A technique for the quantitative measurements of signal intensities in cellulose-based transformer insulators by13C CPMAS NMR

A full investigation of the¹³C CPMAS relaxation times for samples of virgin and aged insulation paper material has revealed the quantitative aspects of the CPMAS technique. We observe, as have others, that the peak due to methyloxy carbon C₆ in the solid-state spectrum is reduced in intensity, compared with the other peaks, by ca. 7%. This is a direct result of the difference in relaxation times for the different carbon nuclei. It is shown that simplifying assumptions concerning the relative magnitude of the relaxation times used in the analysis of cross-polarization dynamics are not valid in these materials. In particular, the¹³C spin-lattice relaxation time in the rotating frame (¹³C T₁     

Characterization of cyclotriveratrylene inclusion compounds by means of solid state13C NMR

Solid inclusion compounds of cyclotriveratrylene (CTV) with benzene, toluene, acetone, tetrahydrofuran, acetic acid and chloroform, as well as two hydrated forms, were prepared and characterized by solid state¹³C NMR. The inclusion process for CTV appears to be quite selective, and in some instance depends critically on the presence or absence of water. A number of different structural types are indicated by the solid state¹³C NMR splitting paterns. For the guests listed above,¹H NMR in solution indicated a guest to host ratio close to 0.5, except for chloroform, for which the ratio is closer to 2.     

EPR and ab-initio study on the solid state radiolysis of aliphatic and aromatic polyesters

Low temperature matrix EPR spectroscopy was employed to investigate the free radical intermediates in the solid state radiolysis of poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) (PEN), poly(1,4-cyclohexanedimethylene terephthalate-co-ethylene terephthalate) (PCT-co-ET) and poly(ethylene adipate) (PEA). With PEA the species identified is consistent with the formation of carbon centred radicals stemming from electron capture, C–O scissions and H abstractions, the most abundant species at room temperature being the -ester radical, –CH₂–CH–COO–. The radiolysis of PEN is characterized by the predominance of radicals arising from the aromatic section, the preferred reaction being the H atom addition at a position adjacent to the ester unit, leading to the formation of cyclohexadienyl type radicals. In the copolymer, PET and PBT, the role played by the aliphatic and aromatic sections with respect to radical forming processes is of comparable importance. The cyclohexadienyl radicals arising from these latter polymers are reckoned with the prominent H addition at the sites adjacent to the esters groups which are also statistically favoured. The reactivity distribution in the aromatic rings with respect to the formation of cyclohexadienyls was analyzed in greater detail by comparing the experimental results with DFT B3LYP M.O. calculations. The DFT method has also been employed for assessing the EPR properties of the cyclohexadienyl and ion-radicals from PEN and PET models.     

Quinuclidine complex with alpha-cyclodextrin: a diffusion and 13C NMR relaxation study

The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased.     

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

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

13C CPMAS NMR investigations of cellulose polymorphs in different pulps

In order to obtain information about the crystallinity and polymorphs of cellulose, and the occurrence of hemicelluloses in pulp fibers, wood cellulose, bacterial cellulose, cotton linters, viscose, and celluloses in different pulps were investigated by solid state ¹³C CPMAS NMR spectroscopy. A mixed softwood kraft pulp and a dissolving-grade pulp were treated under strongly alkaline and acidic conditions and the effect on cellulose crystallinity was studied. The presence of different crystalline polymorphs of cellulose and the amounts of hemicelluloses are considered.     

Non-Kramers' behavior of the chain local dynamics of PVC in dilute solution. Carbon-13 NMR relaxation study

Carbon-13 spin-lattice, spin-spin relaxation times, and NOE values were measured as a function of temperature at two magnetic fields for poly(vinyl chloride) (PVC) in three solvents: chloroform, dioxane, and dimethyl sulfoxide. The relaxation data were interpreted in terms of chain local motions by using the bimodal time-correlation function of the Dejean-Laupretre-Monnerie (DLM) model. Using this model, the correlation times obtained in this study, as well as those from an earlier study in dibutyl phthalate and 1,1,2,2-tetrachloroethane did not follow a linear relationship with solvent viscosity. Instead, the chain local dynamics showed a 0.60 power dependence on solvent viscosity, indicating that PVC deviates from the hydrodynamic Kramers' theory. © 1997 John Wiley & Sons, Inc.     

High-resolution 13C NMR study of free and metal-complexed ionophores in the solid state: Conformation and dynamics of the macrocyclic ring and the effect of intermolecular short contact of methyl groups on spin–lattice relaxation times and displacements of chemical shifts

High-resolution ¹³C NMR spectra of 15 samples of uncomplexed and metal-complexed tetranactin and nonactin were recorded in the solid state, revealing characteristic displacements of peaks due to complex formation and the effect of crystalline packing on the ¹³C chemical shifts and spin–lattice relaxation times of the methyl groups. The C-1 ¹³C chemical shifts of uncomplexed and complexed tetranaction and nonactin are well related to the variation of nearby torsion angles characteristic of the macrocyclic conformation, as determined by x-ray diffraction. The existence of short intermolecular contact of methyl groups (<3.8 Å) at the surface of the molecules results in either prolonged ¹³C spin–lattice relaxation times in the laboratory frame (T₁^C) or substantial upfield displacement of peaks (up to 6 ppm). In addition, significantly reduced T₁^C values in uncomplexed nonactin (one order of magnitude smaller than those of other compounds) was ascribed to the presence of a puckering motion of the tetrahydrofuran ring and fluctuation of the macrocyclic ring in the solid state (with a time scale of 10⁻⁸ _s). Finally, how the conformations of these compounds in the solid are retained in chloroform solution was examined in view of the differences in the ¹³C chemical shifts between the solid and solution.     

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.     

Structural and dynamic study of chemically modified polyHIPE by solid‐state 13C NMR spectroscopy

The structure of laboratory‐made polyHIPEs was successfully characterized by cross‐polarity/magic‐angle spinning, solid‐state ¹³C NMR experiments. The signals of vinyl groups appeared in the spectrum of the polyHIPE precursor PH? CH?CH₂, which was prepared by the polymerization of the divinylbenzene continuous phase from a highly concentrated reverse emulsion. This material was chemically modified by the regioselective free‐radical addition of thiols to the pendant vinyl groups. Spectra of materials modified by the grafting of C₈ and C₁₂ alkyl chains, PH? SC₈ and PH? SC₁₂, respectively, were produced. The signals of the vinyl groups disappeared in favor of methylene groups. This experiment clearly established that the alkyl chains were covalently bound to the polymer. To elucidate the dynamic aspect of long chains in polyHIPE, we measured the ¹³C spin–lattice relaxation times (T₁) of PH? SC₁₂ from 25 to 100 °C with variable‐temperature, solid‐state, high‐resolution ¹³C NMR spectroscopy, revealing a strong variation in T₁ along the alkyl side chain. Furthermore, the crystallinity of a wide range of chemically modified polyHIPEs, including PH? SC₁₂, was studied with pulse ¹H NMR. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 956–963, 2001     

13C and 15N NMR spectra of aminobenzimidazoles in solution and in the solid state

The ¹³C [hexadeutero‐dimethylsulfoxide (DMSO‐d₆), hexamethyl‐phosphoramide (HMPA)‐d₁₈and solid‐state] and ¹⁵N (solid‐state) NMR spectra of six C‐aminobenzimidazoles have been recorded. The tautomerism of 4(7)‐aminobenzimidazoles and 5(6)‐aminobenzimidazoles has been determined and compared with B3LYP/6‐311 + + G(d,p) calculations confirming the clear predominance of the 4‐amino tautomer and the slight preference for the 6‐amino tautomer. GIAO‐calculated absolute shieldings compare well with experimental chemical shifts. Copyright © 2008 John Wiley & Sons, Ltd.     

Magic angle carbon-13 NMR study of solid poly(ethylene naphthalene-2,6-dicarboxylate)

Amorphous (1) and semicrystalline (2) samples of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) have been investigated by cross-polarization/magic angle spinning (CP/MAS) ¹³C NMR at 26°C (1 and 2), 100°C (1) and 120°C (2) in order to study the phase structure and the local motion of polymer chain segments at temperatures below and close to T_g (120°C). The lineshape of the ethylene unit ¹³C signal in sample 2 is consistent with the presence of two components which were assigned to trans and gauche conformations. The first component arises mainly from the crystalline regions and the second one from the amorphous part. Cross-polarization curves were traced by changing the contact time between carbon and proton reservoirs. T_CH (cross relaxation time) and proton T_1p (spin-lattice relaxation time in the rotating frame) values were obtained as best fit parameters by fitting calculated curves to the experimental data. All ¹³C NMR data are consistent with the presence of highly rigid ethylene units in both semicrystalline and amorphous samples within the temperature range (T) investigated. This result is in disagreement with the ¹H NMR wide line spectra which showed a noticeable narrowing of the linewidth with increasing temperature in the same range, hence indicating a great mobility of the chain segments. To account for this discrepancy a qualitative model based on the existence of two distinct dynamic regions, one where motion is highly restricted and the other one where large reorientations of ethylene group torsional angles take place, is suggested. The NMR results led to the conclusion that three structural phases are present in PEN: crystalline, very rigid amorphous, and very mobile amorphous. © 1995 John Wiley & Sons, Inc.     

Co-acquisition of hyperpolarised 13C and 15N NMR spectra

Recent developments in dynamic nuclear polarisation now allow significant enhancements to be generated in the cryo solid state and transferred to the liquid state for detection at high resolution. We demonstrate that the Ardenkjaer-Larsen method can be extended by taking advantage of the properties of the trityl radicals used. It is possible to hyperpolarise 13C and 15N simultaneously in the solid state, and to maintain these hyperpolarisations through rapid dissolution into the liquid state. We demonstrate the almost simultaneous measurement of hyperpolarised 13C and hyperpolarised 15N NMR spectra. The prospects for further improvement of the method using contemporary technology are also discussed.     

13C NMR Investigation of Si-alkylsubstituted 1,3,5-Trisilacyclohexanes

¹³C NMR spectra of Si-alkylsubstituted derivatives of 1,3,5-trisilacyclohexanes have been recorded and analyzed. A systematic preparation of alkyl derivatives with mixed substituents made it possible to evaluate substituent-induced chemical shift (SCS) values for the ring carbon atoms in β and δ position. It is found that the β_e effect decreases in the order Me > Et > i-Pr > t-Bu. For the alkyl groups Me, Et, and i-Pr the β_a effect is smaller than the β_e effect. Axial SCS values for the t-Bu group are not accessible because chair conformations with an axial t-Bu group are unfavourable and tend to escape into a twisted boat form. The observed δ effects are small and do not show any obvious tendencies.     

Layered metal(IV) phosphonate materials: Solid‐state 1H, 13C, 31P NMR spectra and NMR relaxation

Multinuclear solid‐state NMR and powder X‐ray diffraction data collected for phosphonate materials Zr(O₃PC₆H₄PO₃) · 3.6H₂O and Sn(O₃PC₆H₄PO₃)_0.85(O₃POH)_0.30 · 3.09H₂O have resulted in the layered structure, where the phosphonic acids cross‐link the layers. The main structural motif (the 111 connectivity in the PO₃ group) has been established by determination of chemical shift anisotropy parameters for phosphorus nuclei in the phosphonate groups. An analysis of the variable‐temperature ³¹P T₁ measurements and the shapes of the phosphorus resonances in the ³¹P static NMR spectra have resulted in the dipolar mechanism of the phosphorus spin‐lattice relaxation, where the rotating phenylene rings reorient dipolar vectors P^…H as a driving force of the relaxation process. It has been found that water protons do not affect the ³¹P T₁ times. The activation energy of the phenylene rotation in both compounds has been determined as low as 12.5 kJ/mol. The interpretation of the phosphorus relaxation data has been independently confirmed by the measurements of ¹H T₁ times for protons of the phenylene rings.     

CPMAS 13C NMR and X-ray studies of cellooligosaccharide acetates as a model for cellulose triacetate

A series of crystalline oligomers from α-D -cellobiose octaacetate through α-D -cellohexaose eicosaacetate were prepared by homogeneous acetylation of the corresponding cellooligosaccharides and characterized by cross-polarization and magic angle sample spinning (CPMAS) carbon-13 nuclear magnetic resonance (¹³C NMR) spectroscopy and X-ray analysis to obtain the structural models of cellulose triacetate (CTA) in the solid state. Progressing toward the hexamer, the NMR spectral features of the oligomers, in comparison with two allomorphs of CTA I and CTA II, gradually approached those of CTA I. Specifically, chemical shifts of both the hexamer and pentamer were in agreement with those of CTA I. In addition, X-ray diffraction patterns of the oligomers established that the crystalline pentamer and hexamer had a CTA I lattice despite recrystallization from ethylacetate-n-hexane. Therefore, we conclude that the pentamer and hexamer are useful models for the CTA I structure. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4100–4107, 1999     

High resolution solid‐state 13C‐NMR study of as‐cured and irradiated epoxy resins

This article presents the effects of strong ionizing radiations on the physico‐chemical modifications of aliphatic or aromatic amine‐cured epoxy resins based on diglycidyl ether of bisphenol A (DGEBA). Such epoxy resins have a considerable number of applications in the nuclear industrial field and are known to be very stable under moderate irradiation conditions. Using extensively high resolution solid‐state ¹³C‐NMR spectroscopy we show that the aliphatic amine‐cured resin (DGEBA‐TETA) appears much more sensitive to gamma rays than the aromatic amine‐cured one (DGEBA‐DDM). On the one hand, qualitative analyses of the high resolution solid‐state ¹³C‐NMR spectra of both epoxy resins, irradiated under similar conditions (8.5 MGy), reveal almost no change in the aromatic amine‐cured resin whereas new resonances are observed for the aliphatic amine‐cured resin. These new peaks were interpreted as the formation of new functional groups such as amides, acids and/or esters and to alkene groups probably formed in the aliphatic amine skeleton. On the other hand, molecular dynamics of these polymers are investigated by measuring the relaxation times, T_CH, T_1ρH and T_1C , before and after irradiation. The study of relaxation data shows the formation, under irradiation, of a more rigid network, especially for the aliphatic amine‐cured system and confirms that aromatic amine‐cured resin [DGEBA‐4,4′‐diaminodiphenylmethane(DDM)] is much less affected by ionizing radiations than the aliphatic amine‐cured resin [DGEBA‐triethylenetetramine(TETA)]. Moreover, it has been shown that the molecular modifications generated by irradiation on the powder of the aliphatic‐amine‐cured resin appear to be homogeneously distributed inside the polymers as no phase separations can be deduced from the above analyses. Copyright © 2001 John Wiley & Sons, Ltd.     

Oligomeric complexes of some heteroaromatic ligands and aromatic diamines with rhodium and molybdenum tetracarboxylates: 13C and 15N CPMAS NMR and density functional theory studies

Seven new oligomeric complexes of 4,4′‐bipyridine; 3,3′‐bipyridine; benzene‐1,4‐diamine; benzene‐1,3‐diamine; benzene‐1,2‐diamine; and benzidine with rhodium tetraacetate, as well as 4,4′‐bipyridine with molybdenum tetraacetate, have been obtained and investigated by elemental analysis and solid‐state nuclear magnetic resonance spectroscopy, ¹³C and ¹⁵N CPMAS NMR. The known complexes of pyrazine with rhodium tetrabenzoate, benzoquinone with rhodium tetrapivalate, 4,4′‐bipyridine with molybdenum tetrakistrifluoroacetate and the 1 : 1 complex of 2,2′‐bipyridine with rhodium tetraacetate exhibiting axial–equatorial ligation mode have been obtained as well for comparison purposes. Elemental analysis revealed 1 : 1 complex stoichiometry of all complexes. The ¹⁵N CPMAS NMR spectra of all new complexes consist of one narrow signal, indicating regular uniform structures. Benzidine forms a heterogeneous material, probably containing linear oligomers and products of further reactions. The complexes were characterized by the parameter complexation shift Δδ (Δδ = δ_complex ? δ_ligand). This parameter ranged from around ?40 to ?90 ppm in the case of heteroaromatic ligands, from around ?12 to ?22 ppm for diamines and from ?16 to ?31 ppm for the complexes of molybdenum tetracarboxylates with 4,4′‐bipyridine. The experimental results have been supported by a density functional theory computation of ¹⁵N NMR chemical shifts and complexation shifts at the non‐relativistic Becke, three‐parameter, Perdew‐Wang 91/[6‐311++G(2d,p), Stuttgart] and GGA–PBE/QZ4P levels of theory and at the relativistic scalar and spin‐orbit zeroth order regular approximation/GGA–PBE/QZ4P level of theory. Nucleus‐independent chemical shifts have been calculated for the selected compounds. Copyright © 2015 John Wiley & Sons, Ltd.     

A 1H, 13C and 15N NMR study in solution and in the solid state of six N-substituted pyrazoles and indazoles

Three N-substituted pyrazoles and three N-substituted indazoles [1-(4-nitrophenyl)-3,5-dimethylpyrazole (1), 1-(2,4-dinitrophenyl)-3,5-dimethylpyrazole (2), 1-tosyl-pyrazole (3), 1-p-chlorobenzoylindazole (4), 1-tosylinda-zole (5) and 2-(2-hydroxy-2-phenylethyl)-indazole (6)] have been studied by NMR spectroscopy in solution (1H, 13C, 15N) and in the solid state (13C, 15N). The chemical shifts have been compared with GIAO/DFT calculated absolute shieldings. Some discrepancies have been analyzed.     

A 13C NMR study of the effect of γ-irradiation on the chain dynamics of poly(ethylene oxide)

A comparison of solid-state ¹³C nuclear magnetic resonance (NMR) spectra of virgin and vacuum γ-irradiated poly (ethylene oxide) (PEO) evidences marked differences. The unirradiated PEO shows a well-resolved amorphous resonance and a weak, broad envelope of crystalline resonances, while the irradiated PEO presents well-resolved resonances for both the crystalline and amorphous carbons. Upon recrystallization from the melt both PEO samples yield solid-state ¹³C NMR spectra that are closely similar to that of the virgin, unheated sample. Observation of both melt-recrystallized samples at ?60°C yields similar spectra with well-resolved crystalline resonances. Crosslinking is the predominant chemical change occurring during the γ-irradiation of PEO under vacuum and produces a change in the motional character of the crystalline phase. This change is not the result of a reduction in crystallinity as evidenced by differential scanning calorimetry (DSC) observations. The most probable explanation is that the crosslinks are concentrated at the surface of the crystalline lamellae with a resultant change in the low frequency molecular motions of the crystalline chains. This motional change shifts the T_1p^H such that the crystalline carbon nuclei can now be cross-polarized at room temperature and the resonance linewidth is reduced. Following melting and recrystallization the motional characteristics of the irradiated PEO are nearly identical to those of the unirradiated sample, probably as a result of a redistribution of the crosslinks throughout the amorphous phase during recrystallization.