Direct Polarization/magic Angle Spinning Research Articles

Chemical composition of soil humin in an organic soil profile

Humin is identified as the most recalcitrant fraction of soil organic matter, but its synthesis and depth variability in the soil profile are unknown. Here, we suggest that carboxyl-rich alicyclic structures (CRAM) and fused aromatic ring structures (FAR) can process significant components of the humin structure. We conducted molecular-level characterization of humin using advanced solid-state nuclear magnetic resonance spectroscopy (SSNMR) experiments and calculated the proportion of FAR and functionalities of CRAM (OCq, –C(CH)n, –CH3) structures. Dipolar dephased (dd) direct polarization magic angle spinning (DPMAS) NMR experiments provided insights into non-protonated carbon and mobile quaternary carbons. We calculated the proportions of non-protonated aromatic carbon and non-polar aliphatic C in the organic layers of the topsoil (Oa), mid-soil (Om), and mineral subsoil (Cg). It was evident that the total aliphatic carbon constitutes a significant proportion of humin, as is the non-protonated C attached to CH3. The 13C dd/DPMAS NMR spectrum of Cg humin clearly showed the formation and preservation of hydrophobic OCq, which is indicative of CRAM structures. CRAM structures could be attached to FAR, which makes the whole structure relatively more hydrophobic, a critical humin functionality in deeper soil. The findings from our research can be successfully utilized in terrestrial carbon sequestration studies to amplify the potential of humin as a recalcitrant carbon reservoir.

Chemical Reactions and Their Kinetics of atactic-Polyacrylonitrile As Revealed by Solid-State 13C NMR

Inter- and intramolecular chemical reactions and their kinetics for 13C-labeled atactic-polyacrylonitrile (aPAN) powder heat-treated at 220–290 °C under air and vacuum were investigated by various solid-state nuclear magnetic resonance (ssNMR) techniques. By applying 13C direct polarization magic angle spinning (DPMAS) as well as through-bond and through-space double quantum/single quantum ssNMR techniques, it was concluded that aPAN heat-treated under air at 290 °C for 300 min adopted the ladder formation, namely, conjugated six-membered aromatic rings with partially cross-linked and oxidized rings and polyene components. In contrast, aPAN heat-treated under vacuum at the same condition thermally decomposed into oligomeric chains that were mainly composed of isolated aromatic rings connected by alkyl segments. Furthermore, early stages of the chemical reactions were investigated by 13C cross-polarization (CP) and DPMAS spectra. The latter provided quantitative information regarding the kinetics of the chemical reactions. As a result, it was shown that chemical reactions under oxygen occurred homogeneously with a higher activation energy (Ea) of 122 ± 3 kJ/mol compared to that of vacuum at 47 ± 2 kJ/mol. By comparing both chemical structures and kinetics under two different conditions, the chemical reaction mechanisms of aPAN will be discussed in detail.

Stabilization of Atactic-Polyacrylonitrile under Nitrogen and Air As Studied by Solid-State NMR

Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % 13C-labeled atactic-polyacrylonitrile (a-PAN) heat-treated at various temperatures (Ts) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) 13C NMR spectra provided quantitative information about the functional groups of stabilized a-PAN. Two dimensional (2D) refocused 13C–13C INADEQUATE and 1H–13C HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of a-PAN stabilized at different Ts values. By comparing 1D and 2D NMR spectra, it was revealed that the stabilization process of a-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through thi...

Solid-State 1H and 13C Nuclear Magnetic Resonance Spectroscopy of Athabasca Oil Sands Asphaltenes: Evidence for Interlocking π-Stacked Nanoaggregates with Intercalated Alkyl Side Chains

Solid-state 1H and 13C nuclear magnetic resonance (NMR) spectroscopic studies on heptane-extracted Athabasca oil sands asphaltenes show strong support for the “island” or “Yen–Mullins” model. Structural calculations on the deconvolved 13C direct-polarization magic-angle spinning (DP-MAS) NMR spectrum reveal the existence of large, peri-condensed polycyclic aromatic hydrocarbon (PAH) systems. Further support for this model was obtained from the variable contact time and dipolar dephasing experiments. Variable contact time experiments demonstrated decreased mobility of the alkyl groups with increasing proximity to the aromatic core. Furthermore, it was shown that all of the aromatic groups are highly rigid, with no evidence for mobile aromatic moieties pendant from the main aromatic core. The cross-polarization discrimination induced by variable amplitude minipulses (CP-DIVAM) nutation curves enable discrimination between signals from mobile and rigid components and show that the alicyclic groups are conden...

The molecular environment of phosphorus in sewage sludge ash: implications for bioavailability.

Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state P direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil-sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P ≤ 2, SSAs contained combinations of polymorphs of AlPO, β-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.

NMR a critical tool to study the production of carbon fiber from lignin

The structural changes occurring to hardwood Alcell™ lignin as a result of fiber devolatilization/extrusion, oxidative thermo-stabilization and carbonization are investigated in this study by solid-state and solution nuclear magnetic resonance (NMR) spectroscopy techniques. Solution based 1H–13C correlation NMR of the un-spun Alcell™ lignin powder and extruded lignin fiber detected modest changes occurring due to fiber devolatilization/extrusion in the type and proportion of aliphatic side-chain carbons or monolignol inter-unit linkages. Molecular weight analysis by gel permeation chromatography (GPC), along with an additional 31P NMR method used to indicate changes in terminal hydroxyl functionality, suggest fiber devolatilization/extrusion causes both chain scission and condensation reactions. 1H CRAMPS (combined rotation and multiple-pulse spectroscopy) and 13C cross-polarization/magic angle spinning (CP/MAS) spectra of extruded and stabilized lignin fibers indicate stabilization severely reduces the proportion of methoxy groups present, while also increasing the relative proportion of carbonyl and carboxyl-related structures, typically associated with cross-linking chemistries. 13C direct-polarization/magic angle spinning (DP/MAS) analysis of stabilized and carbonized fibers shows an increased relative amount of carbon–carbon bonds on aryl structures and a relative decrease of aryl ethers. DP/MAS dipolar dephasing experiments suggest that a majority of non-protonated carbons convert from carbonyl to aryl and condensed aryl structures during carbonization.

Advanced Solid-State NMR Characterization of Marine Dissolved Organic Matter Isolated Using the Coupled Reverse Osmosis/Electrodialysis Method

Advanced (13)C solid-state techniques were employed to investigate the major structural characteristics of two surface-seawater dissolved organic matter (DOM) samples isolated using the novel coupled reverse osmosis/electrodialysis method. The NMR techniques included quantitative (13)C direct polarization/magic angle spinning (DP/MAS) and DP/MAS with recoupled dipolar dephasing, (13)C cross-polarization/total sideband suppression (CP/TOSS), (13)C chemical shift anisotropy filter, CH, CH(2), and CH(n) selection, two-dimensional (1)H-(13)C heteronuclear correlation NMR (2D HETCOR), 2D HETCOR combined with dipolar dephasing, and (15)N cross-polarization/magic angle spinning (CP/MAS). The two samples (Coastal and Marine DOM) were collected at the mouth of the Ogeechee River and in the Gulf Stream, respectively. The NMR results indicated that they were structurally distinct. Coastal DOM contained significantly more aromatic and carbonyl carbons whereas Marine DOM was markedly enriched in alkoxy carbon (e.g., carbohydrate-like moieties). Both samples contained significant amide N, but Coastal DOM had nitrogen bonded to aromatic carbons. Our dipolar-dephased spectra indicated that a large fraction of alkoxy carbons were not protonated. For Coastal DOM, our NMR results were consistent with the presence of the major structural units of (1) carbohydrate-like moieties, (2) lignin residues, (3) peptides or amino sugars, and (4) COO-bonded alkyls. For Marine DOM, they were (1) carbohydrate-like moieties, (2) peptides or amino sugars, and (3) COO-bonded alkyls. In addition, both samples contained significant amounts of nonpolar alkyl groups. The potential sources of the major structural units of DOM were discussed in detail. Nonprotonated O-alkyl carbon content was proposed as a possible index of humification.

Chemical structures of coal lithotypes before and after CO2 adsorption as investigated by advanced solid-state 13C nuclear magnetic resonance spectroscopy

Abstract Four lithotypes (vitrain, bright clarain, clarain, and fusain) of a high volatile bituminous Springfield Coal from the Illinois Basin were characterized using advanced solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy. The NMR techniques included quantitative direct polarization/magic angle spinning (DP/MAS), cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH n selection, and recoupled C–H long-range dipolar dephasing techniques. The lithotypes that experienced high-pressure CO 2 adsorption isotherm analysis were also analyzed to determine possible changes in coal structure as a result of CO 2 saturation at high pressure and subsequent evacuation. The main carbon functionalities present in original vitrain, bright clarain, clarain and fusain were aromatic carbons (65.9%–86.1%), nonpolar alkyl groups (9.0%–28.9%), and aromatic C–O carbons (4.1%–9.5%). Among these lithotypes, aromaticity increased in the order of clarain, bright clarain, vitrain, and fusain, whereas the fraction of alkyl carbons decreased in the same order. Fusain was distinct from other three lithotypes in respect to its highest aromatic composition (86.1%) and remarkably small fraction of alkyl carbons (11.0%). The aromatic cluster size in fusain was larger than that in bright clarain. The lithotypes studied responded differently to high pressure CO 2 saturation. After exposure to high pressure CO 2 , vitrain and fusain showed a decrease in aromaticity but an increase in the fraction of alkyl carbons, whereas bright clarain and clarain displayed an increase in aromaticity but a decrease in the fraction of alkyl carbons. Aromatic fused-rings were larger for bright clarain but smaller for fusain in the post-CO 2 adsorption samples compared to the original lithotypes. These observations suggested chemical CO 2 –coal interactions at high pressure and the selectivity of lithotypes in response to CO 2 adsorption.

C 1s K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy for characterizing functional group chemistry of black carbon

Black carbon (BC) is considered ubiquitous in soil organic matter (OM) and therefore plays an important role in soil biogeochemistry. Its complexity, particularly within environmental matrices, presents a challenge for research, primarily as a result of techniques which may favor detection of certain functional group types rather than capturing total sample C. The objective of this study was to utilize carbon (C) 1s near edge X-ray absorption fine edge structure (NEXAFS) spectroscopy to characterize the C chemistry of a broad range of BC materials. Characteristic resonances in the NEXAFS spectra allowed direct molecular speciation of the total C chemistry of the reference materials, environmental matrices and potentially interfering materials, obtained from an earlier BC ring trial. Spectral deconvolution was used to further identify the functional group distribution of the materials. BC reference materials and soils were characterized by a large aromatic C region comprising around 40% of total absorption intensity. We were able to distinguish shale and melanoidin from BC reference materials on the basis of their unique spectral characteristics. However, bituminous coal shared chemical characteristics with BC reference materials, namely high aromaticity of more than 40% identified by way of a broad peak. Lignite also shared similar spectra and functional group distributions to BC reference materials and bituminous coal. We compared the results of spectral deconvolution with the functional group distributions obtained by way of direct polarization magic angle spinning (DPMAS) 13C nuclear magnetic resonance (NMR) spectroscopy. Correlations between aromatic type C values for DPMAS 13C NMR and NEXAFS gave r 2 = 0.633 ( p < 0.05) and the values for NEXAFS were around 30–40% lower than for 13C NMR. Correlations were also drawn between the aromatic C/O-alkyl C ratio values for the two methods ( r 2 = 0.49, p < 0.05). Overall, NEXAFS was applicable for a wide range of environmental materials, such as those measured, although some limitations for the technique were addressed.

Characterization of humic substances by advanced solid state NMR spectroscopy: Demonstration of a systematic approach

Characterization of humic substances is challenging due to their structural complexity and heterogeneity. Solid state nuclear magnetic resonance (NMR) is regarded as one of the best tools for elucidating structures of humic substances. The primary solid state NMR technique that has been used so far is the routine 13C cross polarization-magic angle spinning (CP-MAS) technique. Although this technique has markedly advanced our understanding of humic substances, the full potential of NMR for characterizing humic substances has yet to be realized. Recent technical developments and applications of advanced solid state NMR have revealed the promise to provide deeper insights into structures of humic substances. In this paper, we summarized and demonstrated the systematic solid state NMR protocol for characterization of humic substances using a humic acid as an example. This protocol included (1) identification of specific functional groups using spectral editing techniques, occasionally assisted by 1H 13C two-dimensional heteronuclear correlation (2D HETCOR) NMR, (2) quantification of specific functional groups based on direct polarization-magic angle spinning (DP-MAS) and DP-MAS with recoupled dipolar dephasing, combined with spectral editing techniques, (3) determination of connectivities and proximities of specific functional groups by 1H 13C 2D HETCOR or 2D HETCOR combined with spectral editing techniques, and (4) examination of domains and heterogeneities by 1H 13C 2D HETCOR with 1H spin diffusion. We used a soil humic acid as an example to demonstrate how this protocol was applied to the characterization of humic substances step by step. Afterwards, based on typical 13C NMR spectra of humic substances we described how we could combine different NMR techniques to identify specific functional groups band by band from downfield to upfield. Finally, we briefly mentioned the potential new NMR techniques that could be developed to enrich the current systematic protocol. This systematic protocol is not only applicable to humic substances but also to other natural organic matter samples.

Structural Features of a Bituminous Coal and Their Changes during Low-Temperature Oxidation and Loss of Volatiles Investigated by Advanced Solid-State NMR Spectroscopy

Quantitative and advanced 13C solid-state NMR techniques were employed to investigate (i) the chemical structure of a high volatile bituminous coal, as well as (ii) chemical structural changes of this coal after evacuation of adsorbed gases, (iii) during oxidative air exposure at room temperature, and (iv) after oxidative heating in air at 75 °C. The solid-state NMR techniques employed in this study included quantitative direct polarization/magic angle spinning (DP/MAS) at a high spinning speed of 14 kHz, cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH, CH2, and CHn selection, 13C chemical shift anisotropy (CSA) filtering, two-dimensional (2D) 1H−13C heteronuclear correlation NMR (HETCOR), and 2D HETCOR with 1H spin diffusion. With spectral editing techniques, we identified methyl CCH3, rigid and mobile methylene CCH2C, methine CCH, quaternary Cq, aromatic CH, aromatic carbons bonded to alkyls, small-sized condensed aromatic moieties, and aromatic C−O groups. With direct polarization combined with spectral-editing techniques, we quantified 11 different types of functional groups. 1H−13C 2D HETCOR NMR experiments indicated spatial proximity of aromatic and alkyl moieties in cross-linked structures. The proton spin diffusion experiments indicated that the magnetization was not equilibrated at a 1H spin diffusion time of 5 ms. Therefore, the heterogeneity in spatial distribution of different functional groups should be above 2 nm. Recoupled C−H long-range dipolar dephasing showed that the fraction of large charcoal-like clusters of polycondensed aromatic rings was relatively small. The exposure of this coal to atmospheric oxygen at room temperature for 6 months did not result in obvious chemical structural changes of the coal, whereas heating at 75 °C in air for 10 days led to oxidation of coal and generated some COO groups. Evacuation removed most volatiles and caused a significant reduction in aliphatic signals in its DP/MAS spectrum. DP/MAS, but not CP/MAS, allowed us to detect the changes during low-temperature oxidation and loss of volatiles. These results demonstrate the applicability of advanced solid-state NMR techniques in chemical characterization of coal.

Quantitative 13C NMR of whole and fractionated Iowa Mollisols for assessment of organic matter composition

Both the concentrations and the stocks of soil organic carbon vary across the landscape. Do the amounts of recalcitrant components of soil organic matter (SOM) vary with landscape position? To address this question, we studied four Mollisols in central Iowa, two developed in till and two developed in loess. Two of the soils were well drained and two were poorly drained. We collected surface-horizon samples and studied organic matter in the particulate organic matter (POM) fraction, the clay fractions, and the whole, unfractionated samples. We treated the soil samples with 5 M HF at ambient temperature or at 60 °C for 30 min to concentrate the SOM. To assess the composition of the SOM, we used solid-state nuclear magnetic resonance (NMR) spectroscopy, in particular, quantitative 13C DP/MAS (direct-polarization/magic-angle spinning), with and without recoupled dipolar dephasing. Spin counting by correlation of the integral NMR intensity with the C concentration by elemental analysis showed that NMR was ⩾85% quantitative for the majority of the samples studied. For untreated whole-soil samples with <2.5 wt.% C, which is considerably less than in most previous quantitative NMR analyses of SOM, useful spectra that reflected ⩾65% of all C were obtained. The NMR analyses allowed us to conclude (1) that the HF treatment (with or without heat) had low impact on the organic C composition in the samples, except for protonating carboxylate anions to carboxylic acids, (2) that most organic C was observable by NMR even in untreated soil materials, (3) that esters were likely to compose only a minor fraction of SOM in these Mollisols, and (4) that the aromatic components of SOM were enriched to ∼53% in the poorly drained soils, compared with ∼48% in the well drained soils; in plant tissue and particulate organic matter (POM) the aromaticities were ∼18% and ∼32%, respectively. Nonpolar, nonprotonated aromatic C, interpreted as a proxy for charcoal C, dominated the aromatic C in all soil samples, composing 69–78% of aromatic C and 27–36% of total organic C in the whole-soil and clay-fraction samples.

Characterization of sinking particles from the northwest Mediterranean Sea using advanced solid-state NMR

Sinking particles are the major transporter of organic carbon from surface to the deep ocean, and their chemical composition changes dramatically with depth. However, the exact mechanism controlling the chemical transformation as particles sink is not well understood, and little detail is known about the structural changes. This is mainly due to the paucity of techniques available to analyze the major macromolecular components of sinking particles. Here we applied advanced solid-state NMR techniques, including cross polarization/magic angle spinning (CPMAS), direct polarization/magic angle spinning (DPMAS), two-dimensional 1H– 13C heteronuclear correlation (2D HETCOR) and 1H T 1 inversion recovery, on sinking particles collected in the northwest Mediterranean Sea. The CPMAS 13C NMR spectrum of the 200-m particles is significantly different from that of the DPMAS 13C NMR spectrum: CPMAS overestimates the NCH and CHO groups, but underestimates the aliphatic components, which is attributed to the high mobility of polymethylene units. Thus DPMAS is more suitable for quantifying organic composition of sinking particles. Using 2D HETCOR with 1H spin diffusion, we estimated that the size of domains (similar structural entities either physically or chemically grouped together) in the 200-m sinking particles can be as large as tens of nanometers. The results of CPMAS 13C NMR and 1H inversion recovery on sinking particles from 200, 520 and 920 m indicate that the macromolecular heterogeneity observed in surface particles virtually disappears as particles sink into the deep ocean. This suggests that the macromolecular components at depth are different in structural composition than those in surface waters, and may be compositionally homogenized as particles sink.

Investigation of Dynamics of Poly(dimethylsilane) in the Mesophase by Solid-State 29Si NMR:  Evidence for Rotator Phase

The dynamics of poly(dimethylsilane) (PDMS) in the mesophase was investigated by DSC and 29Si solid-state NMR. A solid−solid transition is observed at 166 °C based on DSC measurements. Below the transition temperature, typical chemical shift anisotropy (CSA) spectra are obtained by the 29Si solid-state NMR with cross-polarization (CP) preparation for the static PDMS sample. In contrast, the intensities at the isotropic chemical shift are suppressed for the CSA spectra above the transition temperature. The intensity attenuation, termed “magic angle hole”, in the CSA spectra is theoretically derived for uniaxially rotating solids. Therefore, it was found that PDMS undergoes a uniaxial rotation motion above the transition temperature. Theoretically, both the direct polarization (DP) experiments without the CP process and ultraslow magic angle spinning (MAS) experiments retrieve standard CSA line shapes. Our DP and ultraslow MAS experiments confirm the theoretical consideration. The retrieved CSA spectra have...

Chain Dynamics, Conformations, and Phase Transformations for Form III Polymorph of Isotactic Poly(1-butene) Investigated by High-Resolution Solid-State13C NMR Spectroscopy and Molecular Mechanics Calculations

High-resolution solid-state 13C NMR spectroscopy and MM3 molecular mechanics calculations have been applied to investigate side-chain conformations, side- and main-chain dynamics, and phase transformations of the form III polymorph of isotactic-poly(1-butene) (i-PB). MM3 calculations indicate that the t−g‘ side-chain conformation is much more stable than the g‘−g and g−t side-chain conformations in the crystal lattices of form III. The 13C cross-polarization magic-angle spinning (CPMAS) NMR spectrum at 199 K shows that the methyl group in the side chain adopts only the t−g‘ conformation. Upfield shift for the 13C methyl signal above 338 K indicates dynamic disorder in the side-chain conformations. The 13C two-dimensional exchange NMR reveals that the polymer chains in form III consist of two components. The 74% polymer chains execute the 90° helical jump motion with an activation energy of 79.3 ± 5.6 kJ/mol, while the residual chains are immobile. It is found that the 90° helical jump motion begins to occur at the temperature very close to the glass-transition temperature (Tg). Upfield shifts for the 13C main-chain signals at 338 K indicate the motional mode change from the 90° helical jump motion to “more or less” rotational or jump motion with different angle displacements. Above 338 K, two phase transformations from form III to forms II and I‘ are confirmed in the 13C CPMAS spectra. The 1H spin−lattice relaxation times in the rotating frame show that the transformation from form III to form I‘ proceeds within the form III phase via the crystal-to-crystal process, whereas the line-shape analyses of the 13C direct-polarization magic-angle spinning (DPMAS) NMR spectra indicate that the transformation from form III to form II proceeds via the quasi-melting/recrystallization process. The frozen chain conformations in the amorphous phase below Tg are also investigated on the basis of the 13C chemical shifts. It is suggested that most of the main chains adopt the helical (tg‘)n conformation, and the residual part adopts the (tt) conformation.

Suitability of Different 13C Solid-state NMR Techniques in the Characterization of Humic Acids

Qualitative and quantitative analyses of humic acids (HAs) with five different 13C solid-state NMR techniques were assessed using HAs of various origins and locations. The NMR techniques compared are: (1) direct polarization/magic angle spinning (DP/MAS) at 13 kHz, (2) conventional cross polarization (CP)/MAS at 5 kHz, (3) ramp-CP/MAS at 8 kHz, (4) CP/total sideband suppression (TOSS) at 4.5 kHz, and (5) DP/MAS corrected by CP/spin-lattice relaxation with TOSS. The spectra from the five techniques were first compared qualitatively. Then, each spectrum was divided into eight regions for quantitative evaluation. DP/MAS spectra were used as quantitative references. Ramp-CP/MAS and CP/TOSS spectra gave consistently better results than those of the conventional CP/MAS spectra at a 13C frequency of 75 MHz, which were incorrect due to spinning sidebands. CP/MAS at low magnetic fields (22.6 and 50.6 MHz 13C frequency) indicated improved integration results but lower resolution. Correction factors calculated by comparison with DP/MAS will be useful to convert the non-quantitative peak areas in the CP/TOSS and ramp-CP/MAS spectra into more quantitative results.

Analysis of Cross-Polarization Dynamics between Two Abundant Nuclei, 19F and 1H, Based on Spin Thermodynamics Theory

The phenomenological theory of spin thermodynamics based on the spin temperature hypothesis was employed to describe the cross-polarization (CP) dynamics between two abundant nuclei, 19F and 1H, when the number of fluorine atoms is not substantially less than the number of hydrogens. The influence of T1ρ's of both nuclei and the relative magnitude (heat capacity) of the two spin baths must be incorporated explicitly into the analysis in order to derive values for the parameters involved in the CP dynamics. Numerical calculations were performed to clarify the difference in the evolution of magnetization in variable contact time CP experiments between the 1H → 13C and 1H → 19F cases. A new type of CP-drain experiment was developed for observing the residual 1H magnetization after 1H → 19F CP. 19F direct polarization magic-angle spinning (MAS), 1H → 19F CP, and 1H → 19F CP-drain MAS NMR spectra have been measured for a fluorinated polyimide, 6FDA/ODA. The CP dynamics between 1H and 19F for the polyimide were analyzed on the basis of the spin thermodynamics theory. The constant for polarization transfer (THF) was determined by the analysis using the effective CP parameters, which were directly obtained from the CP and CP-drain experiments, together with independently measured values of TH1ρ and TF1ρ.

Water-soluble copolymers: 51. Copolymer compositions of high-molecular-weight functional acrylamido water-soluble polymers using direct-polarization magic-angle spinning 13C nuclear magnetic resonance

Compositions of several high-molecular-weight (> 10 6 g mol−1) water-soluble copolymers have been determined using direct-polarization magic-angle spinning (d.p.m.a.s.) 13C nuclear magnetic resonance (n.m.r.). Comonomers were studied that comprised acrylamide and sodium 3-acrylamido-3-methylbutanoate, sodium 2-acrylamido-2-methylpropanesulfonate, diacetone acrylamide or isopropylacrylamide. A copolymer of sodium 2-acrylamido-2-methylpropanesulfonate and 2-acrylamido-2-methylpropanetrimethylammonium chloride was also included. The technique is rapid and provides copolymer compositional analysis with accuracies close to those obtained by conventional elemental analysis and solution n.m.r. techniques. Spectral resolution is excellent and d.p.m.a.s. n.m.r. allows shorter acquisition times and smaller sample size. The technique is applied to a variety of acrylamide copolymers including polyelectrolytes, polyampholytes and hydrophobically associating polymers. The results are compared to copolymer compositions obtained from elemental analysis and solution 13C n.m.r. spectroscopy.