Solid state13C NMR spectra of metal carbonyl clusters

The solid state¹³C NMR spectra of four¹³CO enriched carbonyl clusters having a tri-iron metallic core have been analyzed to provide structural and dynamic information. In Fe₃(CO)₁₂ (1), the high temperature spectra suggest the occurrence of large amplitude motions of the CO groups around their position at the vertexes of the coordination polyhedron in addition to the motion involving the Fe₃-triangle previously detected in the VT-¹³C MAS spectra.¹³C and³¹P NMR data of Fe₃(CO)₁₁PPh₃ (2) indicates the presence of one molecule in the asymmetric unit in apparent disagreement with the previously reported X-ray data. Furthermore, we show that structural information can be obtained from the chemical shift tensor components readily available from the analysis of the spinning sideband manifold.     

High-Resolution Solid-State NMR Characterization of Morphology in Annealed Polylactic Acid

Single-pulse ¹³C NMR spectra and spin-lattice relaxation times T₁(¹H), detected indirectly via ¹³C carbons, and T₁(¹³C) were measured at 31°C for virgin pelletized and annealed polylactic acid (PLA) samples using the magic-angle spinning technique. The structural relaxation resulting in more regular crystals with narrower conformation distribution and increase in the lamellae thickness and crystallinity brought about by annealing at 100°C was deduced from the narrowing of the ¹³C NMR lines and proton spin-lattice relaxation times T₁(¹H). The spin-lattice relaxation times T₁(¹³C) related to the respective carbons of the α-polymorph of PLA are also discussed in the study.     

Solid State NMR of Drugs: Soluble Aspirin

Abstract

Natural abundance ¹³C NMR spectra of a soluble aspirin and model mixtures of acetylsalicylic acid with buffering components have been recorded in the solid state by using the combined techniques of cross polarization, high-power decoupling and magic-angle spinning. The solid-state spectrum of the soluble aspirin tablet showed more resonances than the solution spectrum. These multiplicities were originated in the buffer mixture containing citric and tartaric acid, as well as their salts. Solid-state ¹³C NMR was therefore found to provide information that is lost in the solution spectrum due to the fast proton exchange between the organic acids and their conjugated salts.     

Solid-state Na and C NMR characterization of Na3C60

Two separate samples of Na₃C₆₀ were prepared by direct reaction of C₆₀ with sodium metal vapor, and subjected to different annealing times of 10 days and 16 days. Solid-state ¹³C and ²³Na NMR, along with elemental analysis, powder X-ray diffraction (XRD) and Raman spectroscopy, were used to characterize both samples. The Raman spectra of both materials have a single peak at 1447 cm⁻¹ which correspond to the A_g peak of C₆₀³⁻, consistent with the stoichiometry of Na_xC₆₀ with x=3. The powder XRD patterns are also virtually identical for both samples. However, solid-state ²³Na and ¹³C NMR spectra of the two samples are significantly different, suggesting a relationship between annealing times and the final structure of the alkali fulleride. Variable-temperature ²³Na magic-angle spinning (MAS) NMR experiments reveal the existence of two or three distinct sodium species and reversible temperature-dependent diffusion of sodium ions between octahedral and tetrahedral interstitial sites. ¹³C MAS NMR experiments are used to identify resonances corresponding to free C₆₀ and fulleride species, implying that the samples are segregated-phase materials composed of C₆₀ and non-stoichiometric Na₃C₆₀. Variable-temperature ¹³C MAS NMR experiments reveal temperature-dependent motion of the fullerides.     

Solid‐state cross‐polarization magic angle spinning 13C and 15N NMR characterization of Sepia melanin,Sepia melanin free acid and Human hair melanin in comparison with several model compounds

This paper presents the high‐resolution ¹³C and ¹⁵N cross‐polarization magic angle spinning (CP/MAS) NMR spectra of three natural melanin solids: Sepia officinalis melanin, Sepia officinalis melanin free acid (MFA) and Human hair melanin. The functional group characterization of Human hair melanin by NMR is the first to date and the ¹³C CP/MAS NMR spectra reported here show improved resolution of chemically inequivalent sites. The observed spectral regions of the solid melanin samples can be assigned to the postulated structural unit of the polymer chain of Sepia MFA derived from solution‐state NMR studies. To assist in the assignment of functional groups in the spectra, the solid‐state CP/MAS NMR spectra are compared with high‐resolution ¹³C and ¹⁵N CP/MAS spectra of four model compounds, L ‐dopa, dopamine, 2‐methoxycarbonyl‐3‐ethoxycarbonyl‐4‐methylpyrrole and ethyl 5,6‐dimethoxyindole‐2‐carboxylate. To aid further in the assignment of protonated and non‐protonated carbon atoms, CP contact time dependence and non‐quaternary carbon suppression (NQS) experiments were performed on the melanin samples. The ¹⁵N CP/MAS spectra of the melanin samples confirm the presence of indole and pyrrole units in the melanin polymer chain. The NMR peaks observed in all of the melanin samples are relatively broad, presumably owing to the presence of free radicals. Electron spin resonance (ESR) data shows that all three melanin samples contain localized free radicals (g = 2.007), with the Sepia melanin containing a 10‐fold higher free radical density than Human hair melanin. Copyright © 2003 John Wiley & Sons, Ltd.     

A new route for the functionalisation of wood through transesterification reactions

A novel transesterification reaction between acetylated maritime pine sapwood (Pinus pinaster Soland) and methyl benzoate (MB), in the presence of dibutyltin oxide (DBTO) as a catalyst, was performed. Reaction was confirmed by Fourier-transform infrared spectroscopy (FTIR) and ¹³C nuclear magnetic resonance cross-polarisation with magic-angle spinning (NMR CP MAS) analysis: signals corresponding to benzoylated wood were identified and, when transesterification was performed with ethyl trimethylsilylacetate (ETMSA), characteristic trimethylsilyl groups were detected. The acetyl/benzoyl exchange rate was found to increase with increasing amount of DBTO, temperature and reaction time and a concurrent deterioration of the woody material was evidenced.     

The13C NMR spectra of some polychlorinated dibenzo-p-dioxins. A calculation of13C chemical shifts

The¹³C NMR spectra of a number of polychiorinated dibenzo-p-dioxins (PCDD) were measured. These and previously known spectra were used for the development of a method for calculation of¹³C NMR spectra of chloroaromatics in the framework of a two-particle increment scheme for carbon chemical shifts. The scheme one allows to calculate¹³C chemical shifts for all 75 PCDD.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 760–761, April, 1994.     

Secondary structure of peptides 16th. Characterization of proteins by means of13C NMR CP/MAS spectroscopy

50.3 or 75.4 MHz¹³C NMR cross-polarization/magic angle spinning spectra of human hair, horse hair, horse hoof, parrot feather, sperm whale myoglobin, and horse heart cytochrome C were measured. The spectra of human hair and horse hair indicate nearly equal mole fractions of-sheets and-helices and a low percentage of amorphous regions, whereas horse hoof contains a higher fraction of amorphous proteins. The parrot feathers contain a small-helix fraction (ca. 10±5 %) in additon to a large-sheet fraction whereas cytochrome C contains 70–90%-helices. The spectrum of myoglobin could not interpreted in terms of defined secondary structures. The usefulness of the¹³C NMR CP/MAS spectroscopy for the characterization of proteins is compared with that of IR spectroscopy and X-ray diffraction.     

The use of variable temperature 13C solid-state MAS NMR and GIPAW DFT calculations to explore the dynamics of diethylcarbamazine citrate

Experimental ¹³C solid-state magic-angle spinning (MAS) Nuclear Magnetic Resonance (NMR) as well as Density-Functional Theory (DFT) gauge-including projector augmented wave (GIPAW) calculations were used to probe disorder and local mobility in diethylcarbamazine citrate, (DEC)⁺(citrate)⁻. This compound has been used as the first option drug for the treatment of filariasis, a disease endemic in tropical countries and caused by adult worms of Wuchereria bancrofti, which is transmitted by mosquitoes. We firstly present 2D ¹³C─¹H dipolar-coupling-mediated heteronuclear correlation spectra recorded at moderate spinning frequency, to explore the intermolecular interaction between DEC and citrate molecules. Secondly, we investigate the dynamic behavior of (DEC)⁺(citrate)⁻ by varying the temperature and correlating the experimental MAS NMR results with DFT GIPAW calculations that consider two (DEC)⁺ conformers (in a 70:30 ratio) for crystal structures determined at 293 and 235 K. Solid-state NMR provides insights on slow exchange dynamics revealing conformational changes involving particularly the DEC ethyl groups.     

Structural studies on aryl‐substituted enaminoketones and their thio analogues. Part I. Analysis of high‐resolution 1H, 13C NMR and 13C CP MAS spectra combined with GIAO‐DFT calculations

A series of aryl‐substituted enaminoketones and their thio analogues in CDCl₃ solution and in the solid state were studied by the use of high‐resolution ¹H and ¹³C as well as ¹³C cross polarization magic angle spinning (CP MAS) NMR spectra in combination with gauge including atomic orbitals‐density functional theory (GIAO‐DFT) calculations performed at the B3PW91/6–311 + + G(d,p) level of theory using the B3PW91/6‐311 + + G(d,p)‐optimized geometries. The analysis of the ¹³C NMR spectra in solution was done by using the Incredible Natural Abundance DoublE QUAntum Transfer Experiment (INADEQUATE) technique, whereas trends observed in the ¹³C shielding constants, calculated for the compounds studied, were a great help in assigning most of the signals in the ¹³C CP MAS NMR spectra. It was established on the basis of the experimental and theoretical NMR data that both groups of compounds exist in the form of Z‐s‐Z‐s‐E isomers in CDCl₃ solution as well as in the solid state, with the NH hydrogen atom involved in intramolecular hydrogen bonding. This conclusion is in agreement with the fact that some of the compounds studied reveal liquid‐crystalline properties. Three‐bond H, H and C, H coupling constants measured in solution played a crucial role in the structure elucidation. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid-state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside

Three flavonoids of pharmaceutical importance-baicalein, baicalin, and wogonoside-were isolated from a Chinese medicinal plant Scutellaria baicalensis Georgi and studied by 13C NMR in solution and solid state. Two-dimensional (2D) NMR spectroscopy in the liquid phase and dipolar dephasing (DD) experiments in magic-angle spinning (MAS) spectra enabled the assignment of 13C resonances. The cross-polarization (CP) time constants T(CH) and relaxation times T(H) (1rho) were obtained from the variable-contact time experiments. The principal elements of the 13C chemical shift tensor were determined in the spectra recorded under slow sample spinning (2 kHz) using phase-adjusted spinning sideband (PASS)-2D NMR technique, and were verified by density functional theory gauge-independent atomic orbital (DFT GIAO) calculations of shielding constants. Analysis of the 13C delta(ii) and comparison with shielding parameters calculated for different conformers of compounds 1-3 enabled the selection of the most reliable geometry in the solid phase. In all three compounds, an intramolecular hydrogen bond C5--OH...=C4 is formed; the existence of baicalein and baicalin with 'anticlockwise' orientation of OH groups is more probable.     

Critical comments on accuracy of quantitative determination of natural humic matter by solid state (13)C NMR spectroscopy

Structural information of natural organic matter (NOM) at the molecular level is very essential in understanding their nature and reactivity. Nuclear magnetic resonance (NMR) is an excellent tool for estimating the gross chemical composition of the very complex humic matter (HM). A well-known fact is that the solid state ¹³C NMR spectral analysis is very parameter-sensitive especially in the study of the heterogenous HM (e.g. baseline corrections, different pulse techniques and spinning rates of the rotor vs. different disruptive sidebands in the spectra). This being the case, it has been emphasized the importance of qualitative and quantitative analyses for generating as real spectra as possible by means of different pulse and polarization techniques, sampling spinning rates as well as certain correction factors. In the present study a practical accuracy for quantitative determination of NOM type material by solid state ¹³C NMR spectroscopy was assessed using a known HM sample. Different magnetic-field strengths, sampling spinning rates, single and ramped amplitude cross polarization techniques and TOSS pulse sequence were applied for obtaining a more reliable insight into the disruptive effect of the chemical shift anisotropy (CSA), especially the most disturbing first order spinning side bands (SSB). The results demonstrated that the SSB problem is not so significant as sometimes stated, at least in the context of HM samples and in the light of the overall reproducibility and uncertainty connected with the sample itself.     

Characterization of a Water-Dispersed Biodegradable Polyurethane-Silk Composite Sponge Using 13C Solid-State Nuclear Magnetic Resonance as Coating Material for Silk Vascular Grafts with Small Diameters

Recently, Bombyx mori silk fibroin (SF) has been shown to be a suitable material for vascular prostheses for small arteries. In this study, we developed a softer SF graft by coating water-dispersed biodegradable polyurethane (PU) based on polycaprolactone and an SF composite sponge on the knitted SF vascular graft. Three kinds of ¹³C solid-state nuclear magnetic resonance (NMR), namely carbon-13 (¹³C) cross-polarization/magic angle spinning (MAS), ¹³C dipolar decoupled MAS, and ¹³C refocused insensitive nuclei enhanced by polarization transfer (r-INEPT) NMR, were used to characterize the PU-SF coating sponge. Especially the ¹³C r-INEPT NMR spectrum of water-dispersed biodegradable PU showed that both main components of the non-crystalline domain of PU and amorphous domain of SF were highly mobile in the hydrated state. Then, the small-diameter SF artificial vascular grafts coated with this sponge were evaluated through implantation experiments with rats. The implanted PU-SF-coated SF grafts showed a high patency rate. It was confirmed that the inside of the SF grafts was covered with vascular endothelial cells 4 weeks after implantation. These results showed that the water-dispersed biodegradable PU-SF-coated SF graft created in this study could be a strong candidate for small-diameter artificial vascular graft.     

13C and19F NMR study of α,β-difluorostyryl derivatives of transition metal carbonylates. A method of signal assignment based on the carbon—fluorine spin-spin coupling constants

The¹³C and¹⁹F NMR spectra ofZ- andE-isomers of β-X-substituted α,β-difluorostyrenes (X=F, Cl, CpFe(CO)₂, Re(CO)₅, Re₂(CO)₉Na) were studied. Direct and long-range (across 1–5 bonds) spin-spin coupling constants and the (¹³C−¹²C) isotope shifts in the¹⁹F NMR spectra were determined. The study of the¹³C satellites in the¹⁹F NMR spectra of substituted difluorostyrenes permitted assignment of the¹³C NMR signals of the vinylic carbon atoms. Similarly, the signals in¹⁹F NMR spectra were assigned based on coupling constants of fluorine withipso-carbon. These assignments were found to be in good agreement with the data available from the literature (X=F, Cl). The developed approach was applied to the elucidation of the structure ofZ−PhCF=CClFe(CO)₂Cp. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya. No. 8, pp. 1575–1579, August, 1998.     

Solid state N and C NMR study of dioxomolybdenum (VI) complexes of Schiff bases derived from trans-1,2-cyclohexanediamine

The ¹³C, ¹⁵N CP MAS NMR and FT-IR spectra of dioxomolybdenum (VI) complexes of trans-N,N′-bis-(R-salicylidene)-1,2-cyclohexanediamine (R=H, R=3,5-diCl, R=3,5-diBr, R=4,6-diOCH₃), trans-N,N′-bis-(2-OH-naphthylidene)-1,2-cyclohexanediamine and trans-N-(salicylidene)-N′-(2-OH-naphthylidene)-1,2-cyclohexanediamine have been measured. Comparative analysis of the NMR and IR spectra of the complexes with those of the corresponding ligands has shown that the complexation of the di-Schiff bases leads to changes in the conformation of the ligands and the charge redistribution. The asymmetric structure and non-planar structure of the complexes have been suggested.     

Investigation of the chemical modifications of beech wood lignin during heat treatment

Holocellulose, Klason lignin and milled wood lignin (MWL) of beech wood were extracted before and after heat treatment and analysed using CP MAS ¹³C NMR, ¹³C NMR, ³¹P NMR and size exclusion chromatography (SEC). Experimental results showed that the thermal treatment degrades hemicelluloses and affects lignin polymer through depolymerisation due mainly to cleavage of β-aryl-ether linkages and recondensation reactions. The spectroscopic analysis of MWL demonstrated that these recondensation reactions involved mainly guaiacyl units through formation of 5,5′-biphenolic and diarylmethane structures.Analysis of molecular weight distribution of MWL by SEC indicated that average molecular weights of heat treated milled wood lignin were lower than those of native milled wood lignin.     

Enzymatically Synthesized Conducting Polyaniline Nanocomposites: A Solid‐State NMR Study

Abstract

The chiral conducting polyaniline (PANI) nanocomposites [polyacrylic acid/polyaniline/(?) camphorsulphonic acid (CSA)] were synthesized using enzyme, horseradish peroxidase (HRP) in the aqueous buffer solution at pH 4.3. It appears that the enzyme HRP apart being a biocatalyst, plays an important role during the polymerization, which allows PANI to prefer a specific helical conformation whether the induced chirality in the monomer‐CSA complex is either by (+)CSA or (?)CSA. In this paper, we report, the structural characterization of these nanocomposites by solid‐state ¹³C cross‐polarization with magic angle spinning (CP/MAS) NMR techniques. The structural features of PANI in the conducting form of nanocomposite (as‐synthesized) are similar to that of enzymatically and chemically synthesized PANI. Preliminary data also suggest that some portion of nanocomposite samples are not completely doped. Dedoping of as‐synthesized PANI nanocomposite with aqueous NH₄OH shows the spectral features that of the emeraldine base form. Solid‐state ¹³C NMR data suggest that it is possible to detach PAA and CSA from PANI in the nanocomposite material.     

The increment scheme for the prediction of13C NMR chemical shifts in polychlorinated dibenzo-p-dioxins

A topological method for the calculation of¹³C NMR chemical shifts was developed for polychlorinated dibenzo-p-dioxins (PCDD). Based on previous results for polychlorinated benzenes and polyhydroxybenzenes, the collective influence of the substituents on carbon chemical shifts is presented as the sum of two-particle increments. The increments only of two new monosubstituted graphs have to be added to those known for PCDD spectra: 1-Cl-DD and 2-Cl-DD. All structural situations in the¹³C NMR chemical shifts of the whole class of 75 PCDD can be covered with a few model compounds. The coefficients of the increment scheme are independent of the change of CDCl₃ for acetone-d₆, so it may be a new reliable criterion for recognizing PCDD by¹³C NMR, in spite of the close resemblance of NMR spectra of aromatic compounds.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 280–284, February, 1995.     

13C NMR Spectra of 6-Hydroximinosteroids of the Stigmastane Series

¹³ C NMR spectra were studied and signals of C atoms were assigned for 6-keto- and 6-hydroximinosteroids 1-10.     

The effect of silica nanoparticles on the morphology, mechanical properties and thermal degradation kinetics of PMMA

Silica-PMMA nanocomposites with different silica quantities were prepared by a melt compounding method. The effect of silica amount, in the range 1-5 wt.%, on the morphology, mechanical properties and thermal degradation kinetics of PMMA was investigated by means of transmission electron microscopy (TEM), X-ray diffractometry (XRD), dynamic mechanical analysis (DMA), thermogravimetric analyses (TGA), Fourier-transform infrared spectroscopy (FTIR), ¹³C cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (¹³C{¹H} CP-MAS NMR) and measures of proton spin-lattice relaxation time in the rotating frame (T_1ρ(H)), in the laboratory frame (T₁(H)) and cross-polarization times (T_CH). Results showed that silica nanoparticles are well dispersed in the polymeric matrix whose structure remains amorphous. The degradation of the polymer occurs at higher temperature in the presence of silica because of the interaction between the two components.