High‐field 95Mo and 183W static and MAS NMR study of polyoxometalates

The potential of high‐field NMR to measure solid‐state ⁹⁵Mo and ¹⁸³W NMR in polyoxometalates (POMs) is explored using some archetypical structures like Lindqvist, Keggin and Dawson as model compounds that are well characterized in solution. NMR spectra in static and under magic angle spinning (MAS) were obtained, and their analysis allowed extraction of the NMR parameters, including chemical shift anisotropy and quadrupolar coupling parameters. Despite the inherent difficulties of measurement in solid state of these low‐gamma NMR nuclei, due mainly to the low spectral resolution and poor signal‐to‐noise ratio, the observed global trends compare well with the solution‐state NMR data. This would open an avenue for application of solid‐state NMR to POMs, especially when liquid‐state NMR is not possible, e.g., for poorly soluble or unstable compounds in solution, and for giant molecules with slow tumbling motion. This is the case of Keplerate where we provide here the first NMR characterization of this class of POMs in the solid state. Copyright © 2017 John Wiley & Sons, Ltd.     

Nanoscale polyoxometalate‐based inorganic/organic hybrids

The latest advances in the area of polyoxometalate (POM)‐based inorganic/organic hybrid materials prepared by self‐assembly, covalent modification, and supramolecular interactions are presented. This Review is composed of five sections and documents the effect of organic cations on the formation of novel POMs, surfactant encapsulated POM‐based hybrids, polymeric POM/organic hybrid materials, POMs‐containing ionic crystals, and covalently functionalized POMs. In addition to their role in the charge‐balancing, of anionic POMs, the crucial role of organic cations in the formation and functionalization of POM‐based hybrid materials is discussed. DOI 10.1002/tcr.201100002     

The Energy Landscape of Uranyl–Peroxide Species

Nanoscale uranyl peroxide clusters containing UO₂²⁺ groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U₂₈ clusters), with an encapsulated anion in the center, for example, [UO₂(O₂)_3?x(OH)_x^4?], [Nb(O₂)₄^3?], or [Ta(O₂)₄^3?]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U₂₈ compounds, one of which is uranyl centered and the other is peroxotantalate centered. The [(Ta(O₂)₄]‐centered U₂₈ capsule is energetically more stable than the [(UO₂)(O₂)₃]‐centered capsule. These data, along with our prior studies on other uranyl–peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali–uranyl–peroxide systems. It was suggested that the energetic role of charge‐balancing alkali ions and their electrostatic interactions with the negatively charged uranyl–peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the [(Ta(O₂)₄]‐centered U₂₈ capsule is more stable than the uranyl‐centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali‐anion interactions was shown to be important at all length scales of uranyl–peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.     

Polymorphism of N,N'-diacetylbiuret studied by solid-state 13C and 15N NMR spectroscopy, DFT calculations, and X-ray diffraction

The molecular configuration and crystal structure of solid polycrystalline N,N′′‐diacetylbiuret (DAB), a potential nitrogen‐rich fertilizer, have been analyzed by a combination of solid‐ and liquid‐state NMR spectroscopy, X‐ray diffraction, and DFT calculations. Initially a pure NMR study (“NMR crystallography”) was performed as available single crystals of DAB were not suitable for X‐ray diffraction. Solid‐state ¹³C NMR spectra revealed the unexpected existence of two polymorphic modifications (α‐ and β‐DAB) obtained from different chemical procedures. Several NMR techniques were applied for a thorough characterization of the molecular system, revealing chemical shift anisotropy (CSA) tensors of selected nuclei in the solid state, chemical shifts in the liquid state, and molecular dynamics in the solid state. Dynamic NMR spectroscopy of DAB in solution revealed exchange between two different configurations, which raised the question, is there a correlation between the two different configurations found in solution and the two polymorphic modifications found in the solid state? By using this knowledge, a new crystallization protocol was devised which led to the growth of single crystals suitable for X‐ray diffraction. The X‐ray data showed that the same symmetric configuration is present in both polymorphic modifications, but the packing patterns in the crystals are different. In both cases hydrogen bonds lead to the formation of planes of DAB molecules. Additional symmetry elements, a two‐fold screw in the case of α‐DAB and a c‐glide plane in the case of β‐DAB, lead to a more symmetric (α‐DAB) or asymmetric (β‐DAB) intermolecular hydrogen‐bonding pattern for each molecule.     

Heteronuclear Cross‐Relaxation Effects in the NMR Spectroscopy of Hyperpolarized Targets

Dissolution dynamic nuclear polarization (DNP) enables high‐sensitivity solution‐phase NMR experiments on long‐lived nuclear spin species such as ¹⁵N and ¹³C. This report explores certain features arising in solution‐state ¹H NMR upon polarizing low‐γ nuclear species. Following solid‐state hyperpolarization of both ¹³C and ¹H, solution‐phase ¹H NMR experiments on dissolved samples revealed transient effects, whereby peaks arising from protons bonded to the naturally occurring ¹³C nuclei appeared larger than the typically dominant ¹²C‐bonded ¹H resonances. This enhancement of the satellite peaks was examined in detail with respect to a variety of mechanisms that could potentially explain this observation. Both two‐ and three‐spin phenomena active in the solid state could lead to this kind of effect; still, experimental observations revealed that the enhancement originates from ¹³C→¹H polarization‐transfer processes active in the liquid state. Kinetic equations based on modified heteronuclear cross‐relaxation models were examined, and found to well describe the distinct patterns of growth and decay shown by the ¹³C‐bound ¹H NMR satellite resonances. The dynamics of these novel cross‐relaxation phenomena were determined, and their potential usefulness as tools for investigating hyperpolarized ensembles and for obtaining enhanced‐sensitivity ¹H NMR traces was explored.     

Solution dynamics of 5-fluorouracil entrapped in poly lactic-co-glycolic acid (PLGA) microsphere—A study with 1D selective NMR methods

In this report, our main focus is to introduce a set of one-dimensional (1D) NMR methods based on chemical shift, relaxation, and magnetization transfer, namely, NOE and chemical exchange involving selective pulse excitation to study the solution dynamics of drug in free and encapsulated state within polymeric microsphere. In this regard 5-fluorouracil (5-FU) loaded poly lactic-co-glycolic acid (PLGA) microspheres are prepared as model system via standard water-in-oil-in-water emulsification method. One-dimensional ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectra of 5-FU in presence of PLGA microspheres presented a significant change in linewidth and relaxation rates compared with free 5-FU confirming encapsulation. Furthermore, loss of coupling pattern in ¹H and ¹⁹F NMR of PLGA encapsulated 5-FU as compared with free 5-FU suggests an enhanced –NH and –H₂O protons exchange dynamics in the interior of the microsphere indicating hydrated microsphere cavity. Quantification of exchange dynamics in case of free and PLGA-encapsulated 5-FU was attempted employing 1D selective NOESY and 1D multiply selective inversion recovery experiments. Analysis of the exchange rates confirmed existence of more than one kind of water population within the cavity as mentioned in an earlier solid state NMR report.     

Dynamics in the crystalline polymorphic forms I and II and form III of isotactic poly‐1‐butene

Following an earlier study of the ¹H relaxation and NMR line shapes, we have carried out selective one‐dimensional and two‐dimensional ¹³C solid‐state NMR studies that yield to detailed interpretation of the dynamics in form I, II, and III polymorphs of isotactic poly‐1‐butene. A specific defect diffusion along the side group is proposed to account for the temperature dependence of the ¹³C spectra in form I. The backbone of the helix in forms II and III is shown to undergo large angle motions above the glass‐transition temperature. High‐resolution solid‐state ¹³C two‐dimensional exchange NMR under magic‐angle spinning with cross‐polarization techniques demonstrates the existence of slow rotational jumps of the helices in form III with typical jump rates of about 10 s⁻¹. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2611–2624, 2000     

Hexameric Capsules Studied by Magic Angle Spinning Solid‐State NMR Spectroscopy: Identifying Solvent Molecules in Pyrogallol[4]arene Capsules

Powders of pyrogallol[4]arene hexamers were produced by evaporation from organic solvents and were studied, for the first time, by magic angle spinning solid‐state NMR (MAS ssNMR). Evaporation selectively removed non‐encapsulated solvent molecules leaving stable hexameric capsules encapsulating solvent molecules. After exposure of the powder to solvent vapors, ¹H/¹³C heteronuclear correlation MAS ssNMR experiments were used to assign the signals of the external and encapsulated solvent molecules. The formed capsules were stable for months and the process of solvent encapsulation was reversible. According to the ssNMR experiments, the encapsulated solvent molecules occupy different sites and those sites differ in their mobility. The presented approach paves the way for studying guest exchange, guest affinity, and gas storage in hexamers of this type in the solid state.     

Conformational Plasticity in Glycomimetics: Fluorocarbamethyl‐L‐idopyranosides Mimic the Intrinsic Dynamic Behaviour of Natural Idose Rings

Sugar function, structure and dynamics are intricately correlated. Ring flexibility is intrinsically related to biological activity; actually plasticity in L ‐iduronic rings modulates their interactions with biological receptors. However, the access to the experimental values of the energy barriers and free‐energy difference for conformer interconversion in water solution has been elusive. Here, a new generation of fluorine‐containing glycomimetics is presented. We have applied a combination of organic synthesis, NMR spectroscopy and computational methods to investigate the conformational behaviour of idose‐ and glucose‐like rings. We have used low‐temperature NMR spectroscopic experiments to slow down the conformational exchange of the idose‐like rings. Under these conditions, the exchange rate becomes slow in the ¹⁹F NMR spectroscopic chemical shift timescale and allows shedding light on the thermodynamic and kinetic features of the equilibrium. Despite the minimal structural differences between these compounds, a remarkable difference in their dynamic behaviour indeed occurs. The importance of introducing fluorine atoms in these sugars mimics is also highlighted. Only the use of ¹⁹F NMR spectroscopic experiments has permitted the unveiling of key features of the conformational equilibrium that would have otherwise remained unobserved.     

Stacking Structure of Confined 1‐Butanol in SBA‐15 Investigated by Solid‐State NMR Spectroscopy

Understanding the complex thermodynamic behavior of confined amphiphilic molecules in biological or mesoporous hosts requires detailed knowledge of the stacking structures. Here, we present detailed solid‐state NMR spectroscopic investigations on 1‐butanol molecules confined in the hydrophilic mesoporous SBA‐15 host. A range of NMR spectroscopic measurements comprising of ¹H spin–lattice (T₁), spin–spin (T₂) relaxation, ¹³C cross‐polarization (CP), and ¹H,¹H two‐dimensional nuclear Overhauser enhancement spectroscopy (¹H,¹H 2D NOESY) with the magic angle spinning (MAS) technique as well as static wide‐line ²H NMR spectra have been used to investigate the dynamics and to observe the stacking structure of confined 1‐butanol in SBA‐15. The results suggest that not only the molecular reorientation but also the exchange motions of confined molecules of 1‐butanol are extremely restricted in the confined space of the SBA‐15 pores. The dynamics of the confined molecules of 1‐butanol imply that the ¹H,¹H 2D NOESY should be an appropriate technique to observe the stacking structure of confined amphiphilc molecules. This study is the first to observe that a significant part of confined 1‐butanol molecules are orientated as tilted bilayered structures on the surface of the host SBA‐15 pores in a time‐average state by solid‐state NMR spectroscopy with the ¹H,¹H 2D NOESY technique.     

Multidimensional solid‐state NMR studies of the structure and dynamics of pectic polysaccharides in uniformly 13C‐labeled Arabidopsis primary cell walls

Plant cell wall (CW) polysaccharides are responsible for the mechanical strength and growth of plant cells; however, the high‐resolution structure and dynamics of the CW polysaccharides are still poorly understood because of the insoluble nature of these molecules. Here, we use 2D and 3D magic‐angle‐spinning (MAS) solid‐state NMR (SSNMR) to investigate the structural role of pectins in the plant CW. Intact and partially depectinated primary CWs of Arabidopsis thaliana were uniformly labeled with ¹³C and their NMR spectra were compared. Recent ¹³C resonance assignment of the major polysaccharides in Arabidopsis thaliana CWs allowed us to determine the effects of depectination on the intermolecular packing and dynamics of the remaining wall polysaccharides. 2D and 3D correlation spectra show the suppression of pectin signals, confirming partial pectin removal by chelating agents and sodium carbonate. Importantly, higher cross peaks are observed in 2D and 3D ¹³C spectra of the depectinated CW, suggesting higher rigidity and denser packing of the remaining wall polysaccharides compared with the intact CW. ¹³C spin–lattice relaxation times and ¹H rotating‐frame spin–lattice relaxation times indicate that the polysaccharides are more rigid on both the nanosecond and microsecond timescales in the depectinated CW. Taken together, these results indicate that pectic polysaccharides are highly dynamic and endow the polysaccharide network of the primary CW with mobility and flexibility, which may be important for pectin functions. This study demonstrates the capability of multidimensional SSNMR to determine the intermolecular interactions and dynamic structures of complex plant materials under near‐native conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamic Phenomena in Self‐Complementary {2}‐Metallocryptates Probed by Solution 133Cs‐NMR. New Insights into Ion Pairing Processes: X‐Ray Structure and Solid‐State NMR Spectra of a Meandering Species

Two self‐complementary {2}‐metallocryptates, differing in methyl and phenyl substituents, respectively, have been studied by X‐ray analysis, and solid‐state and solution NMR. Mixed Mg/Cs metal methyl complex 2 is a linear polymer in the solid state. The two different Cs sites are confirmed by ¹³³Cs‐solid‐state NMR. By contrast, the analog mixed Mg/Cs metal phenyl complex 4 is a meandering polymer as shown by an actual X‐ray analysis. The four non‐equivalent Cs‐sites in 4 are reflected in the solid‐state NMR spectra. Solution ¹³³Cs‐NMR spectra of 4 reveal two independent dynamic processes: a fast exchange of Cs within contact ion‐pairs and solvent‐separated ion‐pairs (CIP, SSIP), and a slower exchange of ‘inside’ endo Cs, surrounded by three ligands, and ‘outside’ exo Cs involved in the CIP/SSIP equilibrium. Complete line‐shape analysis of variable‐temperature ¹³³Cs‐NMR spectra of 4 yield kinetic parameters of =10.8 kcal/mol for the fast SSIP‐CIP exchange and =13.2 kcal/mol for the slower endo/exo exchange of Cs. DOSY‐NMR Measurements confirm the monomeric nature of 4 in solution.     

Structural Insight into IAPP‐Derived Amyloid Inhibitors and Their Mechanism of Action

Designed peptides derived from the islet amyloid polypeptide (IAPP) cross‐amyloid interaction surface with Aβ (termed interaction surface mimics or ISMs) have been shown to be highly potent inhibitors of Aβ amyloid self‐assembly. However, the molecular mechanism of their function is not well understood. Using solution‐state and solid‐state NMR spectroscopy in combination with ensemble‐averaged dynamics simulations and other biophysical methods including TEM, fluorescence spectroscopy and microscopy, and DLS, we characterize ISM structural preferences and interactions. We find that the ISM peptide R3‐GI is highly dynamic, can adopt a β‐like structure, and oligomerizes into colloid‐like assemblies in a process that is reminiscent of liquid–liquid phase separation (LLPS). Our results suggest that such assemblies yield multivalent surfaces for interactions with Aβ40. Sequestration of substrates into these colloid‐like structures provides a mechanistic basis for ISM function and the design of novel potent anti‐amyloid molecules.     

Investigation of chloromethane complexes of cryptophane‐A analogue with butoxy groups using 13C NMR in the solid state and solution along with single crystal X‐ray diffraction

Host‐guest complexes between cryptophane‐A analogue with butoxy groups (cryptophane‐But) and chloromethanes (chloroform, dichloromethane) were investigated in the solid state by means of magic‐angle spinning ¹³C NMR spectroscopy. The separated local fields method with ¹³C‐¹H dipolar recoupling was used to determine the residual dipolar coupling for the guest molecules encaged in the host cavity. In the case of chloroform guest, the residual dipolar interaction was estimated to be about 19 kHz, consistent with a strongly restricted mobility of the guest in the cavity, while no residual interaction was observed for encaged dichloromethane. In order to rationalize this unexpected result, we performed single crystal X‐ray diffraction studies, which confirmed that both guest molecules indeed were present inside the cryptophane cavity, with a certain level of disorder. To improve the insight in the dynamics, we performed a ¹³C NMR spin‐lattice relaxation study for the dichloromethane guest in solution. The system was characterized by chemical exchange, which was slow on the chemical shift time scale but fast with respect to the relaxation rates. Despite these disadvantageous conditions, we demonstrated that the data could be analyzed and that the results were consistent with an isotropic reorientation of dichloromethane within the cryptophane cavity. Copyright © 2015 The Authors. Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd.     

Catalytically Active and Spectator Ce3+ in Ceria‐Supported Metal Catalysts

Identification of active species and the rate‐determining reaction steps are crucial for optimizing the performance of oxygen‐storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce³⁺ species in a ceria‐supported platinum catalyst during CO oxidation are short‐lived and therefore cannot be observed under steady‐state conditions. Using time‐resolved resonant X‐ray emission spectroscopy, we quantitatively correlated the initial rate of Ce³⁺ formation under transient conditions to the overall rate of CO oxidation under steady‐state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce³⁺ was present as spectators. This approach can be applied to various catalytic processes involving oxygen‐storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.     

Characterizing Methyl‐Bearing Side Chain Contacts and Dynamics Mediating Amyloid β Protofibril Interactions Using 13Cmethyl‐DEST and Lifetime Line Broadening

Many details pertaining to the formation and interactions of protein aggregates associated with neurodegenerative diseases are invisible to conventional biophysical techniques. We recently introduced ¹⁵N dark‐state exchange saturation transfer (DEST) and ¹⁵N lifetime line‐broadening to study solution backbone dynamics and position‐specific binding probabilities for amyloid β (Aβ) monomers in exchange with large (2–80 MDa) protofibrillar Aβ aggregates. Here we use ¹³C_methyl DEST and lifetime line‐broadening to probe the interactions and dynamics of methyl‐bearing side chains in the Aβ‐protofibril‐bound state. We show that all methyl groups of Aβ40 populate direct‐contact bound states with a very fast effective transverse relaxation rate, indicative of side‐chain‐mediated direct binding to the protofibril surface. The data are consistent with position‐specific enhancements of ¹³C_methyl‐${R{{{\rm tethered}\hfill \atop 2\hfill}}}$ values in tethered states, providing further insights into the structural ensemble of the protofibril‐bound state.     

Billion‐Fold Enhancement in Sensitivity of Nuclear Magnetic Resonance Spectroscopy for Magnesium Ions in Solution

β‐nuclear magnetic resonance (NMR) spectroscopy is highly sensitive compared to conventional NMR spectroscopy, and may be applied for several elements across the periodic table. β‐NMR has previously been successfully applied in the fields of nuclear and solid‐state physics. In this work, β‐NMR is applied, for the first time, to record an NMR spectrum for a species in solution. ³¹Mg β‐NMR spectra are measured for as few as 10⁷ magnesium ions in ionic liquid (EMIM‐Ac) within minutes, as a prototypical test case. Resonances are observed at 3882.9 and 3887.2 kHz in an external field of 0.3 T. The key achievement of the current work is to demonstrate that β‐NMR is applicable for the analysis of species in solution, and thus represents a novel spectroscopic technique for use in general chemistry and potentially in biochemistry.     

Synthesis,Solid‐State NMR Characterization,and Application for Hydrogenation Reactions of a Novel Wilkinson’s‐Type Immobilized Catalyst

Silica nanoparticles (SiNPs) were chosen as a solid support material for the immobilization of a new Wilkinson’s‐type catalyst. In a first step, polymer molecules (poly(triphenylphosphine)ethylene (PTPPE); 4‐diphenylphosphine styrene as monomer) were grafted onto the silica nanoparticles by surface‐initiated photoinferter‐mediated polymerization (SI‐PIMP). The catalyst was then created by binding rhodium (Rh) to the polymer side chains, with RhCl₃ ? x H₂O as a precursor. The triphenylphosphine units and rhodium as Rh^I provide an environment to form Wilkinson’s catalyst‐like structures. Employing multinuclear (³¹P, ²⁹Si, and ¹³C) solid‐state NMR spectroscopy (SSNMR), the structure of the catalyst bound to the polymer and the intermediates of the grafting reaction have been characterized. Finally, first applications of this catalyst in hydrogenation reactions employing para‐enriched hydrogen gas (PHIP experiments) and an assessment of its leaching properties are presented.     

Structural characterization of monomeric and oligomeric arylamides by solution and solid‐state NMR spectroscopy

An extensive study of both liquid‐ and solid‐state NMR spectroscopy was undertaken in order to elucidate the structural features of a phenyleneterephthalamide oligomer (OPTA) and of some related diarylamides. 1D‐ and 2D‐COSY measurements allowed us to assign completely the proton signals of the title compounds in solution, while 1D‐, 2D‐HETCOR and 2D‐COLOC measurements were used to assign ¹³C resonances. Solid‐state ¹³C NMR experiments, by conventional cross‐polarization (CP) at different contact times and with the dipolar dephased CP technique, were used to characterize these molecules in the solid state. Such techniques allowed us to differentiate among different carbon atoms; in the resulting spectra it was then possible to observe the selective appearance of signals from protonated and quaternary carbon atoms. It was also ascertained that the limited structural mobility of the insoluble OPTA, existing as a single monophasic species, can be explained in terms of hydrogen‐type bonds present in the solid state. Copyright © 2002 John Wiley & Sons, Ltd.     

A Computational Evaluation of the Evidence for the Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution

Density functional calculations of ¹H NMR spectra and reaction barriers at the ωB97XD/6‐311G(d,p)/continuum water level do not support the claimed identification of encarcerated 1,3‐dimethylcyclobutadiene in either the solid state or aqueous solution, as reported by Barboiu et al (Chem. Eur. 2011 , 17, 10021). Instead, previous suggestions that the species identified in the solid state is in fact 2‐oxabicyclo[2.2.0]hex‐5‐en‐3‐one (the Dewar lactone Me₂2 ) are reaffirmed. Analysis of the ground‐state electronic structure of this species indicates an unusual π‐anomeric effect is promoting a Dunitz‐like chemical reaction pathway leading to the eventual elimination of carbon dioxide and formation of 1,3‐dimethylcyclobutadiene.