Combined Rotation And Multiple Pulse Spectroscopy Research Articles

A combined solid-state NMR and quantum chemical calculation study of hydrogen bonding in two forms of α-d-glucose

Hydrogen bonding plays an important role in the structure and function of a wide range of materials. Solid-state 1H nuclear magnetic resonance (NMR) spectroscopy provides a very sensitive tool to investigate the local structure of hydrogen atoms involved in hydrogen bonding. While there is extensive 1H solid-state NMR data on O–H - - O hydrogen bonding in solid carboxylic acids, there has been no systematic 1H solid-state NMR studies of hydroxyl groups in carbohydrates (and hydroxyl groups in general). With a view to studying the hydrogen bonding in more complex materials such as cellulose polymorphs, we carried out a detailed solid-state 1H NMR investigation of the model compounds α-d-glucose and α-d-glucose monohydrate. Through a combination of fast magic-angle spinning (MAS), combined rotation and multiple pulse spectroscopy (CRAMPS), and two-dimensional (2D) correlation experiments carried out at ultrahigh magnetic fields, it was possible to assign all of the aliphatic (CH), hydroxyl (OH), and water (H2O) 1H chemical shifts in both forms of α-d-glucose. Plane-wave DFT calculations were employed to improve the hydrogen atom positions for α-d-glucose monohydrate and to calculate 1H chemical shifts, providing additional support for the experimentally determined peak assignments. Finally, the relationship between the hydroxyl 1H chemical shifts and their hydrogen bonding geometry was investigated and compared to the well-established relationship for carboxylic acid protons.

Rapid Quantification of Pharmaceuticals via 1H Solid-State NMR Spectroscopy.

The physicochemical properties of active pharmaceutical ingredients (APIs) can depend on their solid-state forms. Therefore, characterization of API forms is crucial for upholding the performance of pharmaceutical products. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is a powerful technique for API quantification due to its selectivity. However, quantitative SSNMR experiments can be time consuming, sometimes requiring days to perform. Sensitivity can be considerably improved using 1H SSNMR spectroscopy. Nonetheless, quantification via 1H can be a challenging task due to low spectral resolution. Here, we offer a novel 1H SSNMR method for rapid API quantification, termed CRAMPS-MAR. The technique is based on combined rotation and multiple-pulse spectroscopy (CRAMPS) and mixture analysis using references (MAR). CRAMPS-MAR can provide high 1H spectral resolution with standard equipment, and data analysis can be accomplished with ease, even for structurally complex APIs. Using several API species as model systems, we show that CRAMPS-MAR can provide a lower quantitation limit than standard approaches such as fast MAS with peak integration. Furthermore, CRAMPS-MAR was found to be robust for cases that are inapproachable by conventional ultra-fast (i.e., 100 kHz) MAS methods even when state-of-the-art SSNMR equipment was employed. Our results demonstrate CRAMPS-MAR as an alternative quantification technique that can generate new opportunities for analytical research.

Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and de Novo Routes.

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) amenable to significant property tuning by altering the functional groups on the imidazolate linkers. Solvent assisted linker exchange (SALE) and de novo synthesis of mixed-linker ZIFs have been demonstrated, but the differences in structural properties-most importantly the linker distributions-and synthesis mechanisms of these two different types of hybrid ZIFs are unknown. In this work, a combination of 1H NMR combined rotation and multiple pulse spectroscopy (CRAMPS), water adsorption, and nitrogen measurements reveal distinct differences in linker mixing between SALE and de novo ZIF-8-90 hybrids. Native-fluorescence confocal microscopy is shown to provide a direct means to visualize these differences. The effects of crystal size, temperature, and SALE duration were studied in detail, and a generalizable mechanism for SALE processes in ZIFs is proposed. The SALE process is found to follow a diffusion-limited behavior leading to core-shell morphologies. Under harsher SALE conditions, deviations from diffusion-limited behavior are found due to etching and partial dissolution of the initial ZIF-8 crystals. With the selection of appropriate reaction conditions, SALE processes appear to be capable of generating controlled core-shell ZIF structures of good morphological quality that complement the well-mixed structures obtained by de novo methods.

Structure Elucidation of Mixed-Linker Zeolitic Imidazolate Frameworks by Solid-State (1)H CRAMPS NMR Spectroscopy and Computational Modeling.

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are nanoporous materials that exhibit continuous and controllable tunability of properties like effective pore size, hydrophobicity, and organophilicity. The structure of mixed-linker ZIFs has been studied on macroscopic scales using gravimetric and spectroscopic techniques. However, it has so far not been possible to obtain information on unit-cell-level linker distribution, an understanding of which is key to predicting and controlling their adsorption and diffusion properties. We demonstrate the use of (1)H combined rotation and multiple pulse spectroscopy (CRAMPS) NMR spin exchange measurements in combination with computational modeling to elucidate potential structures of mixed-linker ZIFs, particularly the ZIF 8-90 series. All of the compositions studied have structures that have linkers mixed at a unit-cell-level as opposed to separated or highly clustered phases within the same crystal. Direct experimental observations of linker mixing were accomplished by measuring the proton spin exchange behavior between functional groups on the linkers. The data were then fitted to a kinetic spin exchange model using proton positions from candidate mixed-linker ZIF structures that were generated computationally using the short-range order (SRO) parameter as a measure of the ordering, clustering, or randomization of the linkers. The present method offers the advantages of sensitivity without requiring isotope enrichment, a straightforward NMR pulse sequence, and an analysis framework that allows one to relate spin diffusion behavior to proposed atomic positions. We find that structures close to equimolar composition of the two linkers show a greater tendency for linker clustering than what would be predicted based on random models. Using computational modeling we have also shown how the window-type distribution in experimentally synthesized mixed-linker ZIF-8-90 materials varies as a function of their composition. The structural information thus obtained can be further used for predicting, screening, or understanding the tunable adsorption and diffusion behavior of mixed-linker ZIFs, for which the knowledge of linker distributions in the framework is expected to be important.

High-resolution proton CRAMPS NMR using narrowband analog filters and postponed data acquisition

Proton linewidths decrease with increasing magic-angle spinning (MAS) rates. However, without spin dilution by deuteration, even with the fastest MAS rates available today, the narrowest proton linewidths are obtained by using the combined rotation and multiple pulse spectroscopy (CRAMPS) method. Direct observation under windowed CRAMPS typically introduces several tens of times more noise, partly because wideband analog filters (e.g. 5MHz) must be used or sometimes even bypassed. Here we report that it is possible to keep using narrowband analog filters (about 50kHz cutoff frequency) in CRAMPS by taking advantage of the time delay caused by the filters, which is inversely proportional to the cutoff frequency. This delay coincides with typical CRAMPS cycle times, enabling acquisition of the data point in the next detection window. The noise of such CRAMPS spectra is only about 5 times larger than MAS-only spectra. This new method allows CRAMPS to be performed on systems that lack wideline hardware (wideband filters and fast ADCs), for example, older spectrometers originally intended for solution NMR.

Host–Guest Interactions in Poly(N-isopropylacrylamide) Hydrogel Seen by One- and Two-Dimensional 1H CRAMPS Solid-State NMR Spectroscopy

Host–guest interactions in poly(N-isopropylacrylamide) (PNIPA) hydrogels with pyrocatechol were investigated by 1H single-pulse as well as one- and two-dimensional combined rotation and multiple-pulse spectroscopy (CRAMPS) 1H solid-state NMR techniques. Differential scanning calorimetry (DSC), swelling measurements, and density functional theory (DFT) calculations were used as supplementary methods. The results are compared with the phenol–PNIPA system to explore possible similarities or differences in the effects of the two phenolic guest molecules on the temperature-induced volume phase transition of the hydrogel. Both small molecules shift the lower critical solution temperature (LCST) of the hydrogel, but with phenol the transition is broader. This observation may reflect changes in the 1H signal from residual water trapped in cavities in the hydrogel above the LCST. Although in the pyrocatechol-containing hydrogel only two signals are observed, the phenol-containing hydrogel displays numerous signals...

Coupling between the protons and PO4 tetrahedra in H+-ion-irradiated TlH2PO4

We studied hydrogen-bonded TlH2PO4 (TDP) ferroelectrics treated with proton-beam bombardment. The TDP material was irradiated with a 1-MeV proton beam at a dose of 1015/cm2. To analyze the local microscopic structure of TDP, we conducted 1H combined rotation and multiple pulse spectroscopy (CRAMPS), nuclear magnetic resonance (NMR), and high-resolution 31P NMR measurements. The isotropic chemical shifts of hydrogen and phosphor nuclei exhibited displacive features throughout the antiferroelectric, ferroelastic, and paraelastic phases that were attributed to the interplay of PO4 lattice deformation and the microscopic hydrogen bond network. The temperature dependence of the isotropic chemical shift σiso in 1H and high resolution 31P NMR revealed that the electronic charge redistribution was induced by proton-beam irradiation and the material’s elastic properties. We examined the proton-lattice relationship in terms of the proton’s location in a local double minimum well and the displacive character of the PO4 lattice.

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.

Identifying Guanosine Self Assembly at Natural Isotopic Abundance by High-Resolution 1H and 13C Solid-State NMR Spectroscopy

By means of the (1)H chemical shifts and the proton-proton proximities as identified in (1)H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)(2) and dG(C3)(2) derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by (15)N refocused INADEQUATE spectra in a previous study of (15)N-labeled derivatives (Pham, T. N.; et al. J. Am. Chem. Soc.2005, 127, 16018). The NH (1)H chemical shift is observed to be higher (13-15 ppm) for ribbon-like self-assembly as compared to 10-11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH(2)(1)H chemical shifts is also inverted, with the NH(2) proton closest in space to the NH proton having a higher or lower (1)H chemical shift than that of the other NH(2) proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)(2) derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, (14)N-(1)H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH(2) chemical shifts for the dG(C3)(2) derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face.

Probing Intermolecular Crystal Packing in γ-Indomethacin by High-Resolution 1H Solid-State NMR Spectroscopy

An NMR crystallography approach that combines experimental solid-state magic-angle-spinning (MAS) NMR with calculation is applied to the γ polymorph of the pharmaceutical molecule, indomethacin. First-principles calculations (GIPAW) for the full crystal structure and an isolated molecule show changes in the 1H chemical shift for specific aliphatic and aromatic protons of over −1 ppm that are due to intermolecular CH-π interactions. For the OH proton, 1H double-quantum (DQ) CRAMPS (combined rotation and multiple-pulse spectroscopy) spectra reveal intermolecular H–H proximities to the OH proton of the carboxylic acid dimer as well as to specific aromatic CH protons. The enhanced resolution in 1H DQ–13C spectra, recorded at 850 MHz, enables separate 1H DQ build-up curves (as a function of the DQ recoupling time) to be extracted for the aromatic CH protons. Supported by eight-spin density-matrix simulations, it is shown how the relative maximum intensities and rates of build-up provide quantitative insight in...

Application of CRAMPS for a Phase Transition in H+-ion irradiated TlH2PO4

We studied the hydrogen-bonded <TEX>$TlH_2PO_4$</TEX> (TDP) ferroelectrics treated with the proton-beam bombardment. The TDP material was irradiated with 1-MeV proton beam at a dose of <TEX>$10^{15}/cm^2$</TEX>. In order to analyze the hydrogen environment in TDP, we carried out the <TEX>$^1H$</TEX> high resolution nuclear magnetic resonance (NMR) - i.e., Combined Rotation And Multiple Pulse Spectroscopy (CRAMPS) measurement. The isotropic chemical shift of hydrogen indicates its displacive property is related to the <TEX>$PO_4$</TEX> lattice deformation which occurs throughout the antiferroelectric-, the ferroelastic- and the paraelastic-phase transitions. The temperature dependence of <TEX>$\sigma_{iso}$</TEX> reveals the electronic charge redistribution is induced by the proton-beam irradiation and the elastic property.

Didanosine Polymorphism in a Supercritical Antisolvent Process

Solid‐state properties of active ingredients are crucial in pharmaceutical development owing to their significant clinical and economical implications. In the present work we investigated the solid‐state properties and the solubility in water of didanosine, DDI, re‐crystallized from a dimethylsulfoxide solution using supercritical CO2 as an antisolvent (SAS process) for comparison with the commercially available drug product. We also applied modern solid‐state NMR (SS NMR) techniques, namely 2D 1H DQ CRAMPS (Combined Rotation And Multiple Pulse Spectroscopy) and 1H–13C on‐ and off‐resonance CP (cross polarization) FSLG‐HETCOR experiments, known for providing reliable information about 1H–1H and 1H–13C intra‐ and intermolecular proximities, in order to address polymorphism issues arising from the crystallization of a new form in the supercritical process. A new polymorph of didanosine was obtained from the supercritical antisolvent process and characterized by means of 1D and 2D multinuclear (1H, 13C, 15N) SS NMR. The particle size of the new crystal phase was reduced by varying the antisolvent density through a pressure increase. The structural differences between the commercial product and the SAS re‐crystallized DDI are highlighted by X‐ray diffractometry and well described by solid‐state NMR. The carbon C6 13C chemical shift suggests that both commercial and re‐crystallized didanosine samples are in the enol form. The analysis of homo‐ and heteronuclear proximities obtained by means of 2D NMR experiments shows that commercial and SAS re‐crystallized DDI possess very similar molecular conformation and hydrogen bond network, but different packing. The new polymorph proved to be a metastable form at ambient conditions, showing higher solubility in water and lower stability to mechanical stress. © 2009 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 99: 1855–1870, 2010

Proton Distributions and Hydrogen Bonding in Crystalline and Glassy Hydrous Silicates and Related Inorganic Materials: Insights from High‐Resolution Solid‐State Nuclear Magnetic Resonance Spectroscopy

Solid‐state 1H nuclear magnetic resonance (NMR) spectroscopy has developed into a versatile, high‐resolution method for elucidating the detailed structures of both crystalline and amorphous materials. Here, we summarize our recent endeavors in applying this method to crystalline and glassy silicates and related inorganic materials. We first present updated correlations between 1H chemical shift and O⋯O and H⋯O distances of O–H⋯O hydrogen bonds, derived from a comprehensive 1H chemical shift database containing both original high‐quality fast magic angle spinning (MAS) and two‐dimensional combined rotation and multiple pulse spectroscopy (CRAMPS)‐MAS NMR data and literature MAS NMR data for a large number of inorganic compounds, including phosphates, silicates, (oxy)hydroxides, borates, sulfates, and carbonates. These correlations may be used to estimate hydrogen‐bonding distances for inorganic oxide materials of unknown structures. We then present case studies for the application of high‐resolution two‐dimensional 1H CRAMPS‐MAS NMR to unravel the order/disorder of proton distributions in crystalline high‐pressure hydrous silicates. Finally, we summarize some of our results on the water speciation in hydrous (alumino)silicate glasses of a range of compositions obtained through comprehensive 1H MAS NMR, and 29Si–1H and 27Al–1H double‐resonance NMR experiments, and analyze them in the framework of a quasi‐chemical model.

Elucidation of Structural Restraints for Phosphate Residues with Different Hydrogen Bonding and Ionization States

Solid state NMR spectroscopy and gauge including atomic orbital (GIAO) theoretical calculations were employed to establish structural restraints (ionization, hydrogen bonding, spatial arrangement) for O-phosphorylated l-threonine derivatives in different ionization states and hydrogen bonding strengths. These structural restraints are invaluable in molecular modeling and docking procedures for biological species containing phosphoryl groups. Both the experimental and the GIAO approach show that 31P delta ii chemical shift tensor parameters are very sensitive to the ionization state. The negative values found for the skew kappa are typical for -2 phosphates. The distinct span Omega values reflect the change of strength of hydrogen bonding. For species in the -1 ionization state, engaged in very strong hydrogen bonds, Omega is smaller than for a phosphate group involved in weak hydrogen bonding. For phosphates in the -2 ionization state, Omega is significantly smaller compared to -1 species, although the kappa for -1 samples never reaches negative values. For -1 phosphate residues, in the case when 1H one pulse and/or combined rotation and multiple pulse spectroscopy (CRAMPS) sequences fail and assignment of proton chemical shift is ambiguous, a combination of 1H-(13)C and 1H-(31)P 2D heteronuclear correlation (HETCOR) correlations is found to be an excellent tool for the elucidation of 1H isotropic chemical shifts. In addition, a 2D strategy using 1H-(1)H double quantum filter (DQF) correlations [a back-to-back (BABA) sequence in this work] is useful for analyzing the topology of hydrogen bonding. In the case of a multicenter phosphorus domain, 2D 31P-(31)P proton driven spin diffusion experiments give information about the spatial arrangement of the phosphate residues.

Practical aspects of Lee–Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: Implementation and applications

Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because 1H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution 1H MAS techniques. 1H Lee–Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic–organic hybrid materials, rich in 1H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the 1H NMR spectra of the different parameters at play, for example LG (Lee–Goldburg) pulses, radio-frequency ( rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H 4pmida] [I], and inorganic–organic hybrid materials (C 4H 12N 2)[Ge 2(pmida) 2OH 2]·4H 2O [II] and (C 2H 5NH 3)[Ti(H 1.5PO 4)(PO 4)] 2·H 2O [III].

Miscibility and morphology of chiral semicrystalline poly‐(R)‐(3‐hydroxybutyrate)/chitosan and poly‐(R)‐(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)/chitosan blends studied with DSC, 1H T1 and T1ρ CRAMPS

AbstractThe phase structure of poly‐(R)‐(3‐hydroxybutyrate) (PHB)/chitosan and poly‐(R)‐(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (P(HB‐co‐HV))/chitosan blends were studied with 1H CRAMPS (combined rotation and multiple pulse spectroscopy). 1H T1 was measured with a modified BR24 sequence that yielded an intensity decay to zero mode rather than the traditional inversion‐recovery mode. 1H T1ρ was measured with a 40‐kHz spin‐lock pulse inserted between the initial 90° pulse and the BR24 pulse train. The chemical shift scale is referenced to the methyl group of PHB as 1.27 ppm relative to tetramethylsilane (TMS) based on 1H liquid NMR of PHB. Single exponential T1 decay is observed for the β‐hydrogen of PHB or P(HB‐co‐HV) at 5.4 ppm and for the chitosan at 3.7 ppm. T1 values of the blends are either faster than or intermediate to those of the plain polymers. The T1ρ decay of β‐hydrogen is bi‐exponential. The slow T1ρ decay component is interpreted as the crystalline phase of PHB or P(HB‐co‐HV). The degree of crystallinity decreases with increasing wt % of chitosan in the blend. The fast T1ρ of β‐hydrogen and the T1ρ of chitosan in the blends either follow the same trend as or faster than the weight‐averaged values based on the T1ρ of the plain polymers. Together with the observation by differential scanning calorimeter (DSC) of a melting point depression and one effective glass transition temperature in the blends, the experimental evidence strongly suggests that chitosan is miscible with either PHB or P(HB‐co‐HV) at all compositions. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1253–1258, 2002

Selective Dephasing of OH and NH Proton Magnetization Based on 1H Chemical-Shift Anisotropy Recoupling

A method for selectively suppressing the signals of OH and NH protons in 1H combined rotation and multiple-pulse spectroscopy (CRAMPS) and in 1H– 13C heteronuclear correlation (HETCOR) solid-state NMR spectra is presented. It permits distinction of overlapping CH and OH/NH proton signals, based on the selective dephasing of the magnetization of OH and NH protons by their relatively large 1H chemical-shift anisotropies. For NH protons, the 14N– 1H dipolar coupling also contributes significantly to this dephasing. The dephasing is achieved by a new combination of heteronuclear recoupling of these anisotropies with 1H homonuclear dipolar decoupling. Since the 180° pulses traditionally used for heteronuclear dipolar and chemical-shift anisotropy recoupling would result in undesirable homonuclear dephasing of proton magnetization, instead the necessary inversion of the chemical-shift Hamiltonian every half rotation period is achieved by inverting the phases of all the pulses in the HW8 multiple-pulse sequence. In the HETCOR experiments, carefully timed 13C 180° pulses remove the strong dipolar coupling to the nearby 13C spin. The suppression of NH and OH peaks is demonstrated on crystalline model compounds. The technique in combination with HETCOR NMR is applied to identify the CONH and NH–CH groups in chitin and to distinguish NH and aromatic proton peaks in a peat humin.

Formula omitted]-NMR solid state investigations of monovalent alkali metal fluorides and tetra-alkylammonium fluorides

The 19 F -NMR chemical shifts of the alkali metal fluorides and of monovalent tetra-alkylammonium fluorides were measured under strictly anhydrous conditions. For reducing the line widths of the broad resonance signals, conventional and ultrafast MAS at 32 kHz rotational frequency as well as CRAMPS (combined rotation and multiple-pulse spectroscopy) techniques were used. Averaging the strong homonuclear dipolar interactions of fluorine by MAS, the dependence of line width on the frequency of sample rotation is given for (CH 3) 4N + F −. The data obtained are compared with CRAMPS results. The broad distribution in shielding of the fluorine nucleus in the cubic alkali fluorides in the column from LiF to CsF is discussed in terms of cation–anion polarization effects, creating a partial covalent bond. The experimental chemical shift values were correlated with the Allred–Rochow electronegativities and with the polarizabilities of ions in the alkali fluoride crystals. The linear fit of both gave R-values better than 0.96. From the latter, a chemical shift of the “free F-ion” at −293 ppm, referred to CCl 3F, was extrapolated.

Reexamining the Molecular/Macromolecular Model of Coal from Comparative in Situ Variable-Temperature 1H NMR Studies of Argonne Premium Coal 601 and Its Pyridine Extraction Residue

We have carried out the first systematic in situ variable-temperature 1H NMR study, using the CRAMPS (combined rotation and multiple pulse spectroscopy) technique, of the residue from pyridine extraction of Argonne premium coal 601. The change of molecular mobility in the residue under various thermal treatments and pyridine swelling conditions were monitored quantitatively through a proton dipolar-dephasing experiment based on CRAMPS detection. Correlations among molecular structure, molecular mobility and solvent-extraction components were established through comparative variable-temperature 1H CRAMPS studies on the C5D5N-saturated original coal and its extraction residue. A critical review of the molecular/macromolecular (M/M) structural model of coal has been made on the basis of the new results, showing that the relationships between the M/M model and molecular dynamics is much more complicated than previously thought. Both the molecular phase and the macromolecular phase can show molecular dynamics behavior ranging from a lack of substantial motion to extremely mobile behavior with correlation times as short as 30 ns. Without systematic variable temperature studies, correlation between molecular mobility and the M/M model cannot be established correctly by NMR.

Interactions between Pyridine and Coal at the Molecular Level: Insights from Variable-Temperature 1H NMR Studies of Pyridine-Saturated Coal

The first systematic, in-situ, variable-temperature 1H NMR study, based on the CRAMPS (combined rotation and multiple-pulse spectroscopy) technique, is reported on pyridine-saturated Argonne premium coals. This work focused on how noncovalent associative interactions are affected at the molecular level by thermal treatment and solvent saturation. With improved proton dipolar-dephasing experiments based on CRAMPS detection, we were able to monitor quantitatively changes of molecular mobility of various molecular structural moieties upon pyridine saturation and temperature variation. It was found that pyridine saturation can dramatically promote fast, random molecular motion with correlation times in the range of 1 μs to 30 ns, even at room temperature, while thermal treatment alone is much less effective in promoting molecular motion even at 230 °C. For pyridine-saturated coal, the molecular mobility of coal can be enhanced further with just a moderate increase in temperature. Correlations between molecula...