High-resolution Solid-state 13C NMR Research Articles

Spectroscopic investigations of polyacrylonitrile thermal degradation

PAN undergoes chemical decomposition in stages on thermal treatment. In the literature, several mechanisms were proposed for the degradation process. However, the decomposition pathway and the structural rearrangement of solid residue in relation to the loss of volatile products are not fully understood. The degradation process has therefore received further attention in this work by employing a combination of FT-IR, high-resolution solid-state 13C-NMR, pyrolysis GC-MS, and microelemental analysis. These investigations have established that PAN decomposes to gaseous and volatile products over a range of temperatures (150–590°C) with concurrent stabilization of the structure of residual matter occurring on a parallel course. While linear polymerization of nitrile group is the principal reaction in the decomposition process, cyclization followed by extended conjugation is the notable exothermic process. No evidence has been obtained for the formation of oxygen-containing chromophores either as intermediates or as part of the chemical structure of the residue. Temperature sensitivity of oligomer formation has been established through pyrolysis–GC-MS studies. The overall decomposition profile of PAN has thus been established. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2503–2512, 1998

2-THIOXO-1,3,2-DITHIAPHOSPHOLANES -SOLID-STATE MAS NMR AND CRYSTAL STRUCTURE INVESTIGATIONS AS WELL AS IGLO CALCULATIONS OF 31P SHIELDING TENSORS

2-Thioxo-1,3,2-dithiaphospholanes with P-substituents R = Me, Et, i-Pr, t-Bu, Ph, 3,5-Me2C6H3, and 4-MeOC6H4 were prepared and characterized by solution and high resolution solid-state 31P and 13C NMR spectroscopy. The influence of different alkyl and aryl substituents at the phosphorus atom on the anisotropy of 31P chemical shift and on the molecular structure is discussed. The orientation of the principal axes with respect to the molecular frame is predicted from the solid-state 31P NMR results. Quantum chemical calculations of the 31P shielding tensors by the IGLO method confirm these results. The crystal structures of 2-tert-butyl-2-thioxo-1,3,2-dithiaphospholane and 2-(3,5-dimethylphenyl)-2-thioxo-1,3,2-dithiaphospholane are presented and discussed.

Phase Structure and Biodegradation of the Bacterial Poly(3-hydroxybutyric acid)/Chemosynthetic Poly(3-hydroxypropionic acid) Blend

Two different aliphatic polyesters, bacterially synthesized polyester poly(3-hydroxybutyric acid) [P(3HB)] and chemically synthesized poly(3-hydoxypropionc acid) [P(3HP)], were blended via solvent casting. By differential scanning calorimetry, polarized optical microscopy, wide-angle X-ray diffractometer and dynamic mechanical thermal analytical instrument, thermal and crystallization behavior and dynamic mechanical relaxation of the as-prepared binary blends were systematically studied. Immiscible properties were ambiguously demonstrated for these blends over the whole range of constituent composition. Glass transition temperature and proton spin diffusion times (T1H and T1ρH) obtained by the high-resolution solid state 13C NMR measurements indicated that the blend system was immiscible not only on an observation scale of DSC (about 10 nm), but on a T1H scale (several tens of nm). Enzymatic biodegradation of the blend samples by extracellular P(3HB)-depolymerase from the bacterium strain A. faecalis T1 showed an approximately linear relation between the degradation rate and the component composition. In addition, on the basis of biochemical oxygen demand (BOD), pure P(3HP) could not be biodegraded in the river water, while environmental biodegradability was confirmed when it presents as one constitutive component of the binary P(3HB)/P(3HP) blend.

Application of Optical Nuclear Polarization Enhanced 13C NMR

A gain in detection sensitivity of more than 3 orders of magnitude has been achieved in high-resolution solid-state 13C NMR of monocrystalline fluorene doped with acridine by applying optical nuclear polarization (ONP) via excited triplet states to protons and transferring this proton polarization to the 13C nuclei. This sensitivity gain was utilized to measure the angular dependence (rotation pattern) of the 13C NMR lines. In this way the principal values and orientations of all 13C chemical shielding tensors were determined. While the 13C shielding tensor of the bridging methylene group exhibits only a small anisotropy, at the aromatic carbon positions the typical strong anisotropy is observed. All tensors belonging to the same molecule have one principal axis, perpendicular to the molecular plane, in common, showing that in the crystal lattice the fluorene molecule is in a planar configuration. The experimental data are compared to ab initio calculations employing optimized geometries and gauge include...

Structure and properties of naphthalene containing polyesters, 1. High resolution solid state13C NMR study of poly(ethylene naphthalene-2,6-dicarboxylate)

Poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) crystallizes in two different modifications depending on the temperature of crystallization. The high resolution solid state 13 C NMR spectra of the α modification, β modification and amorphous PEN have been measured by employing cross polarization with magic angle spinning and high power proton decoupling. By application of both a short contact time pulse sequence and an interrupted decoupling pulse sequence all resonance lines, some of which are overlapped in the original 13 C CP/MAS NMR spectra of both amorphous and β modification PEN, are separated. The α modification PEN, β modification PEN, amorphous PEN and PEN in solution show different chemical shifts for each individual carbon atom. The chemical shift difference between amorphous PEN and PEN in solution is much smaller than the difference between either modification crystalline PEN and PEN in solution. The resonance lines of the naphthalene carbon atoms of crystalline PEN have been found to be at a lower field shift than the corresponding lines of PEN in solution. Conversely, the resonance lines of the other carbon atoms on crystalline PEN are shifted upfield from the ones of PEN in solution. A significant difference between the spectra of α and β form PEN can be observed in resonances of the protonated naphthalene carbon atoms C 1 , C 5 , C 4 , and C 8 . They differ by 1.10 ppm. These chemical shift differences are explained in terms of different conformation and chain packing in the samples.

A structural and dynamic study of poly(vinyl alcohol) in the gel state by solid-state 13C NMR and 1H pulse NMR

1H pulse NMR and high-resolution solid-state 13C NMR spectra of poly(vinyl alcohol) (PVA) gel were measured to clarify the structure and dynamics of the immobile component of the gel. From 1H pulse NMR experiments, it was found that the T 2 signal is mainly composed of three components. The long T 2 component was assigned to the mobile component which comes from the noncrosslinked region, the intermediate T 2 component corresponds to the intermediate immobile component which comes from the vicinity of the crosslinked region, and the short T 2 component corresponds to the immobile component which comes from the crosslinked region in the PVA gel. From high-resolution solid-state 13C NMR experiments, the mechanism of gel formation was satisfactorily elucidated. Further, dynamic viscoelastic modulus measurements were carried out. These experimental results were satisfactorily explained on the basis of the NMR results.

Curing Reactions of Bismaleimidodiphenylmethane with Mono- or Di-functional Allylphenols. High Resolution Solid-State 13C NMR Study

Analysis of the curing reactions of bismaleimidodiphenylmethane (BMI) with diallylbisphenol-A (DABA) or o-allylphenol (AP) with the high resolution solid-state 13C NMR technique revealed that while cured products of BMI and AP are formed through ene-reaction and the sequential Diels-Alder reaction, no Diels-Alder reaction occurs in the case of DABA and only ene-reaction and polymerization proceed. This shows that the crosslinking of such widely used curing system proceeds via a different manner suggested previously.

Structural and motional features of benzene-solvated C 60 as revealed by high-resolution solid state NMR

Structural and motional features of C 60 · 4C 6H 6 solvate has been investigated by high-resolution 13C solid-state NMR spectroscopy. 13C line shape analysis of the cross-polarization spectrum for the static sample at 293 K shows an unusual motional restriction of benzene molecules in this solvate. The existence of highly anisotropic motion of benzene fraction is visualized also by the detection of important homo and heteronuclear dipolar interactions. 13C magic-angle spinning spectra reveal a complex character of the C 60 NMR signal composed of three components with slightly different isotropic chemical shifts indicating the existence of magnetically nonequivalent positions or sites.

Conformational and dynamical behaviors of cross-linked poly(γ-methyl l-glutamate) gel as studied by high-resolution solid-state 13C NMR spectroscopy

Cross-linked poly(γ-methyl l-glutamate) (PMLG) was prepared by the ester-amide exchange reaction using diaminododecane as a cross-linker and was swollen by chloroform (CHCl 3), trifluoracetic acid (TFA) and a mixture of TFA and CHCl 3. The structural and dynamical analyses of the PMLG gel were carried out by means of high-resolution solid-state 13C NMR spectroscopy. From the experimental results on 13C chemical shifts of amide carbonyl and C α carbons of the polypeptide, it was found that the main-chain conformation of cross-linked PMLG was changed from the α-helix form to the random-coil form with an increase in the TFA content. Further, from the experimental results on on 13C spin-lattice relaxation time ( T 1) and 1H spin-spin relaxation time ( T 2) it was found that the molecular motion of the amide carbonyl carbon is in the slow-motion region and the ester carbonyl carbon is in the extreme-narrowing region. The mobility of the cross-linked PMLG becomes much faster with an increase in the TFA content through the helix-coil transition.

Solid-state structure and properties of bacterial copolyesters

In this review article, the solid-state structures and properties of some bacterially synthesized copolyesters, including poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid), poly(3-hydroxybutyric acid-co-4-hydroxybutyric acid) and poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid), are described based mainly on our results obtained by solid-state high-resolution 13C NMR, differential scanning calorimetry as well as molecular mechanics calculation methods. For the binary copolyesters of 3-hydroxybutyric acid, it was found that both 3-hydroxybutyric acid and 3-hydroxyvaleric acid units can cocrystallize in the same crystalline lattice, but the 3-hydroxybutyric acid unit cannot cocrystallize with the 4-hydroxybutyric acid and the 3-hydroxypropionic acid units.

Conformation and dynamics of membrane proteins and biologically active peptides as studied by high-resolution solid-state 13C NMR

We have explored an empirical approach to clarify the three-dimensional structure and dynamics of membrane proteins such as bacteriorhodopsin (bR) based on 13C NMR measurements, utilizing the concept of the conformation-dependent displacements of 13C chemical shifts as determined by cross polarization-magic angle spinning (CP-MAS) and dipolar decoupled-magic angle spinning (DD-MAS) methods. This is possible because 13C chemical shifts of the amino acid residues under consideration are appreciably displaced, up to 8 ppm, depending upon particular conformations from several portions of membrane proteins such as the transmembrane α-helices, loops, N- or C-terminus, etc. as referred to the data accumulated to date of an appropriate model system. It is also possible to distinguish a region characterized by a variety of backbone motions with at least three different time scales from NMR data: rapid motions with correlation times shorter than 10 −8 s, intermediate motions with correlation times of 10 −4 to 10 −5 s, and slow motions with a time scale of 10 −2 s. In addition, we also explored a non-empirical approach to reveal the three-dimensional structure of a smaller molecular system such as biologically active peptides as a messenger molecule in signal transduction, based on accurately determined appropriate sets of interatomic distances as determined by rotational echo double resonance (REDOR). Examination of 13C or 15N chemical shifts before and after REDOR experiments proved to be an indispensable means to examine whether or not conformations of several kinds of 13C, 15N-doubly labeled samples at different positions are not changed all the time. Here, we summarize some illustrative examples to this end, selected from our recent studies on [3- 13C]Ala-labeled bR and biologically active peptides such as enkephalins.

Stability of the C-terminal alpha-helical domain of bacteriorhodopsin that protrudes from the membrane surface, as studied by high-resolution solid-state 13C NMR.

We have recorded 13C NMR spectra of [1-(13)C]Ala- and [3-(13)C]Ala-bacteriorhodopsin (bR), [1-(13)C]Ala- and [3-(13)C]Ala-papain-cleaved bR, and [3-(13)C]Ala-labeled R227Q bR mutant by cross polarization-magic angle spinning (CP-MAS) and dipolar decoupled-magic angle spinning (DD-MAS) methods. The pH and temperature were varied, and Arg 227 was replaced with Gln (R227Q), in order to clarify their effects on the stability of the alpha-helical domain of the C-terminus that protrudes from the membrane surface. The comparative 13C CP- and DD-MAS NMR study of [3-(13)C]Ala-bR, rather than [1-(13)C]Ala-bR, turned out to be the best means to distinguish the 13C NMR signals of the C-terminus from those of the rest of the transmembrane helices or loops. The inner segment of the C-terminus, from Ala 228 to Ala 235, forms an alpha-helical domain (resonated at 15.9 ppm) either at neutral pH and/or at 10 to -10 degrees C. The alpha-helical peak was not seen, however, after either cleavage of the C-terminus with papain or lowering the pH to 4.25. This alpha-helical structure, and a part of the random coil which was produced from the helix at pH 4.25, were further converted to a low-temperature-type alpha-helix, as indicated by an upfield displacement of the 13C NMR signal, when the temperature was lowered to 10- -10 degrees C. Surprisingly, the corresponding helical structure in R227Q is more stable than in the wild type at the acidic pH. This alpha-helical peak was classified as an alphaII-helix from the 13C chemical shifts of Cbeta carbon, although it was ascribed to an alphaI-helix on the basis of the carbonyl shifts. This is in contrast to Ala 53 which adopts the alphaII-helix as judged from the 13C chemical shifts of Cbeta and the carbonyl carbons. Therefore, this discrepancy might be caused by differential sensitivity of the two types of carbon signals to conformation and to modes of hydrogen bonding when motional fluctuation is involved. It is likely that the alphaII-helix form present at the C-terminus is not always the type originally proposed but should be considered as a form undergoing large-amplitude conformational fluctuation around alpha-helix.

NMR studies of polydiacetylenes having alkyl chains. Molecular motions of precursor monomers and the polymers at various stages of the solid-state polymerization

Polydiacetylenes (PDAs) are synthesized from diacetylenes or more conjugated acetylenes having long chains with specific structures by the solid-state polymerization initiated by γ-ray, UV or thermal activation. By using solid-state high-resolution 13C-NMR, we studied the molecular motions of the long alkyl chains ( n-C 14H 29) of the precursor monomers and the polymers for PDAs of a butadiyne ( 2A), an octatetrayne ( 4A), a dodecahexayne ( 6A) and a p-dibutadiynylbenzene ( 2A-Bz-2A). It is shown that the yields of the solid-state polymerization are related to the molecular motions of the precursor monomers. Since the polymers of 4A and 6A have conjugated acetylenic side chains, further reactions occurred, and the intramolecular rearrangement were also monitored by 13C-NMR. The changes of the 13C spectra of the alkyl carbons are considered in detail in this paper.

The structural analysis of Cardo silicone polyimides by high resolution solid-state 13C, 15N and 29Si NMR

It is well-known that polymers containing a fluorene moiety (so-called ‘Cardo-type polymers’) have high thermal stability, good mechanical properties and excellent solubility in common organic solvents. For the benefit of the full use of these polyimides in industrial applications, cross-linked Cardo silicone polyimides, which have both high thermal stability and high resistivity to organic solvents, were designed. The cross-link density was determined by solid-state 13C and 29Si NMR because no other direct method for the determination of the cross-link density is available. Furthermore, natural abundance CPMAS 13C and 15N NMR measurements were made for three different forms of Cardo silicone polyimides which exhibit a high oxygen permselective property. An explanation for the gas separation mechanism is given.

Structural and dynamical studies of poly(vinyl alcohol) gels by high-resolution solid-state 13C NMR spectroscopy

The 13C CP/MAS NMR spectra of isotactic, syndiotactic and atactic poly(vinyl alcohol) (PVA) gels were measured in order to clarify the structure of the immobile component of PVA gel. In the 13C CP/MAS NMR spectra, the three CH carbon peaks I, II and III (at about 77, 71 and 65 ppm) were clearly observed, which originate from the formation of strong intermolecular or intramolecular hydrogen bonds between hydroxyl groups like solid PVA. It has been assigned that these peaks originate from the crosslinked region in the gel state. On the basis of the experimental results, intermolecular hydrogen bonds play an important role in the formation of the crosslinked-region in the gel state. Further, the effect of PVA's tacticity on the amount of the crosslinked regions by intermolecular interactions was discussed. In addition, molecular motion in the immobile and mobile region of PVA gel was discussed through the observation of 13C spin-lattice relaxation time T 1.

Some observations on the high-resolution solid-state CP-MAS 13C-NMR spectra of periodate-oxidised cellulose

Oxidation of cellulose by sodium metaperiodate leads to a decrease in the crystallinity. A wide-angle X-ray diffraction (WAXRD) study of oxidised celluloses [2,3-dialdehyde celluloses (DAC), 2,3-dicarboxycelluloses (DCC) and sodium 2,3-dicarboxylate celluloses (NaDCC)] showed that the crystalline peak at 2Θ = 22.7 ° decreased almost proportionately with the degree of oxidation of the starting cellulose. In the WAXRD spectra, no new crystalline peaks were observed for the oxidised celluloses, further indicating their amorphous nature. High-resolution, solid-state, cross-polarisation-magic angle spinning (CP-MAS) 13C nuclear magnetic resonance (NMR) spectra of these same oxidised cellulose samples were recorded in order to investigate the changes occurring with increasing oxidation levels. For each series of spectra of increasing oxidation levels of DAC, DCC and NaDCC an increasing trend towards the formation of a multiplicity of peaks was observed, owing to the numerous stereochemical configurations and rotameric conformations possible. The 98% oxidised cellulose samples represent almost pure compounds (new, linear, cellulose-derived polymers). The spectra and observed chemical shifts for this series are being reported for the first time in this paper. It is shown that solid-state NMR can be useful in investigations of oxidatively degraded celluloses.

Conformational behaviour of poly(aspartic acid) and its sodium salt in the solid state as studied by high-resolution solid-state 13C-NMR spectroscopy

High-resolution 13C CP/MAS NMR spectra of poly(aspartic acid) (PAA) and its sodium salt (PAANa) obtained with various neutralization numbers, n (=[COONa]/[COOH+COONa], where [ x] is the molar concentration of x), have been measured over a wide range of temperatures, in order to elucidate the conformation and its stability in the solid state. From these experimental results it was found that, at room temperature (25°C), the main chain of PAA takes a mixture of right-handed α ( α R)-helix, ω-helix and β-sheet forms and, with an increase in temperature from 25°C to 180°C, changes from α R-helix form and ω-helix form to β-sheet forms. On the other hand, the main chain of PAANa takes the right-handed α R-helix form in the temperature range from 25°C to 210°C. With a decrease in the neutralization number, PAANa changes to PAA and the NMR spectra are correspondingly changed.

Study on the Structure and the Tensile Property of a 60 mol % p-Hydroxybenzoic Acid/40 mol % Ethylene Terephthalate Liquid Crystalline Copolyester Oriented in a Magnetic Field

A thermotropic liquid crystalline copolyester, one from the Rodrun series, consisting of 60 mol % p-hydroxybenzoic acid and 40 mol % ethylene terephthalate was aligned under a magnetic field of 6 T and by mechanical methods. The tensile properties of the aligned films were different depending on the orientation degree and the means used for the orientation. The magnetically oriented films exhibited a lower ultimate tensile strength than the mechanically oriented films, but their elastic modulus was as high as that of the mechanically oriented films, suggesting that magnetic fields could provide an additional means for orientation in processing thermotropic liquid crystalline copolyesters. The difference in tensile properties was discussed in relation to the oriented structures examined by wide-angle X-ray measurement, the high-resolution solid-state 13C NMR technique, Fourier transform infrared spectroscopy, and polarizing microscopy.

The use of high-resolution solid-state NMR spectroscopy and differential scanning calorimetry to study interactions of anaesthetic steroids with membrane

We have used a combination of high-resolution solid-state 13C-NMR and DSC (differential scanning calorimetry) to study the distinctively different thermotropic and dynamic properties of the anaesthetic steroid alphaxalone and its inactive congener Δ 16-alphaxalone in dipalmitoylphosphatidylcholine (DPPC) model membranes. In the solid-state 13C-NMR, the techniques included cross polarization (CP) and/or magic angle spinning (MAS). The observed data revealed the following important results. (a) DSC as a bulk method showed that the active steroid lowers the main phase transition temperature and broadens the pretransition more significantly than the inactive congener. The 13C-CP/MAS experiments allowed us to detect the pretransition temperature in the alphaxalone-containing preparation, which was not discernible in DSC. (b) The chemical shift values varied with temperature, indicating different degrees of trans-gauche isomerization in the lipid acyl chains when the bilayer is in the liquid crystalline phase. (c) Only specific additional peaks appeared in the 13C-CP/MAS spectra when each of the steroids was present in the preparation. Δ 16-alphaxalone gives rise to more additional peaks than alphaxalone, indicating a different mobility of the corresponding molecular moiety in the phospholipid bilayer environment. (d) The relative intensities of these peaks also confirmed that alphaxalone is fully incorporated in the bilayer, whereas Δ 16-alphaxalone is only partially so. These results suggest that the differential effects of these two analogues in the membrane may, at least in part, explain the reason for their different biological activities.

Orientational Ordering of a Dimer Liquid Crystal by High-Resolution Solid-State 13C NMR Spectroscopy

The ordering of a dimer liquid crystal, α,ω-bis[(4,4′-cyanobiphenylyl)oxy]decane (CBA-10), was studied by variable-temperature high-resolution solid-state 13C NMR. A large transitional change of the 13C chemical shift was observed on going from the isotropic to the nematic phase due to uniaxial alignment of the molecule. The principal values of the 13C chemical shift tensor have been determined by the Herzfeld-Berger method for CBA-10 itself. The molecular axis is assumed to lie parallel to the line connecting the centers of both mesogenic cores, and the evaluation of the order parameters along the molecular axis, (Szz), mesogene axis (Szzring) and Cγ−Cε axis ( S zz C γ - C ε ) was attempted here, using the obtained isotropic 13C chemical shift and principal values of the 13C chemical shift tensor, and taking into account the conformational behavior of the flexible spacer in the frame of rotational isomeric state (RIS) approach. The order parameter of the molecule Szz increased from 0.79 to 0.89 with decrease of 17.5°C in temperature from nearby the nematic-isotropic transition point. Szzring increased from 0.74 to 0.80 with decrease of temperature whereas S zz C γ - C ε and the conformation of the spacer was essentially unaltered in the same temperature range.