Quantitative characterization of coal structure by high-resolution CP/MAS 13C solid-state NMR spectroscopy

A method for quantitatively characterizing the carbon skeletal structure of coal by variable contact time experiment using high-resolution CP/MAS ¹³C solid-state NMR spectroscopy is proposed in this paper. The initial polarization transfer intensity from protons directly bonded with carbons, instead of dipolar-dephasing techniques which had to run on a lower frequency NMR spectrometer (100.02 MHz for proton), was used to divide the bridgehead and protonated aromatic carbons, making all the NMR data in this paper obtained on a high frequency NMR spectrometer (500.12 MHz for proton). On this basis, the fractions of different carbons in coal were further divided by the initial polarization transfer intensity from spin diffusion of protons unbonded with carbons. The structure of Naomaohu coal, a subbituminous coal from China, was measured. The change of polarization transfer with contact time was analyzed quantitatively. The fractions of aromatic, aliphatic, carboxyl and carbonyl carbons, and corrective aromaticity are 0.61, 0.39, 0.1 and 0.51, respectively. The fractions of protonated and bridgehead aromatic carbons are 0.22 and 0.09, respectively. These results agreed with literatures, and bond concentration calculated by the carbon skeletal structure distribution of coal was reasonable.     

Si CP/MAS NMR of phosphorus-bearing organosilicon compounds

While liquid-state ²⁹Si NMR of phosphorus-bearing organosilicon compounds with more than one phosphorus per molecule can take advantage of the presence of J-coupling ⁿJ(³¹P²⁹Si) for purposes of structural assignment from J-coupling patterns, conventional ²⁹Si CP/MAS spectra of such molecular solids do not reveal structural details in a straightforward manner. For such compounds it is necessary to obtain ²⁹Si CP/MAS spectra under conditions of simultaneous ¹H- and ³¹P-high power decoupling in order to derive reliable ²⁹Si chemical shift information. ²⁹Si CP/MAS NMR spectra, obtained with and without ³¹P high power decoupling during the acquisition time, of several organosilicon compounds containing Si_xP_y (x = 1 −10, y = 1 −10) moieties are reported.     

13C CP MAS NMR and GIAO-CHF calculations of coumarins

13C cross-polarization magic-angle spinning NMR spectra were recorded for a series of solid coumarins. Ab initio calculations of shielding constants were performed with the use of GIAO-CHF method. The combined CPMAS NMR and theoretical approach was successful in characterizing solid-state conformations of coumarins; a relationship sigma (ppm) = -1.032 xdelta + 205.28 (R(2) = 0.9845) can be used to obtain structural information for coumarins, for which solid-state NMR or crystal structure data are not available.     

Characteristics of zero-quantum correlation spectroscopy in MAS NMR experiments

Zero-quantum coherence generation and reconversion in magic-angle spinning solid-state NMR is analyzed. Two methods are discussed based on implementations using symmetry-based pulse sequences that utilize either isotropic J couplings or dipolar couplings. In either case, the decoupling of abundant proton spins plays a crucial role for the efficiency of the zero-quantum generation. We present optimized sequences for measuring zero-quantum single-quantum correlation spectra in solids, achieving an efficiency of 50% in ubiquitin. The advantages and disadvantages of zero-quantum single-quantum over single-quantum single-quantum correlation spectroscopy are explored, and similarities and differences with double-quantum single-quantum correlation spectroscopy are discussed. Finally, possible application of zero-quantum single-quantum experiments to polypeptides, where it can lead to better spectral resolution is investigated using ubiquitin, where we find high efficiency and high selectivity, but also increased line widths in the MQ dimension.     

在CP MAS NMR中测量质子自旋扩散速率的方法

自旋扩散在固体核磁共振的许多现象中都起着非常重要的作用。现有几种理论方案以估算扩散系数。然而在实践中这种估算既不实际也不可靠。本文提出了确定自旋扩散速率的新方案,它利用的是CP MAS NMR中的稀核退极化规律。带质子的稀核磁化矢量在退极化中表现出两个阶段,慢衰减的第二阶段是单一指数过程,它提供了自旋扩散速率的直接度量。自旋扩散实质上是极化转移的一种宏观表现形式,这种转移通过一系列成对自旋的flip-flop进行,可以用一维随机走步模型描述。从退磁过程半对数曲线的斜率可以求得平均flip-flop时间。自旋扩散系数可以由此估算。在一些典型的刚性有机固体和结晶高分子聚合物中,求得平均flip-flop的时间是700微秒左右。它比偶极相关时间大一个数量级。这意味着,自旋扩散时间常数与自旋—自旋弛豫时间常数是很不相同的,虽然这两个相应过程虽密切相关的。由质子线宽估计自旋扩散系数是不可靠的。     

Dynamic hydrogen disorder in yttrium-dihydride phases as seen by variable-temperature Y CP MAS NMR

Variable-temperature solid-state MAS NMR studies on some yttrium-dihydride phases YH_2+x are reported and yield evidence that ⁸⁹Y CP MAS NMR techniques are an experimentally feasible route to investigate order–disorder phenomena in such metal-hydride phases.     

Studies of silicophosphate derivatives by 31P-->29Si CP MAS NMR

We show that it is possible to efficiently transfer magnetization from ³¹P to ²⁹Si, using variable amplitude CP MAS experiment. This experiment is demonstrated by using Si₅O(PO₄)₆, the synthesis protocol of which is described. From the obtained results, we show that the experiment allows the spectral edition of ²⁹Si spectra from ³¹P→²⁹Si CP, enabling the studies of derivatives involving Si–O–P linkages, such as phosphosilicate glasses, microporous silicoaluminophosphates (SAPO) and bioactive phosphosilicates.     

CP/MAS NMR in trans-β-carotene

CP/MAS NMR spectra of solid trans-β-carotene were assigned, compared with crystallographic data and discussed in terms of molecular motion.     

尼龙6结晶和动力学的固体高分辨^13C NMR研究

用MSL—400核磁共振(NMR)波谱仪记录了尼龙6的~(13)C交叉极化魔角旋转(CP/MAS)NMR谱,发现固态下ω碳原子有两个峰,尼龙6记为ω_1和ω_2。在同一样品的~(13)C NMR溶液谱中,这两个峰合并成一个峰,证明固态尼龙6中两种晶区是共存的。在固态,ω_2峰比ω_1峰向高场偏移2.2ppm。测量了尼龙6—系列不同接触时间的CP/MAS NMR谱以研究动态行为。用SIMPLEX非线性最小二乘法拟合技术计算了交叉极化速率和质子T_(1ρ)~H用Ngai公式计算了固态尼龙6的运动参数。     

High-field high-speed MAS resolution enhancement in 1H NMR spectroscopy of solids

Resolution in ¹H NMR spectra of solids can be significantly enhanced with fast magic-angle spinning and high magnetic fields. A variable field and spinning speed study up to 25 T and 40 kHz shows that the homogeneous line broadening is inversely proportional to the product of magnetic field strength and spinning speed. The combination of high field and fast speed yields a ¹H linewidth approaching the intrinsic limit determined by anisotropy of magnetic susceptibility. An analysis of the anisotropic magnetic susceptibility line broadening is presented.     

Two-dimensional triple-quantum Na MAS NMR spectroscopy of sodium cations in dehydrated zeolites

Two-dimensional triple-quantum (2D-3Q) ²³Na MAS NMR spectroscopy has been applied for the investigation of sodium cations in dehydrated zeolites NaY, NaEMT, NaZSM-5 and NaMOR. The experiments have shown that the new 2D-3Q technique allows the determination of the isotropic chemical shifts and quadrupolar couplings of sodium cations with SOQE (second-order quadrupolar effect) parameters of up to ca. 4 MHz. In the present work, SOQE parameters of 1.0–1.2 MHz were found for sodium cations located at positions SI in the hexagonal prisms of dehydrated zeolites NaY and NaEMT. The sodium cations located in the 10-ring and 12-ring channels of dehydrated zeolites NaZSM-5 and NaMOR, respectively, are characterized by a SOQE parameter of 2.0 MHz while a value of 3.1 MHz was determined for sodium cations in the sidepockets of the channels in dehydrated NaMOR.     

Experimental method for low-temperature reaction studies in sealed samples by 13C CP/MAS NMR

A new, simple, and inexpensive technique is presented for monitoring high-resolution solid-state NMR of ¹³C at temperatures ranging between 85 and 450 K. In this procedure, the reaction conditions are controlled by preparing samples at 77 K in 5 mm NMR tubes, while attached to a vacuum system. The NMR tubes are prefitted with a rotor for spinning. After preparation, the samples are sealed, transferred to the double-resonance MAS NMR probe, and analyzed, all while the sample temperature is maintained as low as 85 K. The spinning rates vary from 3.0 kHz at 85 K to 5.2 kHz at 300 K using nitrogen drive gas. Probe design and performance, sample-preparation procedure, and details of the low-temperature experiment are described. In general, the technique may be applied in studies of low-temperature reaction mechanisms and kinetics. ¹³C CP/MAS spectra of ethylene adsorbed on silica-supported ruthenium catalyst are presented to illustrate its performance and possible application.     

硫酸交联壳聚糖干膜的13C CP MAS NMR研究

以不同浓度硫酸交联的壳聚糖干膜为研究对象,在国内首次采用¹³C CP MAS NMR技术从分子水平上对壳聚糖膜的结构进行研究,结果表明干膜壳聚糖分子的构象能直观地反映出膜的交联状况,交联状况与膜的分离性能密切相关,但仅对干膜的考察还不够全面.     

硫酸交联壳聚糖湿膜的13C CP MAS NMR研究

首次采用¹³C CP MAS NMR技术对接近膜分离状态时的湿膜进行研究.结果表明:温膜的分子构象和分子运动能较全面地反映出膜的分离特性,弱的交联以及过快或过慢的分子运动均不利于膜的分离.     

Proton-detected heteronuclear single quantum correlation NMR spectroscopy in rigid solids with ultra-fast MAS

In this article, we show the potential for utilizing proton-detected heteronuclear single quantum correlation (HSQC) NMR in rigid solids under ultra-fast magic angle spinning (MAS) conditions. The indirect detection of carbon-13 from coupled neighboring hydrogen nuclei provides a sensitivity enhancement of 3- to 4-fold in crystalline amino acids over direct-detected versions. Furthermore, the sensitivity enhancement is shown to be significantly larger for disordered solids that display inhomogeneously broadened carbon-13 spectra. Latrodectus hesperus (Black Widow) dragline silk is given as an example where the sample is mass-limited and the sensitivity enhancement for the proton-detected experiment is 8- to 13-fold. The ultra-fast MAS proton-detected HSQC solid-state NMR technique has the added advantage that no proton homonuclear decoupling is applied during the experiment. Further, well-resolved, indirectly observed carbon-13 spectra can be obtained in some cases without heteronuclear proton decoupling.     

CP/MAS 13C-NMR spectroscopy applied to structure and interaction studies on cellulose I

Solid-state Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (CP/MAS 13C-NMR) has been used to investigate the structure and interactions of cellulose I. The use of spectral fitting for the extraction of information from CP/MAS 13C-NMR spectra is reviewed and results obtained are discussed. Examples are shown where the method has been used to monitor the structural changes occurring in wood cellulose during kraft pulping. The effects observed on the cellulose and hemicelluloses are further investigated using a model system. Assignments of signal intensities originating from xylan-cellulose interactions are made.     

2H chemical shift anisotropies from high-field 2H MAS NMR spectroscopy

2H chemical shift anisotropies (CSAs) have been determined for the first time for polycrystalline samples employing 2H MAS NMR spectroscopy at high magnetic field strength (14.1 T). The 2H CSA is reflected as distinct asymmetries in the manifold of spinning sidebands (ssbs) observed for the two overlapping single-quantum transitions. Least-squares fitting to the manifold of ssbs allows determination of the 2H CSA parameters along with the quadrupole coupling parameters. This is demonstrated for KD2PO4, ND4D2PO4, KDSO4, KDCO3, alpha-(COOD)2, alpha-(COOD)2.2D2O, and boehmite (AlOOD) which exhibit 2H shift anisotropies in the range 13< or =deltasigma< or =27 ppm. For fixed values of the shift anisotropy and the 2H quadrupole coupling it is shown that the precision of the CSA parameters depends strongly on the asymmetry parameter (etaQ) for the quadrupole coupling tensor, giving the highest precision for etaQ approximately 0. The 2H CSA parameters (deltasigma and etasigma) are in good agreement with 1H CSA data reported in the literature for the corresponding protonated samples from 1H NMR spectra employing various homonuclear decoupling techniques. The determination of 2H quadrupole coupling parameters and 2H (1H) CSAs from the same 2H MAS NMR experiment may be particularly useful in studies of hydrogen bonding since the 2H quadrupole coupling constant and the CSA appear to characterize bond lengths in a hydrogen bond in a different manner.     

Crystal structure and 13C CP/MAS NMR of the p-xylene clathrate of Dianin's compound

Single-crystal X-ray diffraction and ¹³C solid state NMR spectroscopy were used to characterize the structure and dynamics of the p-xylene clathrate of Dianin's compound. In contrast to conclusions obtained from single-crystal ²H NMR and modelling, the diffraction results suggest there is a single (symmetry disordered) guest site without any significant distortion of the host framework. A single xylene guest statistically disordered over six overlapping equivalent positions can account for the ¹³C NMR spectrum at room temperature. The high crystal-lographic symmetry arises from space averaging. At high temperatures the ¹³C spectrum is consistent with the onset of dynamic processes that result in higher effective symmetry.     

Use of SPAM and FAM pulses in high-resolution MAS NMR spectroscopy of quadrupolar nuclei

The merits of SPAM and FAM pulses for enhancing the conversion of triple- to single-quantum coherences in the two-dimensional MQMAS experiment are compared using (87)Rb (spin I=3/2) and (27)Al (I=5/2) NMR of crystalline and amorphous materials. Although SPAM pulses are more easily optimized, our experiments and simulations suggest that FAM pulses yield greater signal intensity in all cases. In conclusion, we argue that, as originally suggested, SPAM and FAM pulses are best implemented in phase-modulated whole-echo MQMAS experiments and that the use of SPAM pulses to record separate echo and antiecho data sets, which are then combined, generally yields lower signal-to-noise ratios.