Solid-state NMR Techniques Research Articles

Stabilizing High-Voltage Layered Oxide Cathode Materials for Li/Na Ion Batteries

Li-ion battery systems have started to extend its roles from portable electronics, power tools to automobile applications such as hybrid and plug-in electric vehicles. However, it is necessary to enhance their electrochemical performance such as energy density and cycle life, for promoting their competition capability as vehicle power system. Obviously, electrode materials, especially positive electrode materials either working voltage or reversible capacity determine the energy density of the Li-ion batteries at mostly[1,2]. In the presentation, some new research progress of stabilizing high-voltage layered oxide cathode materials for Li/Na ion batteries in this group will be reported. Our strategies include two directions[3,4]: one is to improve the interface stability by using high voltage electrolyte additives or surface coating, another is to stabilize the lattice structure by using suitable element doping. For example, when extending the working voltage of LiCoO2 from 4.2V to 4.4V or even more, we must develop some new electrolyte recipes to stabilize corresponding electrode/electrolyte interface at high voltage. Our recent results[5] show that a synergistic effects of suberonitrile-LiBOB binary additives in high-voltage cathode system, i.e. they could improve the cyclic performance of 4.5V LiCoO2 effectively. The corresponding working mechanism of the binary additives will be presented. As to develop high energy density Na-ion battery, it is important to develop high-voltage cathode material. It is found that [6,7] the introduction of Zn2+ in the Na-Ni-Mn-O system can effectively overcome the drawback of voltage decay when charged to a higher cutoff voltage (>4.0 V), and significantly improve capacity retention compared to the unsubstituted material during cycling. In addition, a smoother charge/discharge profile can be observed between 3.0 and 4.0 V for Zn-substituted samples, demonstrating that Na+/vacancy ordering can be suppressed during sodium insertion/extraction. Na0.66Ni0.26Zn0.07Mn0.67O2 can deliver an initial capacity of 132 mAh g–1 at 12 mA g–1 with a high average voltage of 3.6 V and a capacity retention of 89% after 30 cycles. We have used a series of techniques such as in-situ XRD, in-situ XAS, ex-situ solid state NMR and HRTEM techniques to investigate the structural changes during cycling process and working mechanism of Zn2+ in the corresponding Na0.66Ni0.26Zn0.07Mn0.67O2 cathode materials.

Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds.

This work determines the potential of solid-state NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na7HNb6O19·15H2O (1), K8Nb6O19·16H2O (2), and Na8Ta6O19·24.5H2O (3) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). (1)H MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated clusters AxH8-xM6O19·nH2O (A = alkali ion; M = Nb, Ta). (93)Nb MAS NMR studies at different fields and MAS rotation frequencies have been performed on 1. For the first time, the contributions of NbO5Oμ2H sites were clearly distinguished from those assigned to NbO6 sites in the hexaniobate cluster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), (93)Nb WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample 1. The (93)Nb NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO5Oμ2H sites). 1D (23)Na MAS and 2D (23)Na 3QMAS NMR studies of 1 revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The (23)Na MAS NMR study of 3 confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in 1 was also probed by multinuclear MAS NMR ((1)H, (23)Na, and (93)Nb).

Fast Conduction of Organic Cations in Metal Sulfate Frameworks

We demonstrated a new method of synthesizing crystalline organic cation conductors. The conductivities of various organic cations involved in a one-dimensional zinc sulfate framework were studied. The optimized structure (EMIm)2[Zn(SO4)2] exhibited an ionic conductivity of 3.8 × 10–3 S cm–1 at 210 °C, which is comparable to that of highly conductive organic ionic plastic crystals. The high ionic conductivity is attributable to the defect structures of the organic cations in the inorganic frameworks. The pulsed-field gradient solid-state NMR technique revealed that the self-diffusion coefficient of organic cations in the zinc sulfate at 80 °C is comparable to that of the popular ionic liquid EMIm-BF4 at 30 °C, which indicates that liquid-like fast transporting of organic cation is achieved in the robust crystal structure.

New in situ solid-state NMR strategies for exploring materials formation and adsorption processes: prospects in heterogenous catalysis

Solid-state NMR spectroscopy is a powerful technique for studying structural and dynamic properties of solids and has considerable potential to be exploited for in situ studies of chemical processes. However, adapting solid-state NMR techniques and instrumentation for in situ applications are often associated with technical challenges, and for this reason, the opportunities remain underexploited. This paper highlights two experimental strategies that we have developed in recent years for in situ solid-state NMR investigations of solid-state processes. One technique is focused on probing details of the time evolution of materials formation processes, and the other technique is focused on understanding the time evolution of adsorption processes in microporous and mesoporous solid host materials. Each of these in situ solid-state NMR techniques has significant prospects for applications in areas relating to heterogeneous catalysis.

Insight into the PEG-linked bis-imidazolium bridged framework of mesoporous organosilicas as ion exchangers

Development of novel periodic mesoporous organosilicas (PMOs) were carried out by designing precursor, PEG-linked bis-imidazolium chloride bridged silsesquioxanes (PIMAILS, an ionic liquid). The involvement of tetraethyl ortho silicate (TEOS) as a template in an acidic medium led to the formation of a silica hybrid material (PMO-T). Contrary, in the absence of TEOS, PIMAILS self-assembled in a basic environment over the cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) to form the silica hybrid material (PMO-S). The availability of surface silanols and ionic liquid (IL) in both the PMOs were verified using FT-IR and 13C solid NMR. Further, they were quantified using 29Si solid state NMR technique. The results depicted 3 times better loading of the precursor in the framework of PMO-S than PMO-T. Larger surface areas and higher porosities were found in PMO-S than PMO-T while analyzed using BET-BJH technique. Thermogravimetric (TG) analysis revealed nearly 2.2 times higher weight losses (%) from PMO-S than PMO-T, which validate the results of silica solid state NMR. Powder XRD analysis showed well-defined hkl reflections (110) and (200) from PMO-S in contrast of PMO-T, describing the periodic arrangement of IL within PMO-S. Finally ion-exchange capacities of these materials were estimated by employing them for recovery of W(VI). The PMO-S, which has higher percentage of IL in the framework, achieved better separation of W(VI) than PMO-T.

ChemInform Abstract: Acidic Properties and Structure‐Activity Correlations of Solid Acid Catalysts Revealed by Solid‐State NMR Spectroscopy

AbstractReview: 40 refs.

A REDOR ssNMR Investigation of the Role of an N-Terminus Lysine in R5 Silica Recognition.

Diatoms are unicellular algae that construct cell walls called frustules by the precipitation of silica, using special proteins that order the silica into a wide variety of nanostructures. The diatom species Cylindrotheca fusiformis contains proteins called silaffins within its frustules, which are believed to assemble into supramolecular matrices that serve as both accelerators and templates for silica deposition. Studying the properties of these biosilicification proteins has allowed the design of new protein and peptide systems that generate customizable silica nanostructures, with potential generalization to other mineral systems. It is essential to understand the mechanisms of aggregation of the protein and its coprecipitation with silica. We continue previous investigations into the peptide R5, derived from silaffin protein sil1p, shown to independently catalyze the precipitation of silica nanospheres in vitro. We used the solid-state NMR technique 13C{29Si} and 15N{29Si} REDOR to investigate the structure and interactions of R5 in complex with coprecipitated silica. These experiments are sensitive to the strength of magnetic dipole-dipole interactions between the 13C nuclei in R5 and the 29Si nuclei in the silica and thus yield distance between parts of R5 and 29Si in silica. Our data show strong interactions and short internuclear distances of 3.74 ± 0.20 Å between 13C═O Lys3 and silica. On the other hand, the Cα and Cβ nuclei show little or no interaction with 29Si. This selective proximity between the K3 C═O and the silica supports a previously proposed mechanism of rapid silicification of the antimicrobial peptide KSL (KKVVFKVKFK) through an imidate intermediate. This study reports for the first time a direct interaction between the N-terminus of R5 and silica, leading us to believe that the N-terminus of R5 is a key component in the molecular recognition process and a major factor in silica morphogenesis.

Acidic Properties and Structure-Activity Correlations of Solid Acid Catalysts Revealed by Solid-State NMR Spectroscopy.

Solid acid materials with tunable structural and acidic properties are promising heterogeneous catalysts for manipulating and/or emulating the activity and selectivity of industrially important catalytic reactions. On the other hand, the performances of acid-catalyzed reactions are mostly dictated by the acidic features, namely, type (Brønsted vs Lewis acidity), amount, strength, and local environment of acid sites. The latter is relevant to their location (intra- vs extracrystalline), and possible confinement and Brønsted-Lewis acid synergy effects that may strongly affect the host-guest interactions, reaction mechanism, and shape selectivity of the catalytic system. This account aims to highlight some important applications of state-of-the-art solid-state NMR (SSNMR) techniques for exploring the structural and acidic properties of solid acid catalysts as well as their catalytic performances and relevant reaction pathway invoked. In addition, density functional theory (DFT) calculations may be exploited in conjunction with experimental SSNMR studies to verify the structure-activity correlations of the catalytic system at a microscopic scale. We describe in this Account the developments and applications of advanced ex situ and/or in situ SSNMR techniques, such as two-dimensional (2D) double-quantum magic-angle spinning (DQ MAS) homonuclear correlation spectroscopy for structural investigation of solid acids as well as study of their acidic properties. Moreover, the energies and electronic structures of the catalysts and detailed catalytic reaction processes, including the identification of reaction species, elucidation of reaction mechanism, and verification of structure-activity correlations, made available by DFT theoretical calculations were also discussed. Relevant discussions will focus primarily on results obtained from our laboratories in the past decade, including (i) quantitative and qualitative acidity characterization utilizing assorted probe molecules, (ii) probing the spatial proximity and synergy effect of acid sites, and (iii) influence of acid features and pore confinement effect on catalytic activity, transition-state stability, reaction pathway, and product selectivity of solid acid catalysts such as zeolites, metal oxides, and heteropolyacids. It is conclusive that a synergy of acidity (local effect) and pore confinement (environmental effect) tend to strongly dictate the formations of intermediates and transition states, hence, the reaction pathways and catalytic performance of solid acid catalysts. We hope that these information can provide additional insights toward our understanding in heterogeneous catalysis, especially the roles of structural and acidic properties on catalytic performances and reaction mechanism of acid-catalyzed systems, which should be beneficial for rational design of solid acid catalysts.

Solid-State NMR Study of Poly(3-Hydroxybutyrate) and Ecoflex reser Blends

Biodegradable blends of poly(3-hydroxybutyrate) and aromatic-aliphatic co-polyester Ecoflex® were studied by means of basic solid-state NMR techniques. C NMR spectra pointed at existence of individual components in blends, however, existence of regions in which components affect each other was also deduced from changes of shape of some spectral lines. Analysis of proton spin–lattice relaxation process in laboratory frame running in blends with different fractions of components revealed poor miscibility of these polymers. The domain size of components was calculated based on the values of spin–lattice relaxation times in laboratory frame. Spin–lattice relaxation process in the rotating frame of Ecoflex® proton spin system was only slightly influenced by the blending. Incompatibility of these polymers was confirmed by all realized experiments.

Synergy between structure direction and alkalinity toward fast crystallization, controlled morphology and high phase purity of ZSM-12 zeolite

Aiming for a control of the morphology and phase purity of the final product, two commercially available organic templates, supplying tetraethylammonium (TEA+) and methyltriethylammonium (MTEA+) cations in the synthesis mixture, were used for the hydrothermal synthesis of ZSM-12 zeolite. As key factors, alkalinity and water content of the synthesis mixture and silicon and aluminum source types were comprehensively studied using these two organic templates. All the products obtained were characterized by powder XRD, SEM, N2 physisorption, 29Si and 27Al-MAS solid state NMR and TGA techniques. Using TEAOH organic template and organic Si and Al sources along with the proper adjustment of water and sodium hydroxide contents in synthesis mixture, mono-sized polycrystalline particles of ZSM-12 with high crystallinity and purity were synthesized. Alleviating TEAOH drawbacks, MTEA+ organocation in the hydroxide form was used which led to the formation of mono-sized ZSM-12 single crystals. When using MTEAOH organic template, hydroxide ion and Na+ contents and the type of Al source were found to be playing major roles toward nucleation and growth of ZSM-12 crystals. Owing to increased alkalinity of the synthesis mixture, MTEAOH also promoted faster crystallization over a wide-range of Si/Al ratio (30 to ∼infinity). The discrete mono-dispersed synthesized crystals possessed a well-defined cubic morphology with no impurities.

Combining Solution and Solid State NMR Techniques in the Analysis of Arg to Cys Mutations in Phospholamban Associated with Dilated Cardiomyopathy

Dilated cardiomyopathy (DCM) is characterized by the enlargement and weakening of the left ventricle. This dilation of the ventricle prevents the heart from contracting normally and can spread to the right ventricle and atria as the disease progresses. Inherited forms of DCM have been identified in families that carry missense, deletion, or nonsense mutations in the regulatory protein, phospholamban (PLN). Two of the identified missense mutations, R9C and R25C, result in cysteine substitutions for arginine residues in PLN's cytoplasmic domain. In vitro studies with these mutants indicate that they have an altered ability to regulate SERCA, however it is unclear how these mutations lead to dysregulation of SERCA.Here we have applied both solution and solid-state NMR techniques to characterize the structure, dynamics and topology of the monomeric constructs for the R9C and R25C mutants to better understand how these mutations affect PLN's regulation of SERCA in the development of DCM. These studies were pursued under both reducing and oxidizing conditions to better reflect the physiological conditions of DCM progression and for comparison to coupled enzyme activity assays used to quantify the effect of these mutants on SERCA activity. We have utilized chemical shift perturbations as sensitive probes toward the local environment of the cytoplasmic and juxta-membrane domains of PLN which harbor the aforementioned mutations. Dynamics in lipid bilayers have been probed through cross-polarization or J-coupling enabled experiments which are exclusively sensitive toward either the rigid or mobile regions of the protein. Heteronuclear NMR was employed to probe the fast timescale dynamics of these mutants. Finally, separated local field experiments in magnetically aligned bicelles were employed to probe the topology.

Atomic Resolution Studies of Sterol Interactions by Solid-State NMR Spectroscopy

Sterols, vital and abundant components of plasma membranes, are involved in numerous cellular biochemical processes such as regulation of membrane properties and signaling events. However, these sterol interactions are not completely elucidated at the atomic level; this impedes further investigations into disorders related to these biological activities. Combining solid-state NMR (SSNMR), high yield biosynthesis of isotopically labeled sterols, and molecular dynamics, we aim to understand the key interactions that determine the sterol-specificity to amphotericin B (AmB), a powerful but toxic antifungal drug used to treat life-threatening fungal infections. AmB forms large aggregates that are neither crystalline nor soluble, making structural information difficult or impossible to obtain via x-ray crystallography or solution-state NMR. We have recently determined that AmB kills yeast cells primarily by binding to ergosterol1 (Erg) and forming a large extramembranous sterol sponge.2 These findings were enabled by the biosynthesis of fractionally 13C labeled Erg and state-of-the-art SSNMR techniques. The sterol sponge model hypothesizes that interactions of AmB with Erg determine its ability to kill yeast, whereas binding to cholesterol is responsible for determining its toxicity in human cells. SSNMR spectroscopy is uniquely able to detect and quantify the binding of sterols to AmB in the sterol sponge in atomistic detail. These studies provide a roadmap towards an improved therapeutic index3 for this drug while simultaneously elucidating additional details about the molecular mechanism of AmB-sterol interactions.(1)“Amphotericin primarily kills yeast by simply binding ergosterol”, Gray et al. Proc. Natl. Acad. Sci. 2011, 109, 2234-2239.(2)“Amphotericin forms an extramembranous sterol sponge”, Anderson et al. Nat. Chem. Bio. 2014, 10, 400-406.(3)“C2’-OH of amphotericin B plays an important role in binding the primary sterol of human cells but not yeast cells”, Wilcock et al. J. Am. Chem. Soc. 2013, 135, 8488-8491.

Rheological and solid-state NMR assessments of copovidone/clotrimazole model solid dispersions

This study aims to assess several model solid dispersions by using dynamic oscillatory rheology, solid-state NMR and other solid phase characterization techniques, and correlate their viscoelastic responses with processing methods and microstructures. A model active pharmaceutical ingredient (API), clotrimazole, was compounded with copovidone to form solid dispersions via various techniques with different mixing capabilities. Physicochemical characterizations of the resulting solid dispersions demonstrated that simple physical mixing led to a poorly mixed blend manifested by existence of large API crystalline content and heterogeneous distribution. Cryogenic milling significantly improved mixing of two components as a result of reduced particle size and increased contact surface area, but produced limited amorphous content. In contrast, hot melt extrusion (HME) processing resulted in a homogenous amorphous solid dispersion because of its inherent mixing efficiency. Storage modulus and viscosities versus frequency of different solid dispersions indicated that the incorporation of API into the polymer matrix resulted in a plasticizing effect which reduced the viscosity. The crystalline/aggregated forms of API also exhibited more elastic response than its amorphous/dispersed counterpart. Temperature ramps of the physical mixture with high API concentration captured a critical temperature, at which a bump was observed in damping factor. This bump was attributed to the dissolution of crystalline API into the polymer. In addition, heating–cooling cycles of various solid dispersions suggested that cryomilling and HME processing could form a homogeneous solid dispersion at low API content, whereas high drug concentration led to a relatively unstable dispersion due to supersaturation of API in the polymer.

Carbon-13 Solid State NMR Techniques to Evaluate the Morphology of PP/TiO<sub>2</sub> Composites

Solid state NMR was successfully used to determine the proton spin-lattice relaxation time in the rotating frame (T1ρH) for systems based on polypropylene (PP) and PP with titanium dioxide (TiO2) organically modified or unmodified incorporated, in order to understand the molecular behavior of these systems. These techniques were employed in the samples organically modified and unmodified TiO2 to investigate the effect of organic modification on the dispersion and distribution of the particles in the PP matrix. The results were analyzed in terms of the effect of the particles organic modified or not according to the intermolecular interaction in the composites. According to the T1ρH values, all composites showed at least two domains: the short values were related to the rigid part, which included the crystalline and amorphous phase constricted in it, while the longer times were attributed to the amorphous region, which had higher molecular mobility compared to the rigid region of the materials. The increase in the relaxation time parameter in the composites compared to the pure PP was associated to the strong interaction between titanium dioxide particles and the polymer chains. This effect was more pronounced for the systems containing organically modified TiO2. According to the results, it could be inferred that intermolecular interaction occurred in the CH2 and CH groups, being more intense with CH2 groups. Finally, the solid state NMR techniques were able to evaluate the molecular dynamics of those systems.

Characterising the role of water in sildenafil citrate by NMR crystallography

A combination of solid-state NMR techniques, including13C/1H correlation,2H magic-angle spinning NMR and first principles calculation are employed to characterise the role of water in different hydration states of sildenafil citrate.

Crystal Structure of the Glycophorin A Transmembrane Dimer in Lipidic Cubic Phase.

The mechanisms of assembly and function for many important type I/II (single-pass) transmembrane (TM) receptors are proposed to involve the formation and/or alteration of specific interfaces among their membrane-embedded α-helical TM domains. The application of lipidic cubic phase (LCP) bilayer media for crystallization of single-α-helical TM complexes has the potential to provide valuable structural and mechanistic insights into many such systems. However, the fidelity of the interfaces observed in crowded crystalline arrays has been difficult to establish from the very limited number of such structures determined using X-ray diffraction data. Here we examine this issue using the glycophorin A (GpA) model system, whose homodimeric TM helix interface has been characterized by solution and solid-state NMR and biochemical techniques but never crystallographically. We report that a GpA-TM peptide readily crystallized in a monoolein cubic phase bilayer, yielding a dimeric α-helical structure that is in excellent agreement with previously reported NMR measurements made in several different types of host media. These results provide compelling support for the wider application of LCP techniques to enable X-ray crystallographic analysis of single-pass TM interactions.

Effect of PEO molecular weight on the miscibility and dynamics in epoxy/PEO blends.

In this work, the effect of poly(ethylene oxide) (PEO) molecular weight in blends of epoxy (ER) and PEO on the miscibility, inter-chain weak interactions and local dynamics were systematically investigated by multi-frequency temperature modulation DSC and solid-state NMR techniques. We found that the molecular weight (M(w)) of PEO was a crucial factor in controlling the miscibility, chain dynamics and hydrogen bonding interactions between PEO and ER. A critical PEO molecular weight (M(crit)) around 4.5k was found. PEO was well miscible with ER when the molecular weight was below M(crit), where the chain motion of PEO was restricted due to strong inter-chain hydrogen bonding interactions. However, for the blends with high molecular weight PEO (M(w) > M(crit)), the miscibility between PEO and ER was poor, and most of PEO chains were considerably mobile. Finally, polarization inversion spin exchange at magic angle (PISEMA) solid-state NMR experiment further revealed the different mobility of the PEO in ER/PEO blends with different molecular weight of PEO at molecular level. Based on the DSC and NMR results, a tentative model was proposed to illustrate the miscibility in ER/PEO blends.

Effect of iron content on the structure and disorder of iron-bearing sodium silicate glasses: A high-resolution 29Si and 17O solid-state NMR study

Despite its geochemical importance and implications for the properties of natural magmatic melts, understanding the detailed structure of iron-bearing silicate glasses remains among the outstanding problems in geochemistry. This is mainly because solid-state NMR techniques, one of the most versatile experimental methods to probe the structure of oxide glasses, cannot be fully utilized for exploring the structural details of iron-bearing glasses as the unpaired electrons in Fe induce strong local magnetic fields that mask the original spectroscopic features (i.e., paramagnetic effect). Here, we report high-resolution 29Si and 17O solid-state NMR spectra of iron-bearing sodium silicate glasses (Na2O–Fe2O3–SiO2, Fe3+/ΣFe=0.89±0.04, thus containing both ferric and ferrous iron) with varying XFe2O3 [=Fe2O3/(Na2O+Fe2O3)], containing up to 22.9wt% Fe2O3. This compositional series involves Fe–Na substitution at constant SiO2 contents of 66.7mol% in the glasses. For both nuclides, the NMR spectra exhibit a decrease in the signal intensities and an increase in the peak widths with increasing iron concentration partly because of the paramagnetic effect. Despite the intrinsic difficulties that result from the pronounced paramagnetic effect, the 29Si and 17O NMR results yield structural details regarding the effect of iron content on Q speciation, spatial distribution of iron, and the extent of polymerization in the iron-bearing silicate glasses. The 29Si NMR spectra show an apparent increase in highly polymerized Q species with increasing XFe2O3, suggesting an increase in the degree of melt polymerization. The 17O 3QMAS NMR spectra exhibit well-resolved non-bridging oxygen (NBO, Na–O–Si) and bridging oxygen (BO, Si–O–Si) peaks with varying iron concentration. By replacing Na2O with Fe2O3 (and thus with increasing iron content), the fraction of Na–O–Si decreases. Quantitative consideration of this effect confirms that the degree of polymerization is likely to increase with iron content and that Fe3+ is predominantly a network-former. The 17O NMR spectra suggest a moderate degree of preferential partitioning of iron between NBO and BO clusters. The present results bear strong promise for studying iron-bearing silicate glasses using solid-state NMR techniques, constraining the effect of iron content on the degree of polymerization. The observed changes in atomic structures of iron-bearing sodium silicate glasses will be helpful for unraveling atomic origins of the properties of natural silicate melts.

Plant Species Rather Than Climate Greatly Alters the Temporal Pattern of Litter Chemical Composition During Long-Term Decomposition.

A feedback between decomposition and litter chemical composition occurs with decomposition altering composition that in turn influences the decomposition rate. Elucidating the temporal pattern of chemical composition is vital to understand this feedback, but the effects of plant species and climate on chemical changes remain poorly understood, especially over multiple years. In a 10-year decomposition experiment with litter of four species (Acer saccharum, Drypetes glauca, Pinus resinosa, and Thuja plicata) from four sites that range from the arctic to tropics, we determined the abundance of 11 litter chemical constituents that were grouped into waxes, carbohydrates, lignin/tannins, and proteins/peptides using advanced 13C solid-state NMR techniques. Decomposition generally led to an enrichment of waxes and a depletion of carbohydrates, whereas the changes of other chemical constituents were inconsistent. Inconsistent convergence in chemical compositions during decomposition was observed among different litter species across a range of site conditions, whereas one litter species converged under different climate conditions. Our data clearly demonstrate that plant species rather than climate greatly alters the temporal pattern of litter chemical composition, suggesting the decomposition-chemistry feedback varies among different plant species.

Applications of DQ-DRENAR for the structural analysis of phosphate glasses

A new solid state NMR technique entitled DQ-DRENAR (Double-Quantum based Dipolar Recoupling Effects Nuclear Alignment Reduction) has been recently described for measuring homonuclear dipole–dipole interactions in multi-spin-1/2 systems under magic-angle spinning conditions. As in rotational echo double resonance (REDOR), the homonuclear dipole–dipole coupling constant can be extracted from a plot of a normalized difference signal (S0−S′)/S0 versus dipolar mixing time, where S is the signal amplitude with the DQ-Hamiltonian present, and S0 is the signal amplitude in the absence of the DQ-Hamiltonian, which is used for normalization. Within the range of (S0−S)/S0≤0.3–0.5 such “homonuclear REDOR curves” can be approximated by simple parabolae, yielding effective squared dipole–dipole coupling constants ∑bjk2 summed over all the pairwise interactions present. The effect of glassy disorder has been studied by simulations, replacing singular-valued internuclear distances by Gaussian distance distributions with the same central value. This situation results in a systematic over-estimation effect, which tends to compensate the implicit under-estimation effect caused by the parabolic fitting approach. The present contribution describes applications to a number of phosphate-based glasses and glass ceramics. The method turns out to be well suited for the differentiation of the various Q(n) phosphate species, for characterizing the spatial distribution of isolated orthophosphate ions and for the detection of incipient nano-segregation and/or phase separation effects in glass ceramics.