Quantitative Analysis of the Proximities of OH Ligands and Vanadium Sites in a Polyoxovanadate Cluster Using Frequency-Selective ¹H–⁵¹V Solid-State NMR Spectroscopy

We introduce a magic-angle spinning NMR experiment to estimate specific distances in a solid material between a given site occupied by a quadrupolar nucleus and the nearby spin-1/2 nuclei. The new sequence, called DANTE-S-REDOR, consists of a frequency-selective dephasing experiment where heteronuclear dipolar couplings are reintroduced by applying a symmetry-based sequence (S-REDOR). The selectivity is achieved by applying a pulse train, such as Delays Alternating with Nutations for Tailored Excitation (DANTE), to the quadrupolar nucleus. This new method allows quantitative analysis of proximities in the 3–4 Å range of protons in OH ligands and one of the ⁵¹V sites in a complex decavanadate cluster, namely Cs₄[H₂V₁₀O₂₈]·4H₂O. The high selectivity of the DANTE-S-REDOR sequence offers the possibility to investigate a wide range of materials with different quadrupolar nuclei, including polyoxometalates, oxides, zeolites, and aluminophosphates

Advances in Structural Studies on Alkylaluminum Species in the Solid State via Challenging ²⁷Al–¹³C NMR Spectroscopy and X‑ray Diffraction

Advanced multinuclear solid state NMR experiments were developed to probe the structure of two organometallic aluminum derivatives, Li­[Al­(CH₃)₃CH₂Si­(CH₃)₃] (<b>1</b>) and Li­[Al­(CH₃)₄] (<b>2</b>), which are relevant to olefin polymerization processes. For the first time, NMR observation of ²⁷Al–¹³C covalent bonds in solids is performed with the natural abundance material <b>1</b>. Unprecedented triple-resonance (¹H–¹³C–²⁷Al) and quadruple-resonance (¹H–⁷Li–¹³C–²⁷Al) heteronuclear correlation two-dimensional NMR experiments are also introduced to probe ²⁷Al–¹³C and ¹³C–⁷Li proximities for <b>2</b>. High-resolution solid-state NMR spectra thus obtained provide information on the local structure of these representative organometallic derivatives that proved to be most complementary and in full agreement with the structures obtained by X-ray diffraction

Enhanced Solid-State NMR Correlation Spectroscopy of Quadrupolar Nuclei Using Dynamic Nuclear Polarization

By means of a true sensitivity enhancement for a solid-state NMR spectroscopy (SSNMR) experiment performed under dynamic nuclear polarization (DNP) conditions, corresponding to 4–5 orders of magnitude of time savings compared with a conventional SSNMR experiment, it is shown that it is possible to record interface-selective ²⁷Al–²⁷Al two-dimensional dipolar correlation spectra on mesoporous alumina, an advanced material with potential industrial applications. The low efficiency of cross-polarization and dipolar recoupling for quadrupolar nuclei is completely negated using this technique. The important presence of pentacoordinated Al has not only been observed, but its role in bridging interfacial tetra- and hexacoordinated Al has been determined. Such structural information, collected at low temperature (∼103 K) and 9.4 T with the use of DNP, would have been impossible to obtain under standard conditions, even using a higher magnetic field. However, here it is demonstrated that this information can be obtained in only 4 h. This work clearly opens a new avenue for the application of SSNMR to quadrupolar nuclei and notably the atomic-scale structure determination of catalysis materials such as mesoporous alumina

Mechanochemical Synthesis and Study of the Local Structure of NaGaS₂ Glass and Glass–Ceramics

NaGaS2 is a newly discovered compound that has already shown great promise for a variety of applications because of its layered structure and ion exchange properties. In this work, crystalline NaGaS2 has been synthesized by an alternative method to what has been previously published, namely, by mechanochemistry, either by a direct one-step process or by a two-step process. In the one-step process, crystalline NaGaS2 is directly formed by milling sodium sulfide Na2S and gallium(III) sulfide Ga2S3. However, an amorphous material is present in majority together with the crystalline phase. In the two-step process, amorphous NaGaS2 is first obtained by mechanical milling and then heated above its glass transition temperature to obtain a glass–ceramic mainly composed of crystalline NaGaS2. For the two-step process, changes of the local atomic-level structure in amorphous NaGaS2 and after crystallization were analyzed by high-field solid-state nuclear magnetic resonance (NMR) spectroscopy as well as by X-ray total scattering and pair distribution function (PDF) analysis. Based on quantitative analysis on the 23Na NMR spectra, modifying the annealing treatment can promote the formation of the crystalline phase up to a molar fraction of 83.8%

High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies

The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al₅(OH)­(PO₄)₃(PO₃OH)₄] [NH₃(CH₂)₂NH₃]₂ [2H₂O], compound <b>1</b>, and [Al₅(PO₄)₅(PO₃OH)₂] [NH₃(CH₂)₃NH₃]₂ [H₂O], compound <b>2</b>. We have previously reported the structure of the periodic part of <b>1</b> by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocks determined by analysis of the ²⁷Al–³¹P correlation pattern of the two-dimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of <b>2</b>, using the powder synchrotron diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five P and seven Al inequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO₄, AlO₅, and AlO₆, and the interlayer space contains the amines and water molecules. In <b>1</b>, the inorganic layers are stacked on each other with a 4₂ element of symmetry along the <i>c</i>-axis, while they are stacked with a 180° rotation angle in <b>2</b>. By analysis of a set of high-resolution 1D and 2D NMR spectra (³¹P, ²⁷Al, ¹H, ¹⁵N, ¹³C, ²⁷Al–³¹P, ¹H–³¹P, and ¹H–¹⁴N), the structure analysis of <b>1</b> and <b>2</b> is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data

High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies

The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al₅(OH)­(PO₄)₃(PO₃OH)₄] [NH₃(CH₂)₂NH₃]₂ [2H₂O], compound <b>1</b>, and [Al₅(PO₄)₅(PO₃OH)₂] [NH₃(CH₂)₃NH₃]₂ [H₂O], compound <b>2</b>. We have previously reported the structure of the periodic part of <b>1</b> by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocks determined by analysis of the ²⁷Al–³¹P correlation pattern of the two-dimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of <b>2</b>, using the powder synchrotron diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five P and seven Al inequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO₄, AlO₅, and AlO₆, and the interlayer space contains the amines and water molecules. In <b>1</b>, the inorganic layers are stacked on each other with a 4₂ element of symmetry along the <i>c</i>-axis, while they are stacked with a 180° rotation angle in <b>2</b>. By analysis of a set of high-resolution 1D and 2D NMR spectra (³¹P, ²⁷Al, ¹H, ¹⁵N, ¹³C, ²⁷Al–³¹P, ¹H–³¹P, and ¹H–¹⁴N), the structure analysis of <b>1</b> and <b>2</b> is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data

Host–Guest Interactions in Dealuminated HY Zeolite Probed by ¹³C–²⁷Al Solid-State NMR Spectroscopy

Host–guest interactions in dealuminated HY zeolite have been investigated by advanced ¹³C–²⁷Al solid-state NMR experiments. This analysis allows us to report new insights into the adsorption geometry of acetone and its interaction with acid sites in the zeolite channels

High-Resolution Structural Characterization of Two Layered Aluminophosphates by Synchrotron Powder Diffraction and NMR Crystallographies

The syntheses and structure resolution process of two highly complex powdered aluminophosphates with an original 5:7 Al/P ratio are presented: [Al₅(OH)­(PO₄)₃(PO₃OH)₄] [NH₃(CH₂)₂NH₃]₂ [2H₂O], compound <b>1</b>, and [Al₅(PO₄)₅(PO₃OH)₂] [NH₃(CH₂)₃NH₃]₂ [H₂O], compound <b>2</b>. We have previously reported the structure of the periodic part of <b>1</b> by coupling synchrotron powder diffraction and solid-state nuclear magnetic resonance (NMR) crystallographies. With a similar strategy, that is, input of large parts of the building blocks determined by analysis of the ²⁷Al–³¹P correlation pattern of the two-dimensional (2D) NMR spectrum in the structure search process, we first determine the periodic structure of <b>2</b>, using the powder synchrotron diffraction data as cost function. Both <b>1</b> and <b>2</b> are layered materials, in which the inorganic layers contain five P and seven Al inequivalent atoms, with aluminum atoms that are found in three different coordination states, AlO₄, AlO₅, and AlO₆, and the interlayer space contains the amines and water molecules. In <b>1</b>, the inorganic layers are stacked on each other with a 4₂ element of symmetry along the <i>c</i>-axis, while they are stacked with a 180° rotation angle in <b>2</b>. By analysis of a set of high-resolution 1D and 2D NMR spectra (³¹P, ²⁷Al, ¹H, ¹⁵N, ¹³C, ²⁷Al–³¹P, ¹H–³¹P, and ¹H–¹⁴N), the structure analysis of <b>1</b> and <b>2</b> is extended beyond the strict periodicity, to which diffraction is restricted, and provides localization of the hydroxyl groups and water molecules in the frameworks and an attempt to correlate the presence of these latter species to the structural features of the two samples is presented. Finally, the dehydration/rehydration processes occurring in these solids are analyzed. The methodology of the structure determination for these dehydrated forms uses the same principles, combining X-ray powder diffraction and solid-state NMR data

Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning ¹³C and ²⁹Si NMR Enhanced by Dynamic Nuclear Polarization

We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of ¹³C and ²⁹Si nuclei located in mesoporous organic/inorganic hybrid materials, at several hundreds of nanometers from stable radicals (TOTAPOL) trapped in the surrounding frozen disordered water. The approach is demonstrated using mesoporous silica nanoparticles (MSN), functionalized with 3-(<i>N</i>-phenylureido)­propyl (PUP) groups, filled with the surfactant cetyltrimethylammonium bromide (CTAB). The DNP-enhanced proton magnetization is transported into the mesopores via ¹H–¹H spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements ε_on/off of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to ε_on/off ≈ 30 for both nuclei. A quantitative analysis of the signal enhancements in MSN with and without surfactant is based on a one-dimensional proton spin diffusion model. The effect of solvent deuteration is also investigated