imaging techniques with resolution that approaches the atomic level, the diagnosis, description, and qualification of local order in three dimensions is much more challenging. Solid-state nuclear magnetic resonance (NMR) and its panel of continually developing instruments and methods enable the local, atom-selective characterization of structures and assemblies ranging from the atomic to the nanometer length scales. By making use of the indirect J-coupling that distinguishes chemical bonds, researchers can use solid-state NMR to characterize a variety of materials, ranging from crystalline compounds to amorphous or glassy materials. In crystalline compounds showing some disorder, we describe and distinguish the contributions of topology, geometry, and local chemistry in ways that are consistent with X-ray diffraction and computational approaches. We give examples of materials featuring either chemical disorder in a topological order or topological disorder with local chemical order. For glasses, we show that we can separate geometric and chemical contributions to the local order by identifying structural motifs with a viewpoint that extends from the atomic scale up to the nanoscale. As identified by solid state NMR, the local structure of amorphous materials or glasses consists of well-identified structural entities up to at least the nanometer scale. Instead of speaking of disorder, we propose a new description for these structures as a continuous assembly of locally defined structures, an idea that draws on the concept of locally favored structures (LFS) introduced by Tanaka and coworkers. This idea provides a comprehensive picture of amorphous structures based on fluctuations of chemical composition and structure over different length scales. We hope that these local or molecular insights will allow researchers to consider key questions related to nucleation and crystallization, as well as chemically (spinodal decomposition) or density-driven (polyamorphism) phase separation, which could lead to future applications in a variety of materials.

NMR measurements of scalar-coupling distributions in disordered solids

The measurement of scalar (J) couplings by solid-state NMR is a field of great interest, since this interaction is a rich source of local structural information, complementary to dipolar and chemical shift interactions. Here, we first demonstrate that J-coupling distributions exist and can be observed in disordered solids, as illustrated with the observation of a pair-specific distribution of (2)J((31)P-N-(31)P) couplings in a bis-phosphino amine, and we investigate the potential effects of such distributions on the measurement of average J-coupling constants. Second, we show that the measurement of two-dimensional (2D) distributions of J-couplings provides a much richer probe of local structural disorder than one-dimensional distributions, and we introduce new methods that provide different (selective or non-selective) ways of measuring 2D J distributions in a wide range of disordered systems. These methods are finally applied to a slightly disordered polymorphic sample of fully (13)C-enriched cellulose, and then to the bis-phosphino amine sample, from which 2D (2)J(PP)-coupling distributions are clearly identified and interpreted.

Synthesis and structure determination of CaSi1/3B2/3O8/3, a new calcium borosilicate

This article reports on the identification, synthesis, and in-situ structure determination of a new crystalline calcium borosilicate compound of composition CaSi(1/3)B(2/3)O(8/3). Synthesis was carried out by complete crystallization on annealing from a corresponding glassy composition in the widely studied CaO-SiO2-B2O3 ternary system. The crystallographic structure was determined ab initio using electron diffraction information and the charge flipping algorithm performed on synchrotron and neutron powder diffraction data collected in situ at high temperature. CaSi(1/3)B(2/3)O(8/3) is found to crystallize in the Pna2(1) (no. 33) orthorhombic space group, with a = 12.1025(4) Å, b = 5.2676(1) Å, c = 3.7132(1) Å, and V = 236.71(1) Å(3) at 650 °C. Solid-state (29)Si and (11)B NMR experiments have confirmed the existence of finite chains along the c axis, formed by corner-sharing SiO4 tetrahedra and BO3 units. Silicon and boron species share a crystallographic site, and the Si/B distribution induces different possible arrangements of the chains which are discussed in light of DFT calculations. At room temperature, the existence of a superstructure, resulting from the ordering within nanoscale domains, was explored by transmission electron microscopy.

Supercell program: a combinatorial structure-generation approach for the local-level modeling of atomic substitutions and partial occupancies in crystals

BackgroundDisordered compounds are crucially important for fundamental science and industrial applications. Yet most available methods to explore solid-state material properties require ideal periodicity, which, strictly speaking, does not exist in this type of materials. The supercell approximation is a way to imply periodicity to disordered systems while preserving “disordered” properties at the local level. Although this approach is very common, most of the reported research still uses supercells that are constructed “by hand” and ad-hoc.ResultsThis paper describes a software named supercell, which has been designed to facilitate the construction of structural models for the description of vacancy or substitution defects in otherwise periodically-ordered (crystalline) materials. The presented software allows to apply the supercell approximation systematically with an all-in-one implementation of algorithms for structure manipulation, supercell generation, permutations of atoms and vacancies, charge balancing, detecting symmetry-equivalent structures, Coulomb energy calculations and sampling output configurations. The mathematical and physical backgrounds of the program are presented, along with an explanation of the main algorithms and relevant technical details of their implementation. Practical applications of the program to different types of solid-state materials are given to illustrate some of its potential fields of application. Comparisons of the various algorithms implemented within supercell with similar solutions are presented where possible.ConclusionsThe all-in-one approach to process point disordered structures, powerful command line interface, excellent performance, flexibility and GNU GPL license make the supercell program a versatile set of tools for disordered structures manipulations.

Effects of the orientation of the 23Na-29Si dipolar vector on the dipolar mediated heteronuclear solid state NMR correlation spectrum of crystalline sodium silicates.

Dipolar-Heteronuclear Multiple Quantum Correlation (D-HMQC) experiment based on SR4(2)(1) recoupling was shown as a very efficient probe of spatial proximities in ordered or disordered materials. As crystalline sodium silicates have been extensively studied using 1D and 2D MAS NMR experiments and DFT calculations, they have been used as candidate model systems to perform this D-HMQC experiment. In this work, we demonstrate that the combination of (29)Si and (23)Na NMR at high magnetic field and DFT calculations makes it possible to revisit the assignment of the NMR signature of the δ-Na(2)Si(2)O(5) polymorph. A D-HMQC experiment performed on this crystalline sample reveals lineshape distortions on the (23)Na powder patterns extracted from the 2D correlation. Numerical simulations showed that these distortions result from an effect of the relative orientation between the (23)Na quadrupolar tensor and the (23)Na-(29)Si dipolar vector at the origin of the magnetization transfer.

A solid-state NMR study of C70: A model molecule for amorphous carbons

We show that natural abundance, solid-state MAS-NMR (13)C INADEQUATE spectra can be recorded for crystallized C(70), using the through-bond J-coupling for the magnetization transfer. The effect of strong J-coupling can be lessened at high magnetic fields, allowing the observation of cross-peaks between close resonances. DFT calculations of the chemical shifts show an excellent agreement with the experimental values. A correlation is observed between the average CCC bond angles and the (13)C chemical shift, offering a way to understand the dispersion of (13)C chemical shifts in nanoporous activated carbons in terms of local deviations from planarity.

Structure and Dynamics of Nonionic Surfactant Aggregates in Layered Materials

The aggregation of surfactants on solid surfaces as they are adsorbed from solution is the basis of numerous technological applications such as colloidal stabilization, ore flotation, and floor cleaning. The understanding of both the structure and the dynamics of surfactant aggregates applies to the development of alternative ways of preparing hybrid layered materials. For this purpose, we study the adsorption of the triethylene glycol mono n-decyl ether (C10E3) nonionic surfactant onto a synthetic montmorillonite (Mt), an aluminosilicate clay mineral for organoclay preparation with important applications in materials sciences, catalysis, wastewater treatment, or as drug delivery. The aggregation mechanisms follow those observed in an analogous natural Mt, with the condensation of C10E3 in a bilayer arrangement once the surfactant self-assembles in a lamellar phase beyond the critical micelle concentration, underlining the importance of the surfactant state in solution. Solid-state 1H nuclear magnetic resonance (NMR) at fast magic-angle spinning (MAS) and high magnetic field combined with1H-13C correlation experiments and different types of 13C NMR experiments selectively probes mobile or rigid moieties of C10E3 in three different aggregate organizations: (i) a lateral monolayer, (ii) a lateral bilayer, and (iii) a normal bilayer. High-resolution 1H{27Al} CP-1H-1H spin diffusion experiments shed light on the proximities and dynamics of the different fragments and fractions of the intercalated surfactant molecules with respect to the Mt surface. 23Na and 1H NMR measurements combined with complementary NMR data, at both molecular and nanometer scales, precisely pointed out the location of the C10E3 ethylene oxide hydrophilic group in close contact with the Mt surface interacting through ion-dipole or van der Waals interactions.

Structure determination of a partially ordered layered silicate material with an NMR crystallography approach

Structure determination of layered materials can present challenges for conventional diffraction methods due to the fact that such materials often lack full three-dimensional periodicity since adjacent layers may not stack in an orderly and regular fashion. In such cases, NMR crystallography strategies involving a combination of solid-state NMR spectroscopy, powder X-ray diffraction, and computational chemistry methods can often reveal structural details that cannot be acquired from diffraction alone. We present here the structure determination of a surfactant-templated layered silicate material that lacks full three-dimensional crystallinity using such an NMR crystallography approach. Through a combination of powder X-ray diffraction and advanced 29Si solid-state NMR spectroscopy, it is revealed that the structure of the silicate layer of this layered silicate material templated with cetyltrimethylammonium surfactant cations is isostructural with the silicate layer of a previously reported material referred to as ilerite, octosilicate, or RUB-18. High-field 1H NMR spectroscopy reveals differences between the materials in terms of the ordering of silanol groups on the surfaces of the layers, as well as the contents of the inter-layer space.