High-resolution Solid-state 13C NMR Spectroscopy Research Articles

Understanding the Solid-State Structure of Riboflavin through a Multitechnique Approach.

Crystalline riboflavin (vitamin B2) performs an important biological role as an optically functional material in the tapetum lucidum of certain animals, notably lemurs and cats. The tapetum lucidum is a reflecting layer behind the retina, which serves to enhance photon capture and vision in low-light settings. Motivated by the aim of rationalizing its biological role, and given that the structure of biogenic solid-state riboflavin remains unknown, we have used a range of experimental and computational techniques to determine the solid-state structure of synthetic riboflavin. Our multitechnique approach included microcrystal XRD, powder XRD, three-dimensional electron diffraction (3D-ED), high-resolution solid-state 13C NMR spectroscopy, and dispersion-augmented density functional theory (DFT-D) calculations. Although an independent report of the crystal structure of riboflavin was published recently, our structural investigations reported herein provide a different interpretation of the intermolecular hydrogen-bonding arrangement in this material, supported by all the experimental and computational approaches utilized in our study. We also discuss, more generally, potential pitfalls that may arise in applying DFT-D geometry optimization as a bridging step between structure solution and Rietveld refinement in the structure determination of hydrogen-bonded materials from powder XRD data. Finally, we report experimental and computational values for the refractive index of riboflavin, with implications for its optical function.

Mechanochemical synthesis of mechanical bonds in M12L8 poly-[n]-catenanes.

Using mechanochemistry by grinding TPB and ZnBr2, an amorphous poly-[n]-catenane of interlocked M12L8 nanocages is obtained in good yields (∼80%) and within 15 minutes. The mechanical bond among the icosahedral M12L8 cages in the amorphous phase has been demonstrated by single crystal XRD, powder XRD and FT-IR spectroscopy following an amorphous-to-crystalline transformation by guest uptake of the amorphous phase. High-resolution solid-state 13C NMR spectroscopy gives insights into the local structure of the amorphous catenane focusing on TPB aromatic-aromatic interactions.

Evaluation of chitosan crystallinity: A high-resolution solid-state NMR spectroscopy approach

We propose a novel approach relied on high-resolution solid-state 13C NMR spectroscopy to quantify the crystallinity index of chitosans (Ch) prepared with variable average degrees of acetylation (DA¯) from 5% to 60 % and average weight molecular weight (M¯w) ranged in 0.15 × 106 g mol−1–1.2 × 106 g mol−1. The Dipolar Chemical Shift Correlation (DIPSHIFT) curve of the C(6)OH segment revealed increased mobility dynamic, which induced different distribution from trans-to-gauche conformations in relation to C(4). Indeed, 1H-13C Heteronuclear Correlation (2D HETCOR) showed that distinguished C4 chemical shifts correlates with the same aliphatic protons. The short-range ordering can be assigned to C4/C6 signals on 13C CPMAS and, for our case, the deconvolution procedure between disordered and ordered phases revealed increasing crystallinity with DA¯, as confirmed by SVD multivariate analysis. This work extended the knowledge regarding the use of 13C CPMAS technique to predict the crystallinity of chitosans without the use of amorphous standards.

Investigation of the network structure of new bisphenol A non-intent interior food can coatings using high-resolution solid-state 13C-NMR spectroscopy

Facing recent controversial debates about commonly used epoxy resins in food contact applications, an understanding of newly developed polyester-based BPA-non-intent (BPA-NI) interior food can coatings is necessary. So far, little is known about the polymer structure of the polyester-based blends which have been brought to the European market.We report a detailed study on a polyester-phenol blend model system suitable for interior can coating in terms of its mechanical properties such as high flexibility, very good adhesion and hardness. We employ vibrational spectroscopy (FTIR), thermoanalysis (DSC) and an in-depth high-resolution solid-state 13C-NMR analysis to reveal that an overall homogeneity on molecular scale of the two main components in the blend is sustained in the solid state. This is remarkable, taking into account the overall different characteristics of polyesters and phenolic resins and explains the exceptional material properties of this coating.

Structural features of carbon products: an NMR study

Structural characteristics of a series of carbon products and intermediates of carbonization of some polymers were studied by high-resolution solid-state 13C NMR spectroscopy, with comparison of the experimental and calculated spectroscopic data.

ChemInform Abstract: High-Resolution Solid-State NMR Spectroscopy in Studies of Conversions of Hydrocarbons and Alcohols on Zeolites

The review surveys advances in high-resolution solid-state 1H and 13C NMR spectroscopy applied to studies of conversions of hydrocarbons and alcohols on zeolite catalysts of an acidic nature. The potential of NMR spectroscopy in studies of mechanisms of chemical reactions and analysis of compounds formed in situ is considered. The bibliography includes 134 references.

Role of boric acid in the formation of poly(vinyl alcohol)–iodine complexes in undrawn films

The role of boric acid in the formation of poly(vinyl alcohol) (PVA)–iodine complexes in undrawn films has been investigated by using wide-angle X-ray diffraction (WAXD) and high-resolution solid-state 13C NMR spectroscopy. From UV–vis absorption spectroscopy, it is confirmed that boric acid is necessary for the formation of the complexes in films that are treated with I 2/KI aqueous solutions at relatively low I 2 concentrations. The WAXD profiles indicate that, irrespective of the presence of iodine, crystallite sizes perpendicular to the chain axis become smaller by the addition of boric acid in the swelling media. Moreover, small crystallites and surficial parts of larger crystallites may be partially dissolved in the swelling process with water and boric acid suppresses the re-crystallization in the drying process with or without iodine. The 13C spin–lattice relaxation time analysis reveals that there exist two components called the mobile and the less mobile components in the films and the latter component, which contains the complexes and the crystalline component, is increased in the fraction by the presence of boric acid. The evaluation of the CH resonance line shows that some of the intermolecular hydrogen bonds are broken by boric acid, which increases the intramolecular hydrogen bonds. The CH 2 lineshape analysis also reveals that the gauche fraction is appreciably increased in the less mobile component by the addition of boric acid. These facts suggest that boric acid may promote the formation of PVA–iodine complexes particularly in the surficial areas of the crystallites probably by reducing the molecular mobility of the PVA chains by causing cross-linking among them.

Characterization of a Polymorphic System Exhibiting Substantial Variation of Solubility in a Fluorinated Solvent

We report several aspects of the characterization of a material (triacetylated-β-cyclodextrin) that exists in two crystalline polymorphic forms (denoted I and II). A striking feature of this polymorphic system is that polymorph I (MPt 194 °C) and polymorph II (MPt 219 °C) exhibit remarkably different solubility in the fluorinated solvent 2H,3H-decafluoropentane, with the solubility higher by a factor of ca. eight for polymorph I at ambient temperature. Structural information established from powder X-ray diffraction and high-resolution solid-state 13C NMR spectroscopy for polymorphs I and II is reported, including crystal structure determination of polymorph II from the powder X-ray diffraction data. Differential scanning calorimetry and high-temperature powder X-ray diffraction demonstrate that the metastable polymorph I converts to the stable polymorph II over a range of elevated temperatures. The combined evidence of these and other techniques suggests that there is a monotropic relationship between po...

Study on structure and molecular dynamics of starch/poly(sodium acrylate)-grafted superabsorbent by 13C solid state NMR

The domain-structure of samples containing a series of starch/poly(sodium acrylate)-grafted superabsorbents, pure starch, pure poly(sodium acrylate), and blend of starch/poly(sodium acrylate) has been studied by high-resolution solid-state 13C NMR spectroscopy at room temperature. The result shows that the crystallinity of starch decreases greatly in the grafted and blended samples. The values of 1H spin-lattice relaxation time in rotating frame T1ρ and 1H spin-lattice relaxation time T1 shows that starch and poly(sodium acrylate) components in both grafted and blended samples have good compatibility in nanometer scale. In the 13C cross-polarization/magic-angle-spinning (CP/MAS) spectra, the chemical shift of the carbonyl group of poly(sodium acrylate) depends on the composition of the grafting samples, which indicates that the starch and the poly(sodium acrylate) components of the grafting samples exhibit better compatibility with each other than that of blended samples at molecular level.

The Influence of Ser and Tyr Residues on the Structure of Bombyx Mori Silk Fibroin Studied Using High-resolution Solid-state 13C NMR Spectroscopy and 13C Selectively Labeled Model Peptides

The Influence of Ser and Tyr Residues on the Structure of Bombyx Mori Silk Fibroin Studied Using High-resolution Solid-state 13 C NMR Spectroscopy and 13 C Selectively Labeled Model Peptides

Investigation of the structure of poly(vinyl alcohol)–iodine complex hydrogels prepared from the concentrated polymer solutions

The structure and dynamics of poly(vinyl alcohol) (PVA)–iodine complex hydrogels that were prepared from concentrated PVA solutions have been characterized by high-resolution solid-state 13C NMR spectroscopy. The fully relaxed dipolar decoupling (DD)/MAS 13C NMR spectrum indicates that the hydrogels contain at least two components, a highly mobile and broader components. The former is assigned to the soluble or well water-swollen PVA chains that are not closely associated with the PVA–iodine complexes, whereas the latter may be mainly ascribed to the aggregated PVA chains that are produced by the formation of the PVA–iodine complexes because no diffraction peaks due to the conventional PVA crystallites are observed by wide-angle X-ray diffractometry. Furthermore, 13C spin-lattice relaxation time (T1C) analyses reveal that the broader component is composed of the highly restricted component probably assignable to PVA molecular chain aggregates containing the PVA–iodine complexes and the less mobile component. As for the former component, their CH resonance line measured by the T1C-filtering method is successfully resolved into 7 constituent lines by the least-squares curve fitting. The statistical analysis of the integrated intensities of the constituent lines thus obtained also reveals that the probability fa for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probability ft of the trans conformation are, respectively, as high as 0.86 and 0.88. This fact indicates that the PVA molecular chain aggregates containing the PVA–iodine complexes should be composed of PVA segments with the trans-rich conformation and the PVA–iodine complexes therein may also be formed with these several trans-rich segments surrounding the rod-like polyiodine cores in agreement with the so-called aggregation model. Moreover, several new diffraction peaks that should be interpreted in terms of the hexagonal structure are observed for the PVA–iodine complex hydrogels in the low 2θ region in the wide-angle X-ray diffraction (WAXD) profile measured by a strong X-ray source at SPring-8. This suggests the necessity of more detailed WAXD characterization to propose a new structure model, which should be referred to as the hexagonal aggregation model, for the PVA–iodine complexes.

Solid-state 13C NMR investigation of the structure and hydrogen bonding for stereoregular poly(vinyl alcohol) films in the hydrated state

The structure and hydrogen bonding of the hydrated stereoregular poly(vinyl alcohol) (PVA) films have been investigated by high-resolution solid-state 13C NMR spectroscopy. It is found by the 13C spin-lattice relaxation analysis that there exist three components with different T 1C values assigned to the crystalline, less mobile and mobile components for the hydrated syndiotactic PVA (S-PVA) and highly isotactic PVA (HI-PVA) films. The line shape analysis indicates that the probability of intramolecular hydrogen bonding is appreciably increased in the crystalline region for the S-PVA films by the hydration but a slightly helical structure, which is probably allowed by the formation of the successive intramolecular hydrogen bondings along the chains in the crystalline region, seems not to undergo any significant change by the hydration for HI-PVA. This fact indicates that intramolecular hydrogen bonding is more stable in the hydrated state in the crystalline region. As for the less mobile component, the line shape of the CH resonance line for the hydrated S-PVA or HI-PVA films is found to be very similar to that of the corresponding crystalline component, probably being due to the successive formation of intermolecular or intramolecular hydrogen bonding in the interfacial region, which mainly contributes to the less mobile component, for the S-PVA or HI-PVA films even in the hydrated state. The mole fractions of the mm, mr and rr sequences are also estimated for the mobile component that is produced in each stereoregular PVA sample by swelling with water and it is concluded that no prominent preferential partitioning of the mm, mr and rr sequences occurs in the crystalline and noncrystalline regions for the PVA films with different tacticities.

Molecular weight effect on the complexation of poly(methacrylic acid) and poly(ethylene oxide) as studied by high-resolution solid-state 13C NMR spectroscopy

Complexes of poly(methacrylic acid) (PMAA) and poly(ethylene oxide) (PEO) with different PEO molecular weight were studied by solid-state high-resolution 13C NMR spectroscopy, with the emphasis on the PEO molecular weight effect on inter-polymer interaction, morphology and molecular motion. It is found that the crystalline phase of PEO is completely destroyed in the complex. The results of 1H transverse relaxation times and 13C spin-lattice relaxation times indicate that the chain mobility of both PEO and PMAA are greatly restricted by inter-molecular hydrogen-bonding interactions, especially when the molecular weight of PEO is 1500. The bulk structures of the complexes are found to be closely dependent on the molecular weight of PEO. The fraction of “free” PEO segments without forming hydrogen-bonds with PMAA increases with increasing PEO molecular weight.

Leaf litter decomposition in a chaparral ecosystem, Southern California

Decomposition losses from leaves of three evergreen chaparral species, scrub oak ( Quercus dumosa), ceanothus ( Ceanothus crassifolius), and manzanita ( Arctostaphylos glauca), were quantified over a 2-y field exposure using litterbags. Changes in ash-free dry mass, C, and N were monitored at 2- to 6-month intervals at four replicate sites composed of patches of these three chaparral species. Three proximate C fractions were extracted from fresh and decomposing litter samples: polar and non-polar extractives (EXT), acid-solubles (ACID), and acid-insolubles (KLIG). The chemical structure of fresh and decomposed litter was additionally characterized using high-resolution solid-state 13C NMR spectroscopy, while morphological properties were examined by scanning electron microscopy (SEM). After 2 y, the litters had lost between 20.7%±1.2 ( Ceanothus) and 35.2%±6.8 ( Quercus) of their original ash-free dry mass. The manzanita decomposed at a significantly faster rate than the other two litter types during the first few months of field exposure. Yet, after 2 y, mass loss was greater for the oak. Differences in decomposition rates could not be accounted for based on a single litter quality index. Fresh manzanita exhibited a significantly higher N content, which could explain its initially faster decay rate. Fresh oak litter, on the other hand, had a relatively high ACID and O-alkyl C ( O-ALK) content, which may have been responsible for its decay pattern. Fresh ceanothus contained a relatively low KLIG content, yet it decomposed more slowly than the two other species. The solid-state 13C NMR spectra of the ceanothus litter had two peaks characteristic of proanthocyanidins, which likely contributed to the recalcitrance of this litter type. SEM revealed that ceanothus leaf surfaces were left nearly unchanged after field exposure. In comparison, the oak and manzanita leaf surfaces were pitted and covered by microbial growth to the point of being unrecognizable. Taken together, our results indicate that a combination of biological, physical and chemical factors need to be examined to clarify the different decomposition rates and patterns of these three chaparral species.

Strain-induced crystallization of natural rubber as studied by high-resolution solid-state 13C NMR spectroscopy

A series of high-resolution solid-state 13C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the 13C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the 13C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. 13C spin-lattice relaxation time ( T 1) and 1H spin–spin relaxation time ( T 2) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative 13C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.

Strain-induced crystallization of natural rubber as studied by high-resolution solid-state 13C NMR spectroscopy

A series of high-resolution solid-state 13C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the 13C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the 13C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. 13C spin-lattice relaxation time (T1) and 1H spin–spin relaxation time (T2) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative 13C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.

Nanoscale-Confined and Fractional Crystallization of Poly(ethylene oxide) in the Interlamellar Region of Poly(butylene succinate)

The miscibility, morphology, and crystallization behavior of the blend of poly(ethylene oxide) (PEO) and poly(butylene succinate) (PBS) have been investigated using differential scanning calorimetry, high-resolution solid-state 13C NMR spectroscopy, and wide- and small-angle X-ray diffraction. PEO and PBS are found to be miscible in the amorphous phase, and fractional crystallization of PEO component is observed in the blends with low PEO content. Also, it is revealed that the PEO component in the blend with less than 20 wt % PEO content is completely included in the region between PBS lamellae. On the basis of these results, the crystallization behavior of the PEO component has been analyzed and the origin of fractional crystallization has been explored. The blend system of PBS and PEO investigated here is the first instance of confined and fractional crystallization occurring in a miscible blend.

Effect of chemical compositional distribution on solid-state structures and properties of poly(3-hydroxybutyrate- co-3-hydroxyvalerate)

Solid-state structures and crystallization kinetics were compared between poly(3-hydroxybutyrate- co-3-hydroxyvalerate) [PHB-HV] and PHB/PHB-HV blends exhibiting the cocrystallization. As cocrystallizable blends, both the blends showing complete cocrystallization, i.e. the PHB content in the crystalline phase is the same as that of the whole blends, and the blends forming a PHB-rich crystalline phase were used. The PHB and HV content in the cocrystalline phase were determined by high-resolution solid-state 13C NMR spectroscopy. In order to determine these contents with a minimum experimental error, site-specific 13C-labeled PHB/PHB-HV blends and PHB-HV copolymers were used. The crystallinity, lamellar structures, spherulite growth rate, and melting behavior were analyzed by wide-angle X-ray diffraction, small-angle X-ray scattering, polarized microscope, and differential scanning calorimetry, respectively. In these data, no difference was observed between the complete-cocrystallizable PHB/PHB-HV blends and the PHB-HV copolymers with the same overall HV content. On the other hand, the PHB/PHB-HV blends forming a PHB-rich crystalline phase has the amorphous layers thicker than that of the PHB-HV copolymers with the same overall HV content. Based on the collected data, the similarity and differences in the solid-state structures and properties between PHB-HV copolymers and cocrystallizable PHB/PHB-HV blends were discussed.

Study on δ-Form Complex in Syndiotactic Polystyrene-Organic Molecules System III. Fine Structures of Complexes with Xylene Isomers by Means of High-Resolution Solid-State 13C NMR Spectroscopy

Syndiotactic polystyrene (sPS) is known to form a clathrate compound with p-xylene and m-xylene, called δ-form complex. In this study, the structures of the sPS/p-xylene and sPS/m-xylene systems were investigated by high-resolution solid-state 13C nuclear magnetic resonance (13C NMR) and X-ray diffraction. From the Miller indices of the (0,1,0) reflection, it was confirmed that the distance between sPS helical chains in the m-xylene system was longer than that of the p-xylene system. It was concluded that the location of one methyl group of m-xylene in the sPS δ-form complex coincided with that of one methyl group of p-xylene, because the 13C NMR chemical shift and spin-lattice relaxation time of the methyl carbon of p-xylene were similar to those of one methyl carbon of m-xylene in the sPS δ-form complex. The meta substituted another methyl group of m-xylene showed a low-field chemical shift, indicating it close to the two phenyl groups of sPS chains in the δ-form complex.

Biexponential 13C spin–lattice relaxation behavior of the crystalline region of ethylene copolymers and its origin

In this work, two series of ethylene–dimethylaminoethylmethacrylate (EDAM) and ethylene–methyl acrylate copolymers (EMA) with different comonomer content were studied by high-resolution solid-state 13C NMR spectroscopy. Biexponential 13C spin–lattice relaxation behaviors of the crystalline region were observed for all copolymer samples either melt-quenched or isothermally crystallized. The relative content of the shorter 13C T 1 component to that of the longer 13C T 1 component was found to increase with comonomer content. By employing a new pulse sequence which can be considered as a combination of Goldman–Shen's and Torchia's pulse sequences, it was demonstrated that the shorter 13C T 1 component is corresponding to the intermediate part of the crystalline region. The thickness of the intermediate part is estimated to be about 0.85 nm.