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In this study, nuclear magnetic resonance (NMR) is used to investigate the crystallisation behaviour of aspirin within a mesoporous SBA-15 silica material. The potential of dynamic nuclear polarisation (DNP) experiments is also investigated using specifically designed porous materials that incorporate polarising agents within their walls. The formation of the metastable crystalline form II is observed when crystallisation occurs within the pores of the mesoporous structure. Conversely, bulk crystallisation yields the most stable form, namely form I, of aspirin. Remarkably, the metastable form II remains trapped within the pores of mesoporous SBA-15 silica material even 30 days after impregnation, underscoring its persistent stability within this confined environment. [ABSTRACT FROM AUTHOR]

In this study, supercritical CO2 (scCO2) was used to impregnate polymers with paramagnetic polarizing agents to prepare samples for dynamic nuclear polarization (DNP) solid‐state NMR (ssNMR) experiments. As a proof of concept, we impregnated polystyrene samples with bTbK, which stands for bis‐TEMPO‐bisketal where TEMPO is 2,2,6,6‐tetra‐methylpiperindin‐1‐oxyl. Substantial DNP signal enhancements could be measured on DNP‐enhanced 1H → 13C cross‐polarization (CP) magic‐angle spinning (MAS) spectra recorded at 9.4 T and ~100 K, reaching a maximum value of 8 in the most favorable case, which appeared comparable or even higher than what is typically obtained on similar systems for former sample preparation methods. These results highlight the potential of scCO2 impregnation as an efficient and possibly versatile methodology to prepare polymer samples for DNP ssNMR investigations. [ABSTRACT FROM AUTHOR]

Dynamic nuclear polarisation (DNP) can significantly enhance the sensitivity of solid-state nuclear magnetic resonance (SSNMR) experiments by transferring the electron spin polarisation of paramagnetic species to nuclei through microwave irradiation of the sample at cryogenic temperatures. Paramagnetic species required for DNP can be provided in the form of mesoporous silica materials containing nitroxide radicals either located on the porous surface or embedded in the pore walls. The present study focuses specifically on porous materials with wall-embedded radicals that were synthesised using conventional molecular imprinting protocols. More remarkably, by changing the molecular structure of the TEMPO precursor, the theoretical distance between the oxygen atoms in a pair of wall-embedded face-to-face TEMPO radicals was increased stepwise (0.7, 0.9, 1.1, 1.3 and 1.5 nm). The thermal activation of these five series of materials led to 37 TEMPO-functionalised silica materials with different radical concentrations. Their efficiency as DNP polarising agents was subsequently investigated at 9.4 T and ∼110 K under magic-angle spinning conditions (10 kHz) after impregnating them at room temperature with an aqueous solution of isotopically enriched proline. Our results show that the highest DNP efficiency was obtained for the silica materials that exhibited the shortest theoretical oxygen-oxygen distance between the TEMPO rings, suggesting that the design rules accepted for soluble DNP polarising agents may not be transposed to these materials with wall-embedded pairs of nitroxides. [ABSTRACT FROM AUTHOR]

Three anthraquinone-based chromophores (9,10-anthraquinone, alizarin, purpurin) are compared from the point of view of their experimental and computed NMR and UV-visible light absorption spectra. Using a hybrid (explicit/implicit) solvent model, each proton chemical shift can be reproduced with an error of less than 7%, even when such protons are engaged in inter-molecular hydrogen bonds with the solvent or when the analyzed sample contains a significant amount of impurities, for instance, 9,10-anthraquinone in purpurin. All the steady-state UV-visible absorption spectra feature a significant vibrational progression in the first absorption band. The shape of the corresponding computed spectra, including vibronic couplings obtained with the adiabatic Hessian approach and the Franck–Condon and Herzberg–Teller approximation of the transition dipole, are in excellent agreement with the experimental ones. The importance and the nature of the vibronic couplings are different for the three molecules, even if they only differ by the number of hydroxyl groups. [ABSTRACT FROM AUTHOR]

We show here that the Electronic Mixing‐Mediated Annihilation (EMMA) method, previously reported for the suppression of background signals in solid‐state nuclear magnetic resonance spectra, can be successfully applied to remove the solvent signals observed in the case of nuclear magnetic resonance spectra obtained with dynamic nuclear polarization. The methodology presented here is applied to two standard sample preparation methods for dynamic nuclear polarization, namely, glass forming and incipient wetness impregnation. It is demonstrated that the Electronic Mixing‐Mediated Annihilation method is complementary to the different methods for solvent suppression based on relaxation filters and that it can be used to preserve the quantitative information that might be present in the pristine spectra. [ABSTRACT FROM AUTHOR]

Vicinal scalar couplings (3J) are extensively used for the conformational analysis of organic compounds in the liquid state through empirical Karplus equations. In contrast, there are no examples of such use for the structural investigation of solids. With the support of first principles calculations, we demonstrate here that 13C‐13C 3J coupling constants (3JCC) measured on a series of isotopically enriched solid amino acids and sugars can be related to dihedral angles by a simple Karplus‐like relationship, and we provide a parameterized Karplus function for the conformational analysis of organic molecular crystals. Under the experimental conditions discussed, torsional angles can be estimated from the experimental 3JCC values with an accuracy of 10° using this function. These results open new perspectives towards the use of 3JCC as a new analytical tool that could considerably simplify structure determination of functional organic solids. [ABSTRACT FROM AUTHOR]

Vicinal scalar couplings (3J) are extensively used for the conformational analysis of organic compounds in the liquid state through empirical Karplus equations. In contrast, there are no examples of such use for the structural investigation of solids. With the support of first principles calculations, we demonstrate here that 13C‐13C 3J coupling constants (3JCC) measured on a series of isotopically enriched solid amino acids and sugars can be related to dihedral angles by a simple Karplus‐like relationship, and we provide a parameterized Karplus function for the conformational analysis of organic molecular crystals. Under the experimental conditions discussed, torsional angles can be estimated from the experimental 3JCC values with an accuracy of 10° using this function. These results open new perspectives towards the use of 3JCC as a new analytical tool that could considerably simplify structure determination of functional organic solids. [ABSTRACT FROM AUTHOR]

Vicinal scalar couplings (3J) are extensively used for the conformational analysis of organic compounds in the liquid state through empirical Karplus equations. In contrast, there are no examples of such use for the structural investigation of solids. With the support of first principles calculations, we demonstrate here that 13C‐13C 3J coupling constants (3JCC) measured on a series of isotopically enriched solid amino acids and sugars can be related to dihedral angles by a simple Karplus‐like relationship, and we provide a parameterized Karplus function for the conformational analysis of organic molecular crystals. Under the experimental conditions discussed, torsional angles can be estimated from the experimental 3JCC values with an accuracy of 10° using this function. These results open new perspectives towards the use of 3JCC as a new analytical tool that could considerably simplify structure determination of functional organic solids. [ABSTRACT FROM AUTHOR]

Structure determination of functional organic compounds remains a formidable challenge when the sample exists as a powder. Nuclear magnetic resonance crystallography approaches based on the comparison of experimental and Density Functional Theory (DFT)‐computed 1H chemical shifts have already demonstrated great potential for structure determination of organic powders, but limitations still persist. In this study, we discuss the possibility of using 13C‐13C dipolar couplings quantified on powdered theophylline at natural isotopic abundance with the help of dynamic nuclear polarization, to realize a DFT‐free, rapid screening of a pool of structures predicted by ab initio random structure search. We show that although 13C‐13C dipolar couplings can identify structures possessing long range structural motifs and unit cell parameters close to those of the true structure, it must be complemented with other data to recover information about the presence and the chemical nature of the supramolecular interactions. [ABSTRACT FROM AUTHOR]

Abstract: In situ solid‐state NMR spectroscopy is exploited to monitor the structural evolution of a glycine/water glass phase formed on flash cooling an aqueous solution of glycine, with a range of modern solid‐state NMR methods applied to elucidate structural properties of the solid phases present. The glycine/water glass is shown to crystallize into an intermediate phase, which then transforms to the β polymorph of glycine. Our in situ NMR results fully corroborate the identity of the intermediate crystalline phase as glycine dihydrate, which was first proposed only very recently. [ABSTRACT FROM AUTHOR]

In situ solid-state NMR spectroscopy is exploited to monitor the structural evolution of a glycine/water glass phase formed on flash cooling an aqueous solution of glycine, with a range of modern solid-state NMR methods applied to elucidate structural properties of the solid phases present. The glycine/water glass is shown to crystallize into an intermediate phase, which then transforms to the β polymorph of glycine. Our in situ NMR results fully corroborate the identity of the intermediate crystalline phase as glycine dihydrate, which was first proposed only very recently. [ABSTRACT FROM AUTHOR]

Each of the two carbons in alpha to the two nitroxide functions should bear two methyl groups, leading to the following corrected Scheme 1. In the published article SP [ sp [1] SP ] sp above: An error has been made in Scheme 1 on the molecular structure of bTbK (Scheme 1b). Corrigendum: Supercritical CO2 impregnation process applied to polymer samples preparation for dynamic nuclear polarization solid-state NMR. [Extracted from the article]

We present a solid-state NMR methodology capable of investigating the carbon skeleton of natural abundance organic powders. The methodology is based on the 13C–13C dipolar coupling interaction and allows carbon–carbon connectivities to be unambiguously established for a wide range of organic solids. This methodology is particularly suitable for disordered solids, such as natural or synthetic macromolecules, which cannot be studied using conventional diffraction or NMR techniques. [ABSTRACT FROM AUTHOR]

High-field dynamic nuclear polarization (DNP) has emerged as a powerful technique for improving the sensitivity of solid-state NMR (SSNMR), yielding significant sensitivity enhancements for a variety of samples, including polymers. Overall, depending upon the type of polymer, the molecular weight, and the DNP sample preparation method, sensitivity enhancements between 5 and 40 have been reported. These promising enhancements remain, however, far from the theoretical maximum (>1000). Crucial to the success of DNP SSNMR is the DNP signal enhancement ( εDNP), which is the ratio of the NMR signal intensities with and without DNP. It is shown here that, for polymers exhibiting high affinity toward molecular oxygen (e.g., polystyrene), removing part of the absorbed (paramagnetic) oxygen from the solid-state samples available as powders (instead of dissolved or dispersed in a solvent) increases proton nuclear relaxation times and εDNP, hereby providing up to a two-fold sensitivity increase (i.e., a four-fold reduction in experimental time). [ABSTRACT FROM AUTHOR]

A straightforward method is reported to quantitatively relate structural constraints based on 13C-13C doublequantum build-up curves obtained by dynamic nuclear polarization (DNP) solid-state NMR to the crystal structure of organic powders at natural isotopic abundance. This method relies on the significant gain in NMR sensitivity provided by DNP (approximately 50-fold, lowering the experimental time from a few years to a few days), and is sensitive to the molecular conformation and crystal packing of the studied powder sample (in this case theophylline). This method allows trial crystal structures to be rapidly and effectively discriminated, and paves the way to three-dimensional structure elucidation of powders through combination with powder X-ray diffraction, crystal-structure prediction, and density functional theory computation of NMR chemical shifts. [ABSTRACT FROM AUTHOR]

A straightforward method is reported to quantitatively relate structural constraints based on 13C-13C double-quantum build-up curves obtained by dynamic nuclear polarization (DNP) solid-state NMR to the crystal structure of organic powders at natural isotopic abundance. This method relies on the significant gain in NMR sensitivity provided by DNP (approximately 50-fold, lowering the experimental time from a few years to a few days), and is sensitive to the molecular conformation and crystal packing of the studied powder sample (in this case theophylline). This method allows trial crystal structures to be rapidly and effectively discriminated, and paves the way to three-dimensional structure elucidation of powders through combination with powder X-ray diffraction, crystal-structure prediction, and density functional theory computation of NMR chemical shifts. [ABSTRACT FROM AUTHOR]

Melanin is the most widespread pigment in the animal kingdom. Despite its importance, its detailed structure and overall molecular architecture remain elusive. Both eumelanin (black) and pheomelanin (red) occur in the human body. These two melanin compounds show very different responses to UV-radiation exposure, which could relate to their microscopic features. Herein, the structural properties and motional behavior of natural eu- and pheomelanin extracted from black and red human hair are investigated by means of solid-state NMR spectroscopy. Several 1D and 2D NMR spectroscopic techniques were combined to highlight the differences between the two forms of the pigment. The quantitative analysis of the 1H NMR wide-line spectra extracted from 2D 1H-13C LG-WISE experiments revealed the presence of two dynamically distinguishable components in both forms. Remarkably, the more mobile fraction of the pigment showed a higher mobility with respect to the proteinaceous components that coexist in the melanosome, which is particularly evident for the red pigment. An explanation of the observed effects takes into account the different architecture of the proteinaceous matrix that constitutes the physical substrate onto which melanin polymerizes within the eu- and pheomelanosomes. Further insight into the molecular structure of the more mobile fraction of pheomelanin was also obtained by means of the analysis of 2D 1H-13C INEPT experiments. Our view is that not only structural features inherent in the pure pigment, but also the role of the matrix structure in defining the overall melanin supramolecular arrangement and the resulting dynamic behavior of the two melanin compounds should be taken into account to explain their functions. The reported results could pave a new way toward the explanation of the molecular origin of the differences in the photoprotection activity displayed by black and red melanin pigments. [ABSTRACT FROM AUTHOR]

A new solid-state NMR method is presented for estimating homonuclear dipole–dipole couplings for selected groups of nuclear spins in a multiple-spin coupled network. The methodology combines off-magic-angle spinning, frequency selective spin echoes, and multiple quantum filtering. The new method is insensitive to incoherent relaxation effects and may be used to estimate weak couplings. Internuclear 13C–13C couplings are estimated in uniformly 13C-labelled l-Histidine·HCl·H2O. Weak intermolecular couplings between 13C nuclei separated by distances exceeding 6 are estimated. [ABSTRACT FROM AUTHOR]

A new solution-state NMR method is proposed to determine apparent transverse NMR relaxation rates in both weakly and strongly scalar coupled two-spin systems. [ABSTRACT FROM AUTHOR]

Rücktitelbild: A Karplus Equation for the Conformational Analysis of Organic Molecular Crystals (Angew. G. Mollica und Mitarbeiter zeigen in ihrer Zuschrift auf S. 16193 ff., dass Diederwinkel in organischen Festkörpern durch eine einfache Karplus-Gleichung mit skalaren SP 13 sp C- SP 13 sp C-Kopplungen über drei Bindungen in Beziehung stehen, wodurch die Molekülkonformation im Festkörper mittels NMR-Spektroskopie bestimmt werden kann. Diese Ergebnisse ebnen neue Wege für die Strukturbestimmung pulverförmiger Materialien auf atomarer Ebene. [Extracted from the article]