Solid NMR Spectra Research Articles

Composite-pulse and partially dipolar dephased multiCP for improved quantitative solid-state 13C NMR

Improved multiple cross polarization (multiCP) pulse sequences for quickly acquiring quantitative 13C NMR spectra of organic solids are presented. Loss of 13C magnetization due to imperfect read-out and storage pulses in multiCP has been identified as a significant mechanism limiting polarization enhancement for 13C sites with weak couplings to 1H. This problem can be greatly reduced by composite 90° pulses with non-orthogonal phases that flip the magnetization onto the spin-lock field and back to the longitudinal direction for the 1H repolarization period; the observed loss is <3% for over ±10 kHz resonance offset and up to 20% flip-angle error. This composite-pulse multiCP (ComPmultiCP) sequence consistently provides performance superior to that of conventional multiCP, without any trade-off. The longer total CP time enabled by the composite pulses allows for a wider amplitude ramp during CP, which decreases the sensitivity to Hartmann-Hahn mismatch by a factor of two, with a <7% root-mean-square deviation within a 1-dB range for Boc-alanine. In samples with very short T1ρ, under-polarization of non-protonated carbons can be compensated by slight dipolar dephasing of CHn signals resulting from relatively weak decoupling during the Hahn spin echo period before detection. Quantitative spectra have been obtained by ComPmultiCP for low-crystallinity branched polyethylene at 4.5 kHz MAS, and in combination with partial dipolar dephasing for soil organic matter at 14 kHz MAS.

Determination of polymer crystallinity by the multivariable curve resolution method in 13C solid NMR spectrum

Most of the polymers are composed of a crystal part, an amorphous part, and a transitional interfacial part. These components present disparate physical and chemical characteristics. However, it always suffers from peak overlapping in solid NMR spectrum in order to acquire polymer's crystallinity. The polyethylene oxide (PEO) sample was tested using the Torchia pulse sequence combined with the Multivariate Curve Resolution (MCR) method. A two dimensional CP/MAS spectrum containing spin-lattice relaxation time (T1) information was acquired. After the correction based on the reciprocity relation, the overlapped peaks were resolved and quantified together with their T1's. Crystallinity is therefore observed naturally according to components’ content ratios associated with their T1 values.

Crystal Structure, Magnetism, 89Y Solid State NMR, and 121Sb Mössbauer Spectroscopic Investigations of YIrSb

The ternary antimonide YIrSb was synthesized from the binary precursor YIr and elemental antimony by a diffusion controlled solid‐state reaction. Single crystals were obtained by a flux technique with elemental bismuth as an inert solvent. The YIrSb structure (TiNiSi type, space group Pnma) was refined from single‐crystal X‐ray diffractometer data: a = 711.06(9), b = 447.74(5), c = 784.20(8) pm, wR2 = 0.0455, 535 F2 values, 20 variables. 89Y solid state MAS NMR and 121Sb Mössbauer spectra show single resonance lines in agreement with single‐crystal X‐ray data. YIrSb is a Pauli paramagnet.

Enhancing the resolution of 1H and 13C solid-state NMR spectra by reduction of anisotropic bulk magnetic susceptibility broadening.

We demonstrate that natural isotopic abundance 2D heteronuclear correlation (HETCOR) solid-state NMR spectra can be used to significantly reduce or eliminate the broadening of 1H and 13C solid-state NMR spectra of organic solids due to anisotropic bulk magnetic susceptibility (ABMS). ABMS often manifests in solids with aromatic groups, such as active pharmaceutical ingredients (APIs), and inhomogeneously broadens the NMR peaks of all nuclei in the sample. Inhomogeneous peaks with full widths at half maximum (FWHM) of ∼1 ppm typically result from ABMS broadening and the low spectral resolution impedes the analysis of solid-state NMR spectra. ABMS broadening of solid-state NMR spectra has previously been eliminated using 2D multiple-quantum correlation experiments, or by performing NMR experiments on diluted materials or single crystals. However, these experiments are often infeasible due to their poor sensitivity and/or provide limited gains in resolution. 2D 1H-13C HETCOR experiments have previously been applied to reduce susceptibility broadening in paramagnetic solids and we show that this strategy can significantly reduce ABMS broadening in diamagnetic organic solids. Comparisons of 1D solid-state NMR spectra and 1H and 13C solid-state NMR spectra obtained from 2D 1H-13C HETCOR NMR spectra show that the HETCOR spectrum directly increases resolution by a factor of 1.5 to 8. The direct gain in resolution is determined by the ratio of the inhomogeneous 13C/1H linewidth to the homogeneous 1H linewidth, with the former depending on the magnitude of the ABMS broadening and the strength of the applied field and the latter on the efficiency of homonuclear decoupling. The direct gains in resolution obtained using the 2D HETCOR experiments are better than that obtained by dilution. For solids with long proton longitudinal relaxation times, dynamic nuclear polarization (DNP) was applied to enhance sensitivity and enable the acquisition of 2D 1H-13C HETCOR NMR spectra. 2D 1H-13C HETCOR experiments were applied to resolve and partially assign the NMR signals of the form I and form II polymorphs of aspirin in a sample containing both forms. These findings have important implications for ultra-high field NMR experiments, optimization of decoupling schemes and assessment of the fundamental limits on the resolution of solid-state NMR spectra.

Influence of small difference in structure of polyamide charring agents on their flame-retardant efficiency in EVA

Two kinds of polyamide charring agents, poly(piperazinyl malonamide) (PPMA) and poly(piperazinyl succinamide) (PPSA), were synthesized, and their only difference is that PPSA increases one methylene in the main chain compared with PPMA, which were confirmed via Fourier transform infrared spectroscopy (FTIR), Elemental analysis (EA), 1H nuclear magnetic resonance (1H-NMR) spectrum and 13C solid NMR spectrum. Thermogravimetric (TG) analysis showed that the residue of PPMA was remarkably higher than that of PPSA during the thermal decomposition process. When they were used to flame retard EVA, PPMA showed higher efficiency than PPSA for passing the UL-94 V-0 rating, increasing the oxygen index (OI), and decreasing the total heat release (THR) and total smoke production (TSP); while the PPSA was more efficient in decreasing the peak of heat release rate (PHRR) and the peak of smoke production rate (peak SPR). The investigation for the pyrolysis products of PPMA and PPSA demonstrated that various aromatic structures were produced due to the dehydrogenation of piperazine ring during the decomposition of PPMA and PPSA, which endowed them with excellent charring capability, and the charring efficiency of PPMA was higher than that of PPSA. However, the PPSA with lower charring capability tended to decompose and cyclize to the imides and volatilize into the gas phase, which resulted in the more ideal microstructure of PPSA system than that of PPMA system, finally led to the lower PHRR and peak SPR of PPSA system in cone calorimeter test.

Effective transformation of cellulose to 5-hydroxymethylfurfural catalyzed by fluorine anion-containing ionic liquid modified biochar sulfonic acids in water

This article first discloses that the fluorine anion-containing ionic liquids-functionalized biochar sulfonic acids (BCSA-IL-F1–3s), which were simply synthesized by an ionic exchange of 1-trimethoxysilylpropyl-3-methylimidazolium chloride (IL-Cl) grafted on the BCSA with CF3SO3H (HF1), HBF4 (HF2), HPF6 (HF3), respectively, can efficiently catalyze cellulose hydrolysis into reducing sugars (RSs) and 5-hydroxymethyl furfural (HMF) in water under microwave irradiation. This process provides a very high catalysis efficiency (turnover numbers, 4.03–4.89) at mild temperature (80 °C) for 3 h, but also possesses an excellent repeatability. More outstandingly, they can achieve much higher HMF yields (12.70–27.94%) compared to the IL-Cl-functionalized BCSA catalyst (HMF yields are lower than 0.1%) under the same reaction conditions. This is likely because the introduction of IL-F1–3s groups can significantly improve the accessibility, acidity and thermal stability of BCSA’s SO3H sites, as supported by evidence from a solid 31P NMR spectrum and thermogravimetric analysis. It is proposed that the good selectivity for HMF perhaps originates from a co-catalysis action of the IL-F1–3s and SO3H groups on BCSA-IL-F1–3s in the further conversion of RSs to HMF.

Mechanism of Proton Conduction in Perfluorinated Sulfonic Acid Polymers with High Ionic Exchange Capacity at Water-Freezing Temperature Region

Molecular-level understanding of dynamic behaviors of complex mass transports, especially proton and water- in polymer electrolytes is essential to design better membrane materials used for polymer electrolyte fuel cells (PEFCs). NMR is one of the suitable experimental techniques for analyzing such molecular dynamics, since both complex static structures and dynamical motions of molecules can be tracked by high-resolution NMR spectra and a pulsed field gradient NMR (PFG-NMR). This technique is widely used to understand proton dynamics however, it cannot clearly distinguish vehicle and grotthuss mechanisms. Recently, we have developed a combination method of 2H, 17O MAS NMR and 1H PFG NMR measurement to overcome the drawback. We applied the technique to an organic-inorganic composite membrane, Zr-SPP-SPES, which forms controlled configuration of proton functional groups [1,2]. In the paper we proposed enhanced grottuss mechanism in a water-freezing temperature region called "packed acid mechanism”. [6]In the present study we prepared perfluorinated sulfonic acid membranes with different equivalent weight (EW) 500, 600, 700 and 900, to understand relationship between value of ion exchange capacity (IEC = 1000/EW) and proton conduction mechanism in water freezing region. 17O labeled water was used in order to distinguish signals from non-exchangeable protons in the membranes, thus natural abundance 17O (0.04%) NMR signals of water in membrane cannot be detected. 1H and 17O MAS NMR measurements were carried out on Agilent NMR systems 400WB at 9.4 T. A 4 mm diameter air tight rotor was used for sample spin at 12 kHz. 1H and 17O 1D-NMR spectra were obtained by using a single pulse sequence. Sample temperature was calibrated by measuring the 79Br signal of KBr powder under the same measurement condisions as described elsewhere. [3] 1H PFG NMR measurements were carried out on JNM LA-400 at 9.4 T. NMR spectrometer equipped with maximum gradient strength of 12 T/m. The diffusion coefficient (D) was measured by using a pulsed-field-gradient stimulated-echo pulse sequence. [4, 5] 17O NMR spectrum in all the membrane show a sharp peak around 0ppm, from temperature 6 to 42℃. With lowering temperature from -20 to -45 ℃, the width of the spectra of all the membranes is drastically increased then finally the spectrum is vanished at T = -45 ℃. The behavior of 17O NMR in all samples were almost similar. Since the width of the NMR spectrum is inversely proportional to the movement of the observed nucleus, it indicates that absorbed water molecules are completely freezing.In contrast to the 17O NMR, solid 1H NMR spectra show a sharp peak even at the low temperature range, from -25 to -45 ℃. This result indicates that proton conductivity is still remaining, even while absorbed water molecules are freezing. 1H PFG-NMR also revealed that proton conduction is present at low temperature as well as the results of 1H NMR. The sample with highest IEC value ( IEC ~ 2 ) shows highest proton conductivity D = 7 x 10-7 (m2/s) at T = -25 ℃ and D = 0.5 x 10-7 (m2/s) at T = -60 ℃.These three results indicate that protons in the membranes with high IEC move independently like a "packed acid mechanism” in freezing temperature region. This conduction mechanism would happen efficiently because sulfonic acid is tightly-packed, which is a noteworthy property of the high IEC membranes.

Combining solid-state NMR spectroscopy with first-principles calculations - a guide to NMR crystallography.

Recent advances in the application of first-principles calculations of NMR parameters to periodic systems have resulted in widespread interest in their use to support experimental measurement. Such calculations often play an important role in the emerging field of "NMR crystallography", where NMR spectroscopy is combined with techniques such as diffraction, to aid structure determination. Here, we discuss the current state-of-the-art for combining experiment and calculation in NMR spectroscopy, considering the basic theory behind the computational approaches and their practical application. We consider the issues associated with geometry optimisation and how the effects of temperature may be included in the calculation. The automated prediction of structural candidates and the treatment of disordered and dynamic solids are discussed. Finally, we consider the areas where further development is needed in this field and its potential future impact.

Porous carbonaceous solid acids derived from farm animal waste and their use in catalyzing biomass transformation

We demonstrate here the transformation of waste cow manure into N rich porous carbons (NPC) with large amount of micropores and large Brunauer–Emmett–Teller (BET) surface areas. The transformation process was achieved via carbonization of cow manure in presence of ZnCl2 and FeCl3 additives, without using additional templates. The resultant NPC could be converted into microporous carbonaceous solid acids (NPC-[C4N][X]) by quaternary ammonization of samples with 1,4-butane sultone, and ion exchanging with strong acids such as HSO3CF3 and H2SO4. X-ray diffraction patterns and Raman spectra showed that NPC-[C4N][X] exhibit amorphous carbon network. N2 isotherms and SEM images showed that NPC-[C4N][X] have abundant nanopores with large BET surface areas. XPS, FT-IR and elemental analysis results showed the N rich network in NPC support, has been successfully and homogeneously functionalized with strongly acidic ionic liquid and sulfonic groups. 31P solid NMR spectra confirmed that NPC-[C4N][X] have typical Brønsted acid centers with strong and controllable acidity. Catalytic tests showed that NPC-[C4N][X] exhibits excellent activity and reusability in production of biofuels and fine chemicals through esterifications, transesterifications, and depolymerization of microcrystalline cellulose. This approach may provide a novel and cost effective way to recycle the waste cow manure into efficiently porous carbon based solid acids for heterogeneous acid catalysis.

Highly crosslinked poly(ethyleneglycol dimethacrylate)-based microspheres via solvothermal precipitation polymerization in alcohol–water system

The preparation of highly crosslinked polymer microspheres through the polymerization reaction in an alcohol–water solution without any stabilizer is still a difficult task. Here, we first reported a facile preparation of monodispersed highly-crosslinked microspheres through the polymerization of ethyleneglycol dimethacrylate (EGDMA) in an alcohol–water solution in the presence of a small amount of hydrophilic comonomer with a high reactivity ratio in a sealed autoclave at above the boiling point of the solvent, which can be considered as a solvothermal precipitation polymerization process. The formation mechanism of the poly(ethyleneglycol dimethacrylate) (PEGDMA)-based microspheres involving an initial nucleation process and the grow process of the microspheres was proposed and confirmed by the composition and distribution of the hydrophilic comonomer in the oligomers and microspheres, which were analyzed by 1H and 13C CP-MAS solid NMR spectra, elemental mapping, and XPS. This work opens a new economic and environmental-friendly way to prepare highly-crosslinked functional PEGDMA-based microspheres.

Proton chemical shift tensors determined by 3D ultrafast MAS double-quantum NMR spectroscopy.

Proton NMR spectroscopy in the solid state has recently attracted much attention owing to the significant enhancement in spectral resolution afforded by the remarkable advances in ultrafast magic angle spinning (MAS) capabilities. In particular, proton chemical shift anisotropy (CSA) has become an important tool for obtaining specific insights into inter/intra-molecular hydrogen bonding. However, even at the highest currently feasible spinning frequencies (110-120 kHz), (1)H MAS NMR spectra of rigid solids still suffer from poor resolution and severe peak overlap caused by the strong (1)H-(1)H homonuclear dipolar couplings and narrow (1)H chemical shift (CS) ranges, which render it difficult to determine the CSA of specific proton sites in the standard CSA/single-quantum (SQ) chemical shift correlation experiment. Herein, we propose a three-dimensional (3D) (1)H double-quantum (DQ) chemical shift/CSA/SQ chemical shift correlation experiment to extract the CS tensors of proton sites whose signals are not well resolved along the single-quantum chemical shift dimension. As extracted from the 3D spectrum, the F1/F3 (DQ/SQ) projection provides valuable information about (1)H-(1)H proximities, which might also reveal the hydrogen-bonding connectivities. In addition, the F2/F3 (CSA/SQ) correlation spectrum, which is similar to the regular 2D CSA/SQ correlation experiment, yields chemical shift anisotropic line shapes at different isotropic chemical shifts. More importantly, since the F2/F1 (CSA/DQ) spectrum correlates the CSA with the DQ signal induced by two neighboring proton sites, the CSA spectrum sliced at a specific DQ chemical shift position contains the CSA information of two neighboring spins indicated by the DQ chemical shift. If these two spins have different CS tensors, both tensors can be extracted by numerical fitting. We believe that this robust and elegant single-channel proton-based 3D experiment provides useful atomistic-level structural and dynamical information for a variety of solid systems that possess high proton density.

High-resolution NMR of hydrogen in organic solids by DNP enhanced natural abundance deuterium spectroscopy

We demonstrate that high field (9.4T) dynamic nuclear polarization (DNP) at cryogenic (∼100K) sample temperatures enables the rapid acquisition of natural abundance 1H–2H cross-polarization magic angle spinning (CPMAS) solid-state NMR spectra of organic solids. Spectra were obtained by impregnating substrates with a solution of the stable DNP polarizing agent TEKPol in tetrachloroethane. Tetrachloroethane is a non-solvent for the solids, and the unmodified substrates are then polarized through spin diffusion. High quality natural abundance 2H CPMAS spectra of histidine hydrochloride monohydrate, glycylglycine and theophylline were acquired in less than 2h, providing direct access to hydrogen chemical shifts and quadrupolar couplings. The spectral resolution of the 2H solid-state NMR spectra is comparable to that of 1H spectra obtained with state of the art homonuclear decoupling techniques.

Effects of inherent rf field inhomogeneity on heteronuclear decoupling in solid-state NMR

Efficient heteronuclear decoupling is essential to obtain high-resolution NMR spectra of organic and biological solids containing protons and low-gamma nuclei such as carbon-13. All modern decoupling sequences employ pulse lengths that are optimized for a given radio-frequency (rf) power. However, there is an unavoidable distribution of rf field amplitudes across the volume of the sample. Inevitably, the effect of the rf field distribution manifests itself for fully packed rotors during any decoupling irradiation. In this study we present a detailed analysis of the effect of the inherent rf field inhomogeneity on decoupling by the low-power phase-inverted supercycled sequence for attenuation of rotary resonance (PISSARRO). This reveals a potential for further improvements of its efficiency.

PIC microcontroller based external fast analog to digital converter to acquire wide-lined solid NMR spectra by BRUKER DRX and Avance-I spectrometers

Concerning many former liquid or hybrid liquid/solid NMR consoles, the built in Analog-to-Digital Converters (ADCs) are incapable of digitizing the fids at sampling rates in the MHz range. Regarding both strong anisotropic interactions in the solid state and wide chemical shift dispersion nuclei in solution phase such as 195Pt, 119Sn, 207Pb etc., the spectrum range of interest might be in the MHz range. As determining the informative tensor components of anisotropic NMR interactions requires nonlinear fitting over the whole spectrum including the asymptotic baseline, it is prohibited by low sampling rates of the ADCs. Wide spectrum width is also useful in solution NMR, since windowing of wide chemical shift ranges is avoidable. We built an external analog to digital converter with 10MHz maximal sampling rate, which can work simultaneously with the built in ADC of the spectrometer. The ADC was tested on both Bruker DRX and Avance-I NMR consoles. In addition to the analog channels it only requires three external digital lines of the NMR console. The ADC sends data to PC via USB. The whole process is controlled by software written in JAVA which is implemented under TopSpin.

Freezing of Molecular Motions Probed by Cryogenic Magic Angle Spinning NMR.

Cryogenic magic angle spinning makes it possible to obtain the NMR spectra of solids at temperatures low enough to freeze out most molecular motions. We have applied cryogenic magic angle spinning NMR to a crystalline small-molecule solid (ibuprofen sodium salt), which displays a variety of molecular dynamics. Magic angle (13)C NMR spectra are shown for a wide range of temperatures, including in the cryogenic regime down to 20 K. The hydrophobic and hydrophilic regions of the molecular structure display different behavior in the cryogenic regime, with the hydrophilic region remaining well-structured, while the hydrophobic region exhibits a broad frozen conformational distribution.

Design and synthesis of hydrophobic and stable mesoporous polymeric solid acid with ultra strong acid strength and excellent catalytic activities for biomass transformation

Novel and efficient mesoporous polydivinylbenzene (PDVB) based solid strong acid (PDVB-SO3H-SO2CF3) has been successfully prepared by grafting of strong electron withdrawing group of SO2CF3 onto the network of performed mesoporous solid acid of PDVB-SO3H, which could be synthesized from sulfonation of superhydrophobic mesoporous PDVB or copolymerization of DVB with sodium p-styrene sulfonate. Characterizations of N2 sorption isotherms, TG curves and contact angle tests show that PDVB-SO3H-SO2CF3 has large Brunauer–Emmett–Teller (BET) surface area, superior thermal stability, good hydrophobicity and oleophilicity. Solid 31P NMR spectra and NH3–TPD curves show that the Brønsted acidic site in PDVB-SO3H-SO2CF3 has been significantly enhanced and rather homogeneously distributed when compared with that of PDVB-SO3H. Catalytic tests show that PDVB-SO3H-SO2CF3 exhibits excellent catalytic activities and good recyclability in biomass transformation toward transesterification to biodiesel and depolymerization of crystalline cellulose to sugars when compared with those of PDVB-SO3H, solid strong acids of SO4/ZrO2 and Nafion NR50. The excellent catalytic activity and good recyclability of PDVB-SO3H-SO2CF3 result from its unique characters such as large surface area, ultra strong acid strength, adjustable hydrophobic–oleophilic and stable network.

Synthesis, spectral, and structural characterizations of imidazole oxalato molybdenum(iv/v/vi) complexes

Substitutions of trans-Na(Him)[Mo(2)O(4)(ox)(2)(H(2)O)(2)]·H(2)O (1) and trans-(Him)(2)[Mo(2)O(4)(ox)(2)(H(2)O)(2)] (2) with imidazole result in the formation of the mixed-ligand molybdenum complexes cis-Na(2)[Mo(2)O(4)(ox)(2)(im)(2)]·4.5H(2)O (3), cis-K(2)[Mo(2)O(4)(ox)(2)(im)(2)]·3H(2)O (4), respectively (H(2)ox = oxalic acid; im = imidazole). Further reduction of cis-K(2)[Mo(2)O(4)(ox)(2)(im)(2)]·3H(2)O (4) gives a trinuclear molybdenum(IV) complex K(Him)[Mo(3)O(4)(ox)(3)(im)(3)]·3H(2)O (5), which contains an incomplete cubane cluster [Mo(IV)(3)O(4)](4+). Two novel trinuclear mixed-valence imidazole compounds [Mo(3)O(8)(im)(4)](im)·H(2)O (6) and [Mo(3)O(8)(im)(4)]·H(2)O (7) were obtained by the reduction of (Him)(4)[Mo(8)O(26)(im)(2)] (8). Both 6 and 7 contain a novel Mo(VI)O(4)(Mo(V)(2)O(4)) center, where the [Mo(V)(2)O(4)](2+) unit is linked by [Mo(VI)O(4)](2-) anion. The Mo-Mo bond distances in 1-7 decrease with the decrease of oxidation state of molybdenum. Solid and solution NMR spectra show that imidazole molybdenum compounds 6-8 fully dissociate in solution, where solvated imidazole and imidazolium groups in 6 and 8 could be served as internal references in their solid (13)C NMR spectra. Furthermore, mixed-ligand molybdenum species 3 and 4 are stable in water. Stabilities of 3 and 4 in solution may be attributed to the strong coordination of bidentate oxalate and the formation of hydrogen bond. Dimers 2 and 4 display quasi-reversible redox process, while trimer 6 is irreversible. Bond valence calculations for 1-8 are consistent with their oxidation states of molybdenum atoms. Calculation of the oxidation state in recent structure of iron molybdenum cofactor [MoFe(7)S(9)C(R-homocit)] (FeMo-co) is 3.318.

Solid and solution evidences on the temperature effect of N-chelated zinc phthalates

Progressive reactions of zinc(II) phthalate in the presence of 2,2-bipyridine result in the isolations of three complexes, [Zn(phth)(bpy)(H2O)]2·2H2O (1), [Zn(phth)(bpy)(H2O)]2 (2) and {[Zn(phth)(bpy)(H2O)][Zn(phth)(bpy)]·H2O}n (3) (bpy=2,2-bipyridine). Heating the reaction mixture reduces the coordination numbers from distorted octahedron, trigonal–bipyramid to tetrahedron configurations. The metal configurations of the isolated complexes depend on the reaction temperature. Detailed comparisons in solid and solution NMR spectra reveal that dimers 1 and 2 show no decomposition in solution and retain their coordinated structures in solid. Obvious downfield shifts (Δδ=3.19 and 2.31ppm) of carboxy groups in 2 are observed compared with those of dihydrates 1 in solid and solution respectively, which are consistent with the bond strengths of their structures.

Mechanism of oxidation of low rank coal by nitric acid

The pretreatment of low rank coal with nitric acid oxidation can promote its bio-liquefaction. However, the detailed mechanism of which remains an unresolved problem. In the present work, the characteristics of Fushun coal before and after oxidation by nitric acid were investigated combined with elemental composition, pore volume and pore size, Zeta potential, FT-IR, and 13C solid NMR spectrum analysis. The results show that the inorganic substance inlaid in coal are dissolved by nitric acid, which results in the decrease of coal ash content and increase of pore volume and pore size. Furthermore, there exist obvious chemical reactions between nitric acid and the functional groups of coal including aromatic ring carboxylation, side chain alkyl of aromatic ring oxidation and aromatic ring nitration. Among these reactions, some led to the increase in content of carboxyl, aliphatic carbon connected with O and humic acid carbon, while others caused the reduction of aromaticity, methyl carbon, substituted aryl carbon and side chain.

Coherent averaging in the frequency domain.

Quantum-mechanical evolution of systems with periodic time-modulated Hamiltonians is often described by effective interactions. Such average Hamiltonians, calculated as few terms of an expansion in powers of the interaction, are sometimes difficult to relate to experimental observations. We propose a frequency-domain approach to this problem, which offers certain advantages and produces an approximate solution for the density matrix, better linked to measurable quantities. The formalism is suitable for calculating the intensities of narrowed spectral peaks. Fast magic-angle-spinning NMR spectra of solids are used to experimentally illustrate the method.