Solid-state 13C Nuclear Magnetic Resonance Spectra Research Articles

Vinyl and methyl-ester groups in the insoluble polymer drug patiromer identified and quantified by solid-state NMR

Patiromer (Veltassa®) is a crosslinked, insoluble co-polymer drug used as a nonabsorbent potassium binder, approved for treatment of hyperkalemia. Quantitative solid-state 13C nuclear magnetic resonance (NMR) analysis with comprehensive peak assignment, component quantification, and calculation of mole and weight fractions of monomer units was performed on three doses of patiromer. The workflow is documented in detail. Spectrally edited solid-state 13C NMR spectra of patiromer show =CHn peaks of matching intensity at 116 and 141 ppm, characteristic of -CH=CH2 vinyl groups. Similar spectral features can be observed in earlier studies but were previously ignored. In this study, the vinyl signals are well-resolved in a 2-s direct polarization (DP) spectrum without and with dipolar dephasing, which confirms that these sp2-hybridized carbons are bonded to hydrogen and partially mobile, consistent with vinyl side groups from incompletely reacted divinyl crosslinkers. The vinyl groups account for 1.6% of all carbon, 3% of the monomer units, and nearly 1/3 of the crosslinkers. Furthermore, an unexpected OCH3 moiety accounting for ∼1.2% of all carbons was identified by spectral editing; its chemical shift of 54 ppm is more consistent with a methyl ester than with a methyl ether. It can originate from incomplete hydrolysis of ∼6% of methyl-2-fluoroacrylate, the main monomer of patiromer. Characteristic cross peaks in two-dimensional 1H–13C heteronuclear correlation NMR confirm the presence of the vinyl and OCH3 groups. Trace amounts of xanthan gum are also detected. The quantitative 13C NMR spectrum of patiromer has been matched in a simulation using a model with five monomer units.

Solid-state NMR spectroscopy of roasted and ground coffee samples: Evidences for phase heterogeneity and prospects of applications in food screening

The advancement in the use of spectroscopic techniques to investigate coffee samples is of high interest especially considering the widespread problems with coffee adulteration and counterfeiting. In this work, the use of solid-state nuclear magnetic resonance (NMR) is investigated as a means to probe the various chemically-distinct phases existent in roasted coffee samples and to detect the occurrence of counterfeiting or adulterations in coffee blends. Routine solid-state 1H and 13C NMR spectra allowed the distinction between different coffee types (Arabica/Robusta) and the evaluation of the presence of these components in coffee blends. On the other hand, the use of more specialized solid-state NMR experiments revealed the existence of phases with different molecular mobilities (e.g., associated with lipids or carbohydrates). The results illustrate the usefulness of solid-state NMR spectroscopy to examine molecular mobilities and interactions and to aid in the quality control of coffee-related products.

Modeling of Antioxidant Activity, Polyphenols and Macronutrients Content of Bee Pollen Applying Solid-State 13C NMR Spectra.

An application of solid 13C nuclear magnetic resonance (NMR) spectroscopy for the determination of macronutrients, total polyphenols content, antioxidant activity, N C S elements, and pH in commercially available bee pollens is reported herein. Solid-state 13C NMR spectra were recorded for homogenized pollen granules without chemical treatment or dissolution of samples. By combining spectral data with the results of reference analyses, partial least squares models were constructed and validated separately for each of the studied parameters. To characterize and compare the models’ quality, the relative standard errors of prediction (RSEP) were calculated for calibration and validation sets. In the case of the analysis of protein, fat and reducing sugars, these errors were in the 1.8–2.5% range. Modeling the elemental composition of bee pollen on the basis of 13C NMR spectra resulted in RSEPcal/RSEPval values of 0.3/0.6% for the sum of NHCS elements, 0.3/0.4% for C, 1.8/1.9% for N, and 4.2/6.1% for S quantification. Analyses of total phenolics and ABTS antioxidant activity resulted in RSEP values in the 2.7–3.5% and 2.8–3.8% ranges, respectively, whereas they were 1.4–2.1% for pH. The obtained results demonstrate the usefulness of 13C solid-state NMR spectroscopy for direct determination of various important physiochemical parameters of bee pollen.

Enhanced NMR Discrimination of Pharmaceutically Relevant Molecular Crystal Forms through Fragment-Based Ab Initio Chemical Shift Predictions.

Chemical shift prediction plays an important role in the determination or validation of crystal structures with solid-state nuclear magnetic resonance (NMR) spectroscopy. One of the fundamental theoretical challenges lies in discriminating variations in chemical shifts resulting from different crystallographic environments. Fragment-based electronic structure methods provide an alternative to the widely used plane wave gauge-including projector augmented wave (GIPAW) density functional technique for chemical shift prediction. Fragment methods allow hybrid density functionals to be employed routinely in chemical shift prediction, and we have recently demonstrated appreciable improvements in the accuracy of the predicted shifts when using the hybrid PBE0 functional instead of generalized gradient approximation (GGA) functionals like PBE. Here, we investigate the solid-state 13C and 15N NMR spectra for multiple crystal forms of acetaminophen, phenobarbital, and testosterone. We demonstrate that the use of the hybrid density functional instead of a GGA provides both higher accuracy in the chemical shifts and increased discrimination among the different crystallographic environments. Finally, these results also provide compelling evidence for the transferability of the linear regression parameters mapping predicted chemical shieldings to chemical shifts that were derived in an earlier study.

Synthesis of graphite oxide from milled graphite studied by solid-state 13C nuclear magnetic resonance

In the present work we investigate the influence of the degree of crystallinity of graphite on the production of graphite oxide (GO). Graphite precursors with varying degrees of crystallinity were produced by milling a high-purity synthetic graphite powder during different times and the milled materials were used for the synthesis of GO, using a modified Hummers method. The products were characterized by X-ray diffraction (XRD), thermogravimetry, solid-state 13C nuclear magnetic resonance (NMR), elemental and textural analysis. 13C NMR spectra revealed the presence of epoxy, hydroxyl, carbonyl and lactol groups in the oxides. However, the oxidation degree was found to be largely dependent on the milling time, with the best GO samples being produced from graphites with intermediate milling times. These results were rationalized in terms of the structural changes brought about by the milling process, as revealed by the solid-state 13C NMR spectra and the XRD patterns obtained for the milled graphites.

Characterization of the ionic clathrate hydrate of tetra-n-butylammonium acrylate

The ionic clathrate hydrate of tetra-n-butylammonium (TBA) acrylate was characterized using single-crystal X-ray diffraction, elemental analysis, and nuclear magnetic resonance (NMR) spectroscopy. The crystal structure of TBA acrylate was Jeffrey’s type III and tetragonal P42/n, with a 33.076(7) × 33.076(7) × 12.170(2) Å3 unit cell. The volume of the unit cell was 13315(5) Å3, which is almost twice that of the ideal structure. The TBA cation was disordered and located in two types of fused cages. Although the acrylate anion was located in a pentagonal dodecahedral cage neighboring the TBA cation, there is a residual acrylate anion that could be around the other TBA cation in the unit cell. Solid-state 13C NMR spectra showed that the TBA cation was clearly disordered at 173 K, but not at 239 K. NMR peaks from the acrylate anion were not observed at either temperature. This is probably because of the strong restriction on the acrylate anion by hydrogen bonding with the lattice water. Some of the characteristics of the anion and cation of the ionic guest incorporated in the hydrate structure have yet to be defined. Further research is needed to clarify complexation of the ionic clathrate hydrate and the ionic guest, and the resulting structure.

Synthesis and Characterization of Rare Earth Siloxide Complexes, M[OSi(OtBu)3]3(L)x where L is HOSi(OtBu)3 and x = 0 or 1

The reaction of lanthanide and group 3 amides, M[N(SiMe3)3]3, with 3 equiv of tris(tert-butoxy)silanol, (tBuO)3SiOH, gives M[OSi(OtBu)3]3 complexes. Their solid-state structure depends on the size of the lanthanide metal; for M = Sc, Yb, and Lu, the primary coordination sphere is a square-based pyramid in which two siloxides are κ2-coordinated and one is κ1-coordinated. For La, Nd, and Sm, dimeric structures in which the bridging -OSi(OtBu)3 group is κ3-coordinated are formed. When 4 equiv of silanol is used, the resulting molecules crystallize as 4:1 Si:M adducts, in which a neutral silanol remains coordinated, i.e., M[(OSi(OtBu)3)3](HOSi(OtBu)3). For M = Sc, Y, Eu, Yb, and Lu, the solid-state structures are based on five-coordinate M, M(OSi(OtBu)3)3(κ2-HOSi(OtBu)3). For M = Ce, Nd, and Sm, the coordination number of the metal reaches six in molecules with the general formula M(OSi(OtBu)3)2(κ2-OSi(OtBu)3)(κ2-HOSi(OtBu)3). In solution, the 1H nuclear magnetic resonance (NMR) spectra are fluxional, resulting in all tBu groups being chemically equivalent. The solid-state 1H, 13C, and 29Si NMR spectra are consistent with the stereochemistry found in the X-ray crystal structure.

Transformations of the chemical compositions of high molecular weight DOM along a salinity transect: Using two dimensional correlation spectroscopy and principal component analysis approaches

In a study of chemical transformations of estuarine high-molecular-weight (HMW, >1000Da) dissolved organic matter (DOM) collected over a period of two years along a transect through the Elizabeth River/Chesapeake Bay system to the coastal Atlantic Ocean off Virginia, USA, δ13C values, N/C ratios, and principal component analysis (PCA) of the solid-state 13C NMR (nuclear magnetic resonance) spectra of HMW-DOM show an abrupt change in both its sources and chemical structural composition occurring around salinity 20. HMW-DOM in the lower salinity region had lighter isotopic values, higher aromatic and lower carbohydrate contents relative to that in the higher salinity region. These changes around a salinity of 20 are possibly due to introduction of a significant amount of new carbon (autotrophic DOM) to the transect. PC-1 loadings plot shows that spatially differing DOM components are similar to previously reported 13C NMR spectra of heteropolysaccharides (HPS) and carboxyl-rich alicyclic molecules (CRAM).Applying two dimensional correlation spectroscopy techniques to 1H NMR spectra from the same samples reveals increases in the contribution of N-acetyl amino sugars, 6-deoxy sugars, and sulfated polysaccharides to HPS components along the salinity transect, which suggests a transition from plant derived carbohydrates to marine produced carbohydrates within the HMW-DOM pool. In contrast to what has been suggested previously, our combined results from 13C NMR, 1H NMR, and FTIR indicate that CRAM consists of at least two different classes of compounds (aliphatic polycarboxyl compounds and lignin-like compounds).

The role of biodegradation and photo-oxidation in the transformation of terrigenous organic matter

Microbial and photochemical decomposition are two major processes regulating organic matter (OM) transformation in the global carbon cycle. However, photo-oxidation is not as well understood as biodegradation in terms of its impact on OM alteration in terrigenous environments. We examined microbial and photochemical transformation of OM and lignin derived phenols in two plant litters (corn leaves and pine needles). Plant litter was incubated in the laboratory over 3 months and compositional changes to OM were measured using nuclear magnetic resonance (NMR) and gas chromatography–mass spectrometry. We also examined the susceptibility of soil organic matter (SOM) to ultraviolet (UV) radiation. Solid-state 13C NMR spectra showed that O-alkyl type structures (mainly from carbohydrates) decreased during biodegradation and the loss of small carbohydrates and aliphatic molecules was observed by solution-state 1H NMR spectra of water extractable OM from biodegraded litters. Photochemical products were detected in the aliphatic regions of NaOH extracts from both litter samples by solution-state 1H NMR. Photo-oxidation also increased the solubility of SOM, which was attributed to the enhanced oxidation of lignin derived phenols and photochemical degradation of macromolecular SOM species (as observed by diffusion edited 1H NMR). Overall, our data collectively suggests that while biodegradation predominates in litter decomposition, photo-oxidation alters litter OM chemistry and plays a role in destabilizing SOM in soils exposed to UV radiation.

Influence of the nature of soil organic matter on the sorption behaviour of pentadecane as determined by PLS analysis of mid-infrared DRIFT and solid-state 13C NMR spectra

The nature of soil organic matter (SOM) functional groups associated with sorption processes was determined by correlating partitioning coefficients with solid-state 13C nuclear magnetic resonance (NMR) and diffuse reflectance mid-infrared (DRIFT) spectral features using partial least squares (PLS) regression analysis. Partitioning sorption coefficients for n-pentadecane ( n-C 15) were determined for three alternative models: the Langmuir model, the dual distributed reactive domain model (DRDM) and the Freundlich model, where the latter was found to be the most appropriate. NMR-derived constitutional descriptors did not correlate with Freundlich model parameters. By contrast, PLS analysis revealed the most likely nature of the functional groups in SOM associated with n-C 15 sorption coefficients ( K F) to be aromatic, possibly porous soil char, rather than aliphatic organic components for the presently investigated soils. High PLS cross-validation correlation suggested that the model was robust for the purpose of characterising the functional group chemistry important for n-C 15 sorption.

Quantitative Determination of Site-Specific Conformational Distributions in an Unfolded Protein by Solid-State Nuclear Magnetic Resonance

Solid-state nuclear magnetic resonance (NMR) techniques are used to investigate the structure of the 35-residue villin headpiece subdomain (HP35) in folded, partially denatured, and fully denatured states. Experiments are carried out in frozen glycerol/water solutions, with chemical denaturation by guanidine hydrochloride (GdnHCl). Without GdnHCl, two-dimensional solid-state 13C NMR spectra of samples prepared with uniform 13C labeling of selected residues show relatively sharp cross-peaks at chemical shifts that are consistent with the known three-helix bundle structure of folded HP35. At high GdnHCl concentrations, most cross-peaks broaden and shift, qualitatively indicating disruption of the folded structure and development of static conformational disorder in the frozen denatured state. Conformational distributions at one residue in each helical segment are probed quantitatively with three solid-state NMR techniques that provide independent constraints on backbone ϕ and ψ torsion angles in samples with sequential pairs of carbonyl 13C labels. Without GdnHCl, the combined data are well fit by α-helical conformations. At [GdnHCl]=4.5 M, corresponding to the approximate denaturation midpoint, the combined data are well fit by a combination of α-helical and partially extended conformations at each site, but with a site-dependent population ratio. At [GdnHCl]=7.0 M, corresponding to the fully denatured state, the combined data are well fit by a combination of partially extended and polyproline II conformations, again with a site-dependent population ratio. Two entirely different models for conformational distributions lead to nearly the same best-fit distributions, demonstrating the robustness of these conclusions. This work represents the first quantitative investigation of site-specific conformational distributions in partially folded and unfolded states of a protein by solid-state NMR.

Seeded growth of β-amyloid fibrils from Alzheimer's brain-derived fibrils produces a distinct fibril structure

Studies by solid-state nuclear magnetic resonance (NMR) of amyloid fibrils prepared in vitro from synthetic 40-residue beta-amyloid (Abeta(1-40)) peptides have shown that the molecular structure of Abeta(1-40) fibrils is not uniquely determined by amino acid sequence. Instead, the fibril structure depends on the precise details of growth conditions. The molecular structures of beta-amyloid fibrils that develop in Alzheimer's disease (AD) are therefore uncertain. We demonstrate through thioflavin T fluorescence and electron microscopy that fibrils extracted from brain tissue of deceased AD patients can be used to seed the growth of synthetic Abeta(1-40) fibrils, allowing preparation of fibrils with isotopic labeling and in sufficient quantities for solid-state NMR and other measurements. Because amyloid structures propagate themselves in seeded growth, as shown in previous studies, the molecular structures of brain-seeded synthetic Abeta(1-40) fibrils most likely reflect structures that are present in AD brain. Solid-state (13)C NMR spectra of fibril samples seeded with brain material from two AD patients were found to be nearly identical, indicating the same molecular structures. Spectra of an unseeded control sample indicate greater structural heterogeneity. (13)C chemical shifts and other NMR data indicate that the predominant molecular structure in brain-seeded fibrils differs from the structures of purely synthetic Abeta(1-40) fibrils that have been characterized in detail previously. These results demonstrate a new approach to detailed structural characterization of amyloid fibrils that develop in human tissue, and to investigations of possible correlations between fibril structure and the degree of cognitive impairment and neurodegeneration in AD.

Characterization of plant-derived water extractable organic matter by multiple spectroscopic techniques

Water extractable organic matter (WEOM) derived from fresh- or early-stage decomposing soil amendment materials may play an important role in the process of organic matter accumulation. In this study, eight WEOM samples extracted with a 40:1 (v/w) water to sample ratio from alfalfa (Medicago sativa L.), corn (Zea mays L.), crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa L.), lupin (Lupinus albus L.), soybean (Glycine max L. Merr.), wheat (Triticum aestivum L.), and dairy manure were investigated using ultraviolet (UV)–visible, Fourier transform infrared (FT-IR), solution 31P nuclear magnetic resonance (NMR), and solid state 13C NMR spectroscopies. UV–visible and FT-IR spectra of the plant-derived WEOM samples were typical for natural organic matter, but possessed less humic-like characteristics than dairy manure-derived WEOM. Solution 31P NMR spectra indicated that WEOM samples extracted from alfalfa, corn, and soybean shoots contained both orthophosphate and monoester P. Of the monoester P in WEOM from soybean shoot, 70% was phytate P. WEOM from crimson clover, hairy vetch, lupin, and wheat shoots contained orthophosphate only. The solid-state 13C NMR spectra of the seven plant-derived WEOM samples indicated that they all were primarily composed of sugars, amino acids or peptides, and low molecular mass carboxylic acids. Carbohydrates were dominant components with very few aromatics present in these samples. In addition, WEOM from crimson clover and lupin, but not other three leguminous plant WEOM samples, contained significant asparagine. On the other hand, WEOM from corn and wheat contained less amino acids or peptides. The spectra of WEOM of dairy manure revealed the presence of significant amounts of nonprotonated carbons and lignin residues, suggesting humification of the manure-derived WEOM. Significant carbohydrates as well as aromatics were present in this WEOM. The P and C bonding information for these WEOM samples may be useful for understanding the effects of WEOM on soil nutrient availability to plants.

Condensation degree of burnt peat and plant residues and the reliability of solid-state VACP MAS 13C NMR spectra obtained from pyrogenic humic material

Charred organic matter is assumed to contain heavily condensed polycyclic aromatic domains with a considerable proportion of core carbons. To examine their possible underestimation using variable amplitude (VA) cross polarization (CP) magic angle spinning (MAS) 13C nuclear magnetic resonance (NMR), the condensation degree of a peat subjected to thermal oxidation at 350 °C for up to 180 s was examined by means of elemental analysis and various NMR techniques. For comparison, commercially available barbeque charcoal and charred organic residues collected from a fire-affected soil from Southern Spain were analyzed. The atomic H/C ratios of the aromatic moiety of the chars revealed that on average every second to third C is connected to a proton. Dipolar dephasing NMR data confirm the high extent of and are in agreement with relatively small clusters of six aromatic rings connected by approximately two bridging CS. Alternatively, benzanthracene-like structures with two to five bridging CS are possible. The CP time ( T CH) and the 1H and 13C spin lattice relaxation times in the rotating frame ( T 1ρH; T 1ρC) of the chars suggest the presence of crystalline and amorphous domains. Both fractions fulfill the condition T CH < t c < T 1ρH which is required for C-detection with the CP-technique. Reliability of the CP-technique was supported by experiments with standards consisting of charred material and untreated peat mixed in defined ratios. The analysis of 13C spin lattice relaxation indicated saturation of some O-alkyl and alkyl C in the Bloch decay (BD) NMR spectrum of the untreated peat. This may explain its higher aromatic C intensity compared to the respective CP MAS NMR spectrum.

Intimate association between O/N-alkyl carbon and iron oxides in clay fractions of forest soils

Iron oxides with high specific surface areas promote the stabilisation of soil organic matter. We compared solid state 13C nuclear magnetic resonance (NMR) spectra of clay fractions from A horizons before and after demineralisation with 10% hydrofluoric acid (HF) to study selective associations between Fe oxides and functional groups in soil organic matter. The samples were collected from soils under temperate forest conditions, classified as Cambisols, Leptosols, Luvisols and Phaeozem. After removal of Fe oxides by demineralisation, solid state 13C NMR spectra of clay fractions from Leptosols and Luvisols showed an increase in the relative O/N-alkyl C intensity that ranged from 3% to 20%. A variable contact time experiment with a sample from a Luvisol without HF treatment showed that protons in the vicinity of O/N-alkyl C relaxed faster than in the vicinity of other functional groups. Therefore, part of the O/N-alkyl C was invisible unless very short contact times were applied. This demonstrated that Fe oxides and O/N-alkyl C are preferentially associated in soils. The O/N-alkyl C signal loss in the untreated samples was strongly correlated with the ratio of dithionite-extractable iron oxides and the organic carbon (OC) concentration. Our study showed that OC:Fe ratios >1 do not necessarily assure that reasonable solid state 13C NMR spectra will be obtained. In fact, clay fractions with OC:Fe ratios between 2.2 and 3.8 were affected by association between Fe oxides and O/N-alkyl C and had to be treated with HF to circumvent a selective signal loss of O/N-alkyl C.

Solid‐State Carbon‐13 Nuclear Magnetic Resonance of Humic Acids at High Magnetic Field Strengths

Use of solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has become commonplace in studies of humic substances in soils and sediments, but when modern high-field spectrometers are employed, care must be taken to ensure that the data obtained accurately reflect the chemical composition of these complex materials in environmental systems. In an effort to evaluate the quality of solid-state 13C NMR spectra obtained with modern high-field spectrometers, we conducted a series of experiments to examine spectra of various humic acids taken under a variety of conditions. We evaluate conditions for obtaining semiquantitative cross polarization magic angle spinning (CPMAS) 13C NMR spectra of humic acids at high magnetic field and spinning frequency. We examine the cross polarization (CP) dynamics under both traditional and ramp CP conditions on Cedar Creek humic acid. Fitted equilibrium intensities from these CP dynamic studies compare to within 3.4% of the intensities determined from a Bloch decay spectrum of the same sample. With a 1-ms contact time, ramp CP and traditional CP spectra were acquired on this sample and were found to compare to within 5.4% of the Bloch decay spectrum; however, the signal-to-noise ratio per hour of data acquisition was found to double under ramp CP conditions. These results demonstrate the power of applying modern solid-state NMR techniques at high magnetic field strengths. With these techniques, high-quality, semiquantitative spectra can be quickly produced, allowing the application of solid-state NMR techniques to more environmentally relevant samples, especially those where the quantity is limited.

Conversion of polycarbosilane (PCS) to SiC-based ceramic Part 1. Characterisation of PCS and curing products

A commercial polycarbosilane (PCS) preceramic polymer has been characterised as-received and following curing under a variety of conditions. Elemental analysis, gel permeation chromatography (GPC), infra-red spectroscopy (FT-IR), simultaneous thermogravimetric analysis-differential thermal analysis (TG-DTA) and solid state nuclear magnetic resonance (NMR) have been employed. A number average molar mass of 1200 was found with a broad molar mass distribution ( $$\overline M _{\text{w}} $$ / $$\overline M _{\text{n}} $$ = 2.97). Elemental analysis gave an empirical formula of SiC2.2H5.3O0.3. IR and Solid state 29Si and 13C NMR spectra showed the presence of Si-O-Si, SiC4, SiC3H, Si-Si, Si-CH3 and Si-CH2 groups. Simultaneous TG-DTA performed under an argon flow showed that there was a weight gain which started at approximately 240 °C. DTA showed an exotherm starting at this temperature showing that there was oxidation of the polymer even in an inert atmosphere. This is perhaps due to the oxygen in the PCS and there may also be some impurities in the inert atmosphere. Evidently the PCS is very sensitive to oxygen. Above 500 °C, weight loss dominated although the exotherm continued to approximately 700 °C. The effect of heating rate and dwell time at 200 °C on the changes in the chemical composition during curing have been explored using IR and solid state NMR spectroscopies, and elemental analysis. The longer the cure time the higher was the weight gain and greater was the extent of the oxidation reactions. Elemental analysis showed that the ratio of H and C to Si decreased with holding time at the cure-temperature while the amount of oxygen increased. Use of a higher heating rate resulted in a lower weight gain when the same holding time was used. From this it is clear that curing starts below the holding temperature.

Soil organic matter transformations induced by Hieracium pilosella L. in tussock grassland of New Zealand

To study the effect of Hieracium pilosella L. invasion on the transformations of soil organic matter of New Zealand tussock grassland soils (Ustochrepts), plant material and soils underneath Hieracium, the surrounding halo, and the adjacent herbfield (depleted tussock grassland) were examined for their chemical composition. An attempt was made to reveal possible changes in chemical composition of the soil organic matter induced by H. pilosella invasion. Small differences were detected by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy in the composition of the plant and soil materials from these zones. Most of the differences in soil organic matter occurred due to differences in the amount and quality of plant-residue inputs. Comparable amounts of phenolic C were detected in the solid-state 13C NMR spectra of H. pilosella and herbfield vegetation, while alkaline CuO oxidation yielded considerable lower lignin oxidation products for H. pilosella. A slightly higher proportion of these compounds in H. pilosella soil revealed an accumulation and a low degradation rate of lignin compounds under H. pilosella. The HCl hydrolysis and solid-state 15N NMR spectroscopy showed similar chemical compositions of the N fractions of the three different soils. The absence of 15N NMR signal intensity assignable to aniline derivatives or aromatic heterocyclic N indicates that the condensation of phenolic compounds with N groups plays a minor role in N sequestration in these soils.

The use of spin counting for determining quantitation in solid state 13C NMR spectra of natural organic matter : 1. Model systems and the effects of paramagnetic impurities

The degree of quantitation achieved in the solid state 13C nuclear magnetic resonance (NMR) spectra of a number of organic materials, an HF-treated soil, and a whole soil, was determined for both cross-polarisation (CP) and Bloch decay (BD) techniques using spin-counting experiments. The percentage of potential 13C NMR signal, which was actually observed ( C obs) was in the range 79–107% for the BD technique and in the range 29–103% for the CP technique. A number of materials, including cellulose, pectin, lignin, and palmitic acid gave quantitative spectra using both CP and BD. A second group, including chitin, collagen, and the HF-treated soil gave quantitative BD spectra, but significantly diminished CP spectra ( C obs-CP=66–75%). A third group including charcoal, a commercial humic acid, and the whole soil gave significantly diminished BD spectra ( C obs-BD=79–87%) and severely diminished CP spectra ( C obs-CP=29–35%). Signal losses in the CP spectra were attributed to rapid relaxation rates (short T 1ρH) and/or slow magnetisation build-up rates (long T CH). The spin dynamics of the CP experiment were studied and a new method for correcting for differences in T 1ρH between the sample and the reference in CP spin-counting experiments was developed. Signal produced by the Kel-F rotor end-caps was significant for the BD spectra and a correction for the end-cap spectrum was required prior to spin counting. The low 1H content of the fluorinated Kel-F polymer ensured that the contribution of end-caps to the CP spectra was insignificant. The effects of paramagnetic cations on quantitation in solid state NMR spectra was investigated by doping model compounds with paramagnetic impurities. Three mechanisms, which bring about signal loss and operate on three different length scales, were identified. The magnitude of the signal loss brought about by a paramagnetic material was shown to be dependent on both the type of cation involved and on the intimacy of contact with the organic matrix. Chemically bound paramagnetic cations were shown to result in large signal losses in the CP spectrum, whereas paramagnetic salts in a physical mixture with an organic material affected both CP and BD spectra equally.

Curing chemistry of phenylethynyl-terminated imide oligomers: Synthesis of13C-labeled oligomers and solid-state NMR studies

4-(Phenylethynyl-α,β-13C)phthalic anhydride (PEPA) and 13C-labeled phenylethynyl-terminated imide (PETI) oligomers were synthesized, and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was used to determine the structure of cured oligomers. Solid-state 13C NMR spectra were collected before and after thermal curing. Using solid-state 13C NMR difference spectroscopy, several cure products were identified. The observed 13C NMR resonances were assigned to four different classes of cure products: aromatics, products from backbone addition (substituted stilbenes and tetraphenylethanes), polyenes, and cyclobutadiene cyclodimers. The effects of postcuring and oligomer chain length on the structure of the cured resins were examined. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3486–3497, 2000