13C Magic Angle Spinning Nuclear Magnetic Resonance Research Articles

Aluminosilicate-Supported Catalysts for the Synthesis of Cyclic Carbonates by Reaction of CO2 with the Corresponding Epoxides.

Functionalized aluminosilicate materials were studied as catalysts for the conversion of different cyclic carbonates to the corresponding epoxides by the addition of CO2. Aluminum was incorporated in the mesostructured SBA-15 silica network. Thereafter, functionalization with imidazolium chloride or magnesium oxide was performed on the Al_SBA-15 supports. The isomorphic substitution of Si with Al and the resulting acidity of the supports were investigated via 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and NH3 adsorption microcalorimetry. The Al content and the amount of MgO were quantified via inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The anchoring of the imidazolium salt was assessed by 29Si and 13C MAS NMR spectroscopy and quantified by combustion chemical analysis. Textural and structural properties of supports and catalysts were studied by N2 physisorption and X-ray diffraction (XRD). The functionalized systems were then tested as catalysts for the conversion of CO2 and epoxides to cyclic carbonates in a batch reactor at 100 or 125 °C, with an initial CO2 pressure (at room temperature) of 25 bar. Whereas the activity of the MgO/xAl_SBA-15 systems was moderate for the conversion of glycidol to the corresponding cyclic carbonate, the Al_SBA-15-supported imidazolium chloride catalysts gave excellent results over different epoxides (conversion of glycidol, epichlorohydrin, and styrene oxide up to 89%, 78%, and 18%, respectively). Reusability tests were also performed. Even when some deactivation from one run to the other was observed, a comparison with the literature showed the Al-containing imidazolium systems to be promising catalysts. The fully heterogeneous nature of the present catalysts, where the inorganic support on which the imidazolium species are immobilized also contains the Lewis acid sites, gives them a further advantage with respect to most of the catalytic systems reported in the literature so far.

Dynamic heterogeneity and nanophase separation in rubber-toughened amine-cured highly cross-linked polymer networks

Solid state nuclear magnetic resonance (NMR) spectroscopy and small-to wide-angle X-ray scattering (SWAXS) methods were used to characterize the heterogeneous dynamics and polymer domain structure in rubber modified thermoset materials containing the diglycidyl ether of bisphenol A (DGEBA) epoxy resin and a mixture of Jeffamine reactive rubber and 4,4-diaminodicyclohexylmethane (PACM) amine curing agent. The polymer chain dynamics and morphologies as a function of the PACM/Jeffamine ratio were determined. Using dipolar-filtered NMR experiments, the resulting networks are shown to be composed of mobile and rigid regions that are separated on nanometer length scales, along with a dynamically immobilized interface region. Proton NMR spin diffusion experiments measured the dimensions of the mobile phase to range between 9 and 66 nm and varied with the relative PACM concentration. Solid state 13C magic angle spinning NMR experiments show that the highly mobile phase is composed entirely of the dynamically flexible polyether chains of the Jeffamine rubber, the immobilized interface region is a mixture of DGEBA, PACM, and the Jeffamine rubber, with the PACM cross-linked to DGEBA predominantly residing in the rigid phase. The SWAXS results showed compositional nanophase separation spanning the 11–77 nm range. These measurements of the nanoscale compositional and dynamic heterogeneity provide molecular level insight into the very broad and controllable glass transition temperature distributions observed for these highly cross-linked polymer networks.

Water Modulated Framework Flexibility in NH2-MIL-125: Highlights from 13C Nuclear Magnetic Resonance

The influence of adsorbed water on the dynamics of the organic linker 1,4-benzenedicarboxylate (BDC) in the metal organic framework NH2-MIL-125 was examined by applying 13C Nuclear Magnetic Resonance (NMR) spectroscopy on samples loaded with different amounts of water. In particular, the analysis of (i) cross-polarization (CP) 13C Magic Angle Spinning (MAS) NMR spectra in terms of chemical shift and line width of the carbon signals, (ii) variable contact time 13C CP-MAS experiments, and (iii) longitudinal 13C relaxation times indicated that, upon hydration, a dynamic process occurring on the microseconds timescale accelerates. This process could be identified with the rotation of the BDC benzene ring about its C2 axis, with water competing with the carboxylic oxygen for hydrogen bonding with the aminic group. Other motions occurring at frequencies on the order of the 13C Larmor frequency, i.e. 75 MHz, which contribute to the flexibility of the three-dimensional network, were detected, and identified with the twisting, libration and translation of the BDC linker.

Polymer Networks Synthesized from Poly(Sorbitol Adipate) and Functionalized Poly(Ethylene Glycol).

Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.

Thermal property, structural characterization, and physical property of cation and anion in organic–inorganic perovskite [(CH2)3(NH3)2]CdCl4 crystal

Structural characteristics and molecular dynamics of the [(CH2)3(NH3)2] cation and CdCl6 anion in organic-inorganic perovskite [(CH2)3(NH3)2]CdCl4 near the phase transition temperature (TC) were studied by magic angle spinning (MAS) 1H nuclear magnetic resonance (NMR), MAS 13C NMR, and static 113Cd NMR experiments at different temperatures. The chemical shifts of 1H for CH2 and NH3 were nearly constant without anomalous change, whereas the 13C and 113Cd chemical shifts were changed with increasing temperature. These observations imply that the chemical environments of 1H do not change with temperature, while those of 13C and 113Cd were changed. In addition, from the spin-lattice relaxation times in the rotating frame (T1ρ) for 13C in the middle of the N–C–C–C–N bond, and the spin-lattice relaxation times in the laboratory frame (T1) for 113Cd in octahedron CdCl6, the molecular motions of these atoms below and above TC were discussed. The main effect causing the phase transition is a change in the environments of the 113Cd nuclei in the CdCl6 octahedra and in the environments of N–C–C–C–N groups in the [(CH2)3(NH3)2] cations. The study of the molecular dynamics was conducted to improve the relatively weak stability compared to the high efficiency of perovskite solar cells.

Thermodynamic Properties, Structural Characteristics, and Cation Dynamics of Perovskite-Type Layer Crystal [NH3(CH2)2NH3]ZnCl4.

In this study, we investigated the structural dynamic features of the [NH3(CH2)2NH3]ZnCl4 crystal as a function of temperature through magic angle spinning (MAS) 1H nuclear magnetic resonance (NMR), MAS 13C NMR, and static 14N NMR. From the chemical shifts, changes in the structural environments of 13C and 14N were evident. The 1H spin–lattice relaxation time (T1ρ) values at high temperatures undergo molecular motion according to the Bloembergen–Purcell–Pound theory, and the 13C T1ρ value also varied abruptly with increasing temperature. Although the phase-transition temperature was not detected from the differential scanning calorimetry result, the chemical shifts and T1ρ results showed discontinuities around 300 K. Herein, the activation energies of molecular motion for 1H and 13C obtained from T1ρ are discussed. In addition, we compare the structural dynamics of diammonium-type [NH3(CH2)2NH3]ZnCl4 obtained in this study and monoammonium-type [CH3NH3]2ZnCl4 previously reported. The findings reported herein can provide important insights for potential applications of [NH3(CH2)2NH3]ZnCl4 crystals.

Structural dynamics of CH3NH3+ and PbBr3\u2212 in tetragonal and cubic phases of CH3NH3PbBr3 hybrid perovskite by nuclear magnetic resonance

Understanding the structural dynamics of lead-halide perovskites is essential for their advanced use as photovoltaics.Here, the structural dynamics of the CH3NH3 cation and PbBr6 octahedra in the perovskite CH3NH3PbBr3 were studied via nuclear magnetic resonance (NMR) to determine the mechanism of the transition from the tetragonal to cubic phase. The chemical shifts were obtained by 1H, 13C, and 207Pb magic angle spinning NMR and 14N static NMR. The chemical shifts of the 1H nuclei in CH3 and NH3 remained constant with increasing temperature, whereas those of the 13C and 207Pb nuclei varied near the phase transition temperature (TC = 236 K), indicating that the structural environments of 13C and 207Pb change near TC. The spin–lattice relaxation time T1ρ values for 1H, 13C, and 207Pb nuclei increased with increasing temperature and did not exhibit an abrupt change near TC. In addition, the two lines in the 14N NMR spectra superposed into one line near TC, indicating the occurrence of a phase transition to a cubic phase with higher symmetry than tetragonal. Consequently, the main factor causing the phase transition from the tetragonal to cubic phase near TC is a change in the surroundings of the 207Pb nuclei in the PbBr6 octahedra and of the C–N groups in the CH3NH3 cations.

Zinc-tetraphenylporphyrin grafted on functionalised mesoporous SBA-15: synthesis and utilisation for nitroaldol condensation

Functional free-tetraphenylporphyrin (TPP) molecule was prepared by simple one-step method and subsequently zinc ion was introduced in the cavities of TPP (Zn-TPP). The prepared functional free Zn-TPP was grafted by covalent bonding of zinc with amine functionality present on the internal surface of organo-functionalised mesoporous SBA-15 molecular sieves through axial position. The resultant material was completely characterised using various analytical and spectroscopic techniques including Fourier-transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy, proton nuclear magnetic resonance, X-ray powder diffraction, thermo-gravimetry–differential thermal analysis, 13C magic-angle spinning nuclear magnetic resonance (MAS-NMR), N2 physisorption studies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The presence of organo-functionality and Zn-TPP on the surface was confirmed by conducting 13C MAS-NMR and FT-IR studies. Often, zinc containing functional porphyrin derivatives have shown as potential biological, luminescent properties, whereas in the present studies the functional free Zn-TPP and heterogenised functional-free Zn-TPP materials (SBA-15-AM-Zn-TPP) explored for the first time as catalytic materials for nitroaldol condensation under varying reaction conditions. The present functional free Zn-TPP has an advantage of ease on preparation and strong interaction on the surface linker through axial position, which made it a potential catalyst for aldol condensation and exhibit promising catalytic activity.

Assessing mesoscopic organization in copolymer-templated silica hybrid films via solid-state nuclear magnetic resonance

In the context of increasing use of nanostructured materials, finding innovative characterization methods able to assess precisely the level of ordering is essential. To this end, a range of model organic-inorganic copolymer-silica films is synthesized using poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) amphiphilic triblock copolymer as supramolecular template. Upon increasing copolymer concentration (10–100 wt%), the order can be gradually enhanced from short-range to long-range as proved by conventional techniques such as X-ray diffraction and electron microscopy. To evaluate the level of mesoscopic organization, the same series of samples is also analyzed by solid-state nuclear magnetic resonance (NMR) spectroscopy. The disorder-to-order transition is probed by 1H and 13C magic angle spinning NMR spectra through the increase in chemical environment uniformity and chain mobility respectively, which both result from the self-assembly mechanism. 1H NMR relaxation measurements (T2) using low-field NMR spectroscopy are instrumental to identify the threshold template concentration where ordering takes place.

Membrane Stability in the Presence of Methacrylate Esters.

Bioproduction of poly(methyl methacrylate) is a fast growing global industry that is limited by cellular toxicity of monomeric methacrylate intermediates to the producer strains. Maintaining high methacrylate concentrations during biofermentation, required by economically viable technologies, challenges bacterial membrane stability and cellular viability. Studying the stability of model lipid membranes in the presence of methacrylates offers unique molecular insights into the mechanisms of methacrylate toxicity, as well as into the fundamental structural bases of membrane assembly. We investigate the structure and stability of model membranes in the presence of high levels of methacrylate esters using solid-state nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). Wide-line 31P NMR spectroscopy shows that butyl methacrylate (BMA) can be incorporated into the lipid bilayer at concentrations as high as 75 mol % without significantly disrupting membrane integrity and that lipid acyl chain composition can influence membrane tolerance and ability to accommodate BMA. Using high resolution 13C magic angle spinning (MAS) NMR, we show that the presence of 75 mol % BMA lowers the lipid main transition temperature by over 12 degrees, which suggests that BMA intercalates between the lipid chains, causing uncoupling of collective lipid motions that are typically dominated by chain trans-gauche isomerization. Potential uncoupling of the bilayer leaflets to accommodate a separate BMA subphase was not supported by the SAXS experiments, which showed that membrane thickness remained unchanged even at 80% BMA. Reduced X-ray scattering contrast at the polar/apolar interface suggests BMA localization in that region between the lipid molecules.

MAS-NMR studies of carbonate retention in a very wide range of Na2O-SiO2 glasses

Glasses that contain carbon are of geological interest, and the form of that carbon can be probed by Magic-Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy. Previous studies of the Na2O-SiO2 glass system could only reach 56 mol% Na2O. Here we reproduce and extend those studies to cover a very wide compositional range, from 20 to 70 mol% Na2O, by using a combination of conventional melt-quench and twin roller quenching technologies on natural and 99% 13C-enriched sodium silicate glasses. 13C MAS-NMR reveals that measurable levels of carbon retention occur above at least 40 mol% Na2O, and takes the form of CO32− ions incorporated in the glass structure. These CO32− anions are surrounded by Na+ cations, forming nanoscale domains in a sodium silicate glass network. 23Na MAS-NMR showed a linear decrease in mean Na—O bond length with increasing Na2O content, up to 60 mol% Na2O, above which the mean Na—O bond length increased. Elemental analysis detected significant (>5%) carbon dioxide by mass in the 65 and 70 mol% Na2O glasses. For the 70 mol% Na2O glass, 13C and 23Na MAS-NMR detected ordered nanoscale domains composed of only Na2O and CO2. These results have shown the quantity and nature of carbon retention in the archetypal sodium silicate glass system, which will better inform structural models and carbonate solubility limits.

Study on Paramagnetic Interactions of (CH3NH3)2CoBr4 Hybrid Perovskites Based on Nuclear Magnetic Resonance (NMR) Relaxation Time.

The thermal properties of organic–inorganic (CH3NH3)2CoBr4 crystals were investigated using differential scanning calorimetry and thermogravimetric analysis. The phase transition and partial decomposition temperatures were observed at 460 K and 572 K. Nuclear magnetic resonance (NMR) chemical shifts depend on the local field at the site of the resonating nucleus. In addition, temperature-dependent spin–lattice relaxation times (T1ρ) were measured using 1H and 13C magic angle spinning NMR to elucidate the paramagnetic interactions of the (CH3NH3)+ cations. The shortening of 1H and 13C T1ρ of the (CH3NH3)2CoBr4 crystals are due to the paramagnetic Co2+ effect. Moreover, the physical properties of (CH3NH3)2CoBr4 with paramagnetic ions and those of (CH3NH3)2CdBr4 without paramagnetic ions are reported and compared.

Ruthenium-Based Macromolecules as Potential Catalysts in Homogeneous and Heterogeneous Phases for the Utilization of Carbon Dioxide.

Ruthenium-containing tetraphenylporphyrin (Ru-TPP) molecule was prepared, and the structural elucidation was confirmed using 1H nuclear magnetic resonance (NMR), CHN, and mass spectral analyses. The incorporation of ruthenium ion into the cavities of the macromolecule was confirmed from the disappearance of the 1H NMR signal, characteristic of the N–H bond (−2.72 ppm in TPP). The CHN and mass spectral analyses of the ligand and metallomacromolecules are consistent with the theoretically calculated values. The homogeneous Ru-TPP macromolecule is grafted on the surface of aminosilane-, diaminosilane-, and iodosilane-functionalized SBA-15 molecular sieves. The successful grafting of Ru-TPP on functionalized mesoporous molecular sieve materials was evident from low-angle powder X-ray diffraction, 13C magic angle spinning NMR, and scanning electron microscopy–energy-dispersive X-ray analyses. The resultant homogeneous and heterogenized Ru-TPP catalysts were used for the utilization of carbon dioxide (CO2) under moderate reaction conditions. The homogeneous Ru-TPP catalyst showed first-order kinetics with respect to epoxide with the exclusive formation of cyclic carbonate (about 98%) and an activation energy of 16.07 kg/mol, which is much lower than some of the reported catalysts. Ru-TPP grafted on aminosilane- and iodosilane-functionalized materials showed better catalytic activity (above 90% conversion and 83–96% cyclic carbonate selectivity) and reusability for the chosen reaction.

Capping Structure of Ligand-Cysteine on CdSe Magic-Sized Clusters.

Ligand molecules capping on clusters largely affect the formation and stabilization mechanism and the property of clusters. In semiconductor CdSe clusters, cysteine is used as one of the ligands and allows the formation of ultrastable (CdSe)34 magic-sized clusters. Cysteine has sulfhydryl, amine, and carboxylate groups, all of which have coordination ability to the CdSe surface, and the bonding states of the three functional groups of ligand–cysteine on the CdSe core have not been determined. In this work, the capping structure of ligand–cysteine is examined by performing Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. FT-IR, XPS, and 1H, 13C, and 23Na magic-angle spinning NMR show that the sulfhydryl group of ligand–cysteine forms a sulfur–cadmium bond with a cadmium atom at the CdSe surface, while the carboxylate group does not contribute to the protection of the CdSe core and binds to a sodium ion contained as a counterion. 15N–{77Se} through-bond J-single quantum filtered NMR experiment reveals that the amine group of ligand–cysteine has no coordination to selenium atoms. By considering the N–Cd bond forming ratio (∼43%) revealed in our previous work, which is confirmed in this work by analyzing 13Cα signal intensity (∼42%), we concluded that cysteine capping on (CdSe)34 occurs in two ways: one involves both the sulfur–cadmium and nitrogen–cadmium bonds, and the other bears only the sulfur–cadmium bond.

Solid-state NMR studies for the development of non-woven biomaterials based on silk fibroin and polyurethane

Silk fibroin (SF) possesses several characteristics that are favorable for tissue engineering. However, the mechanical properties must be modified in some cases. For instance, soft tissues such as those of the circulatory system are more elastic than tissue-engineered materials. The development of polymer blends is a simple method with the potential to provide materials with extended useful properties. In this study, we developed a non-woven SF and thermoplastic polyurethane-blend sheet from a polymer solution. The structure and miscibility of the blend sheet was evaluated using solid-state nuclear magnetic resonance (NMR) methods. In the 13C cross-polarization and magic angle spinning NMR spectra, no peak shift was observed between the pure and blended samples. The miscibility of the sample was investigated using proton spin-lattice relaxation times in the laboratory frame (T1H). The T1H measurement clearly revealed that the molecular chains of SF and Pellethane exist in close proximity of several tens of nanometers. The development of polymer blends is a simple method with the potential to provide materials with extended useful properties. Here, the SF and Pellethane were observed to be highly miscible with each other even though SF maintained the strong β-sheet crystal structure. This result suggests that SF/Pellethane-blended sheets possess strength that is derived from SF and elasticity that is derived from Pellethane. Therefore, this blended sheet should be a good medical material for soft tissue engineering.

Light weight, mechanically strong and biocompatible α-chitin aerogels from different aqueous alkali hydroxide/urea solutions

Light weight and mechanically strong α-chitin aerogels were fabricated using the sol-gel/self-assembly method from α-chitin in different aqueous alkali hydroxide (KOH, NaOH and LiOH)/urea solutions. All of the α-chitin solutions exhibited temperature-induced rapid gelation behavior. 13C nuclear magnetic resonance (NMR) spectra revealed that the aqueous alkali hydrox- ide/urea solutions are non-derivatizing solvents for α-chitin. Fourier transform infrared (FT-IR), X-ray diffraction (XRD) and cross-polarization magic angle spinning (CP/MAS) 13C NMR confirmed that α-chitin has a stable aggregate structure after undergoing dissolution and regeneration. Subsequently, nanostructured α-chitin aerogels were fabricated by regeneration from the chitin solutions in ethanol and then freeze-drying from t -BuOH. These α-chitin aerogels exhibited high porosity (87% to 94%), low density (0.09 to 0.19 g/cm3), high specific surface area (419 to 535 m2/g) and large pore volume (2.7 to 3.8 cm3/g). Moreover, the α-chitin aerogels exhibited good mechanical properties under compression and tension models. In vitro studies showed that mBMSCs cultured on chitin hydrogels have good biocompatibility. These nanostructured α-chitin aerogels may be useful for various applications, such as catalyst supports, carbon aerogel precursors and biomedical materials.

Structural Study of Mg-Based Metal–Organic Frameworks by X-ray Diffraction, 1H, 13C, and 25Mg Solid-State NMR Spectroscopy, and First-Principles Calculations

Understanding the formation principles, structure, and stability of magnesium-based metal–organic frameworks is a difficult task that requires application of a set of experimental and theoretical methods. In this work, three polycrystalline Mg–benzene–1,3,5-tricarboxylates, differing in the dimensionality of their frameworks, were studied by X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles calculations based on the density functional theory (DFT). XRD provided information on the average periodic structures of the three materials. Natural abundance 25Mg, 13C, and 1H magic-angle spinning NMR spectra confirmed the proposed structural models, elucidated hydrogen bonds within two of the materials, and detected the strong effect of the anisotropy of the bulk magnetic susceptibility in one. The calculations of 13C and 25Mg chemical shifts and 25Mg quadrupolar coupling parameters with the gauge-including projector-augmented wave approach enabled assignment of carbon and magnesium NMR signals to carbon and magnesium sites within the three metal–organic framework structures. The detected 13C and 25Mg NMR parameters could not be simply related to the geometry of the environments of these nuclei. DFT-based calculation of formation energies of the materials enabled the prediction of the thermodynamical stability of their structures.

Hydrophobic materials based on cotton linter cellulose and an epoxy-activated polyester derived from a suberin monomer

AbstractSuberin is a natural hydrophobic material that could be used to improve the water repellency of cellulose surfaces. It is also abundant in the outer bark of birch (Betula verrucosa); birch bark is a side-stream product in Scandinavia from the forest industry, which is generally burned for energy production. A suberin monomer, cis-9,10-epoxy-18-hydroxyoctadecanoic acid, was isolated from birch outer bark and polymerized via lipase (immobilizedCandida antarcticalipase B). The resulting epoxy-activated polyester was characterized by nuclear magnetic resonance (NMR) imaging, matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry, and size exclusion chromatography. Then the polyester was cured with tartaric or oxalic acid, and the crosslinked polyesters were characterized by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry. Hydrophobic materials were prepared by compression molding of polyester-impregnated cellulose sheets, and the final products were characterized by FTIR, cross-polarization magic angle spinning13C NMR, and field-emission scanning electron microscopy. The water contact angle was significantly increased from 0° for the original cellulose sheets to over 100° for the produced hydrophobic materials.

Investigation of the chemical and morphological structure of thermally rearranged polymers

Reacting ortho-functional poly(hydroxyimide)s via a high-temperature (i.e., 350 °C–450 °C) solid-state reaction produces polymers with exceptional gas separation properties for separations such as CO2/CH4, CO2/N2, and H2/CH4. However, these reactions render these so-called thermally rearranged (TR) polymers insoluble in common solvents, which prevent the use of certain experimental characterization techniques such as solution-state nuclear magnetic resonance (NMR) from identifying their chemical structure. In this work, we seek to identify the chemical structure of TR polymers by synthesizing a partially soluble TR polymer from an ortho-functional poly(hydroxyamide). The chemical structure of this TR polymer was characterized using 1-D and 2-D NMR. By use of cross-polarization magic-angle spinning 13C NMR, the structure of the polyamide-based TR polymer was compared to that of a polyimide-based TR polymer with a nearly identical proposed structure. The NMR spectra suggest that oxazole functionality is formed for both of these TR polymers. Furthermore, gas permeation results are provided for the precursor polymers and their corresponding TR polymer. The differences in transport properties for these polymers result from differences in the isomeric nature of oxazole-aromatic linkages and morphological differences related to free volume and free volume distribution.

Solid state nuclear magnetic resonance with magic-angle spinning and dynamic nuclear polarization below 25 K

We describe an apparatus for solid state nuclear magnetic resonance (NMR) with dynamic nuclear polarization (DNP) and magic-angle spinning (MAS) at 20–25K and 9.4Tesla. The MAS NMR probe uses helium to cool the sample space and nitrogen gas for MAS drive and bearings, as described earlier [1], but also includes a corrugated waveguide for transmission of microwaves from below the probe to the sample. With a 30mW circularly polarized microwave source at 264GHz, MAS at 6.8kHz, and 21K sample temperature, greater than 25-fold enhancements of cross-polarized 13C NMR signals are observed in spectra of frozen glycerol/water solutions containing the triradical dopant DOTOPA-TEMPO when microwaves are applied. As demonstrations, we present DNP-enhanced one-dimensional and two-dimensional 13C MAS NMR spectra of frozen solutions of uniformly 13C-labeled l-alanine and melittin, a 26-residue helical peptide that we have synthesized with four uniformly 13C-labeled amino acids.