Thibault Charpentier

The PAW/GIPAW approach for computing NMR parameters: a new dimension added to NMR study of solids.

In 2001, Mauri and Pickard introduced the gauge including projected augmented wave (GIPAW) method that enabled for the first time the calculation of all-electron NMR parameters in solids, i.e. accounting for periodic boundary conditions. The GIPAW method roots in the plane wave pseudopotential formalism of the density functional theory (DFT), and avoids the use of the cluster approximation. This method has undoubtedly revitalized the interest in quantum chemical calculations in the solid-state NMR community. It has quickly evolved and improved so that the calculation of the key components of NMR interactions, namely the shielding and electric field gradient tensors, has now become a routine for most of the common nuclei studied in NMR. Availability of reliable implementations in several software packages (CASTEP, Quantum Espresso, PARATEC) make its usage more and more increasingly popular, maybe indispensable in near future for all material NMR studies. The majority of nuclei of the periodic table have already been investigated by GIPAW, and because of its high accuracy it is quickly becoming an essential tool for interpreting and understanding experimental NMR spectra, providing reliable assignments of the observed resonances to crystallographic sites or enabling a priori prediction of NMR data. The continuous increase of computing power makes ever larger (and thus more realistic) systems amenable to first-principles analysis. In the near future perspectives, as the incorporation of dynamical effects and/or disorder are still at their early developments, these areas will certainly be the prime target.

Origin and consequences of silicate glass passivation by surface layers

Silicate glasses are durable materials, but are they sufficiently durable to confine highly radioactive wastes for hundreds of thousands years? Addressing this question requires a thorough understanding of the mechanisms underpinning aqueous corrosion of these materials. Here we show that in silica-saturated solution, a model glass of nuclear interest corrodes but at a rate that dramatically drops as a passivating layer forms. Water ingress into the glass, leading to the congruent release of mobile elements (B, Na and Ca), is followed by in situ repolymerization of the silicate network. This material is at equilibrium with pore and bulk solutions, and acts as a molecular sieve with a cutoff below 1 nm. The low corrosion rate resulting from the formation of this stable passivating layer enables the objective of durability to be met, while progress in the fundamental understanding of corrosion unlocks the potential for optimizing the design of nuclear glass-geological disposal.

27Al and 29Si solid-state NMR characterization of calcium-aluminosilicate-hydrate.

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.

Influence of glass chemical composition on the Na-O bond distance: a 23Na 3Q-MAS NMR and molecular dynamics study

The sodium environment in oxide glasses was investigated by 23Na multiple-quantum magic-angle spinning (MQ-MAS) NMR spectroscopy and compared with molecular dynamics simulations. In the experimental approach, a spectrum-inversion was employed taking into account the transfer efficiency involved in the MQ-MAS experiment. This allowed the reconstruction of the underlying two-dimensional distribution of the isotropic chemical shift correlated with the quadrupolar interaction. The isotropic chemical shift distributions were extracted from the MQ-MAS spectra to infer Na–O distance distributions. First, a Na2O–2SiO2 glass and its crystal analogue were characterized by this method to observe the disorder effect in the glass through the Na–O distance distribution. Thereafter, in order to study the influence of the chemical composition on the Na–O distance and distribution, additional glasses were investigated with NMR and simulation: Na2O–5SiO2, Na2O–2CaO–3SiO2, Na2O–Al2O3–3SiO2 and Na2O–B2O3–3SiO2. The molecular dynamics results are in good agreement with the experimental findings. The mean Na–O distance is higher when network formers are added to the sodium silicate glass. The effects on the Na–O distance distribution are also discussed. The simulation relates these results to the existence of several types of Na: near the non-bridging oxygen of the silicon, or as aluminum or boron charge compensator. This can be explained through charge and geometric effects.

The structure of fluoride-containing bioactive glasses: new insights from first-principles calculations and solid state NMR spectroscopy

Fluoride-containing bioactive glasses are attracting particular interest in many fields of dentistry and orthopedics because they combine the bone-bonding ability of bioactive glasses with the anticariogenic protection provided by fluoride ions. Since the biomedical applications of these materials critically depend on the release of ionic species in the surrounding physiological environment, a deep knowledge of their environments is required. In this paper, density functional theory calculations and spin effective Hamiltonians have been employed to analyse the NMR signatures of the various environments of 19F, 29Si, 31P and 23Na atoms in fluorinated bioglasses structural models previously generated by Car–Parrinello molecular dynamics simulations. Comparison with experimental spectra expressly recorded in this work shows a good agreement and allows the enlightenment of some longstanding issues about the atomic structure of fluorinated bioglasses, such as the presence of Si–F and Si–O–P bonds. In particular, it is shown that Si–F bonds cannot be resolved by using MAS NMR experiments only, and 29Si{19F} REDOR experiments, that probes directly spatial proximities among atoms, must be employed. Our results show that F is coordinated entirely to the modifier ions Na and Ca, and that no Si–F bonds are present in the real glass structure. Thus, the addition of fluorine to the 45S5 Bioglass® increases the polymerization of the silicate network by removing modifiers from the siliceous matrix and reducing its reactivity. Finally, the computed isotropic chemical shifts of the various environments of phosphorus show that, if present, Si–O–P bonds should be clearly noticeable in the 31P static NMR experimental spectrum. Instead, the latter show that P is present as isolated orthophosphate units and does not enter into the siliceous matrix by forming Si–O–P bonds as conjectured by molecular dynamics simulations.

Numerical and theoretical analysis of multiquantum magic-angle spinning experiments

Using a recent investigation of the Floquet’s theorem for magic-angle spinning nuclear magnetic resonance simulations (NMR), a procedure for computing multiquantum magic-angle spinning spectra is derived. The general formalism which is introduced here can be applied more generally to any solid-state NMR two-dimensional experiments. All interactions and their time dependency are considered during the pulses. Furthermore, for powder patterns, a formal average is possible on γ (the third component of the Euler angle describing the orientation of the crystallite) which leads to great simplifications and to an improved computing efficiency. As an application, the intensity of the spinning sidebands in the two-dimensional multiquantum magic-angle spinning spectrum is investigated. The recently reported appearance of numerous spinning sidebands in the multiquantum dimension is discussed. Such effects appear naturally in the present formalism which provides a theoretical framework for further investigations. Simu...

17O 3Q-MAS NMR characterization of a sodium aluminoborosilicate glass and its alteration gel

Abstract A quantitative approach was applied to the 3Q-MAS spectra of 17 O to characterize a four-oxide glass composition and the alteration film that formed on its surface during aqueous alteration. The intensity of the oxide sites was corrected according to the quadrupolar interaction based on the efficiency of the coherence transfers induced by a 3Q-MAS sequence The restructuring of a boron-depleted aluminosilicate gel was clearly visible during alteration by H 2 17 O .

A 93Nb Solid‐State NMR and Density Functional Theory Study of Four‐ and Six‐Coordinate Niobate Systems

A variable B(0) field static (broadline) NMR study of a large suite of niobate materials has enabled the elucidation of high-precision measurement of (93)Nb NMR interaction parameters such as the isotropic chemical shift (delta(iso)), quadrupole coupling constant and asymmetry parameter (C(Q) and eta(Q)), chemical shift span/anisotropy and skew/asymmetry (Omega/Deltadelta and kappa/eta(delta)) and Euler angles (alpha, beta, gamma) describing the relative orientation of the quadrupolar and chemical shift tensorial frames. These measurements have been augmented with ab initio DFT calculations by using WIEN2k and NMR-CASTEP codes, which corroborate these reported values. Unlike previous assertions made about the inability to detect CSA (chemical shift anisotropy) contributions from Nb(V) in most oxo environments, this study emphasises that a thorough variable B(0) approach coupled with the VOCS (variable offset cumulative spectroscopy) technique for the acquisition of undistorted broad (-1/2<-->+1/2) central transition resonances facilitates the unambiguous observation of both quadrupolar and CSA contributions within these (93)Nb broadline data. These measurements reveal that the (93)Nb electric field gradient tensor is a particularly sensitive measure of the immediate and extended environments of the Nb(V) positions, with C(Q) values in the 0 to >80 MHz range being measured; similarly, the delta(iso) (covering an approximately 250 ppm range) and Omega values (covering a 0 to approximately 800 ppm range) characteristic of these niobate systems are also sensitive to structural disposition. However, their systematic rationalisation in terms of the Nb-O bond angles and distances defining the immediate Nb(V) oxo environment is complicated by longer-range influences that usually involve other heavy elements comprising the structure. It has also been established in this study that the best computational method(s) of analysis for the (93)Nb NMR interaction parameters generated here are the all-electron WIEN2k and the gauge included projector augmented wave (GIPAW) NMR-CASTEP DFT approaches, which account for the short- and long-range symmetries, periodicities and interaction-potential characteristics for all elements (and particularly the heavy elements) in comparison with Gaussian 03 methods, which focus on terminated portions of the total structure.

Contribution of first‐principles calculations to multinuclear NMR analysis of borosilicate glasses

Boron‐11 and silicon‐29 NMR spectra of xSiO2 (1 − x)B2O3 glasses (x = 0.40, 0.80 and 0.83) have been calculated using a combination of molecular dynamics (MD) simulations with density functional theory (DFT) calculations of NMR parameters. Structure models of 200 atoms have been generated using classical force fields and subsequently relaxed at the PBE‐GGAlevel of DFT theory. The gauge including projector augmented wave (GIPAW) method is then employed for computing the shielding and electric field gradient tensors for each silicon and boron atom. Silicon‐29 MAS and boron‐11 MQMAS NMR spectra of two glasses (x = 0.40 and 0.80) have been acquired and theoretical spectra are found to well agree with the experimental data. For boron‐11, the NMR parameter distributions have been analysed using a Kernel density estimation (KDE) approach which is shown to highlight its main features. Accordingly, a new analytical model that incorporates the observed correlations between the NMR parameters is introduced. It significantly improves the fit of the 11B MQMAS spectra and yields, therefore, more reliable NMR parameter distributions. A new analytical model for a quantitative description of the dependence of the silicon‐29 and boron‐11 isotropic chemical shift upon the bond angles is proposed, which incorporates possibly the effect of SiO2B2O3 intermixing. Combining all the above procedures, we show how distributions of SiOT and BOT (TSi, B) bond angles can be estimated from the distribution of isotropic chemical shift of silicon‐29 and boron‐11, respectively. Copyright

Investigation of Al–O–Si bond angle in glass by 27Al 3Q-MAS NMR and molecular dynamics

Abstract The Al–O–Si bond angles were investigated in sodium aluminosilicate glasses with variable calcium content by two complementary methods. We developed the spectrum inversion for the 27 Al MQ-MAS to infer quantitative distributions of isotropic chemical shifts, enabling us to estimate an angular distribution. The experimental data were then compared with molecular dynamics simulations. In both cases, the Al–O–Si angle was observed to decrease as the calcium concentration increased. Structural analysis accounts for this effect by the gradually diminishing distance – and therefore the increasing interaction – between the network modifiers and oxygen atoms near aluminum. A similar but less intense effect is predicted for the Si–O–Si angles.