Chemical shift referencing in MAS solid state NMR

Solid state 13C magic angle spinning (MAS) NMR spectra are typically referenced externally using a probe which does not incorporate a field frequency lock. Solution NMR shifts on the other hand are more often determined with respect to an internal reference and using a deuterium based field frequency lock. Further differences arise in solution NMR of proteins and nucleic acids where both 13C and 1H shifts are referenced by recording the frequency of the 1H resonance of DSS (sodium salt of 2,2-dimethyl-2-silapentane-5-sulphonic acid) instead of TMS (tetramethylsilane). In this note we investigate the difficulties in relating shifts measured relative to TMS and DSS by these various approaches in solution and solids NMR, and calibrate adamantane as an external 13C standard for solids NMR. We find that external chemical shift referencing of magic angle spinning spectra is typically quite reproducible and accurate, with better than +/-0.03 ppm accuracy being straight forward to achieve. Solid state and liquid phase NMR shifts obtained by magic angle spinning with external referencing agree with those measured using typical solution NMR hardware with the sample tube aligned with the applied field as long as magnetic susceptibility corrections and solvent shifts are taken into account. The DSS and TMS reference scales for 13C and 1H are related accurately using MAS NMR. Large solvent shifts for the 13C resonance in TMS in either deuterochloroform or methanol are observed, being +0.71 ppm and -0.74 ppm from external TMS, respectively. The ratio of the 13C resonance frequencies for the two carbons in solid adamantane to the 1H resonance of TMS is reported.     

Study of diamond film by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Three chemical vapor deposited diamond films were studied by dynamic nuclear polarization (DNP)-enhanced high-resolution solid-state¹³C nuclear magnetic resonance (NMR) spectroscopy. Enhanced¹³C direct-polarization spectra of diamond films were obtained by irradiating the samples with microwaves at or near electron spin resonance Larmor frequency of carbon center free radicals. No NMR signal for sp² hybridized carbons could be observed. From the curve of the DNP enhancement as a function of frequency, it is found that the dominant DNP mechanism is the solid-state effect. The¹³C cross-polarization spectrum, which is an evidence for existence of the proton defect in the lattice of diamond films, is much broader than the¹³C single pulse spectrum. The reason is discussed shortly.     

葡糖衍生手性N-杂环卡宾前体波谱学数据分析

以溴代正丁烷与1-（2,3,4,6-四-O-乙酰基-β-D-吡喃葡萄糖基）咪唑（化合物1）为原料合成了溴化1-（2,3,4,6-四-O-乙酰基-β-D-吡喃葡萄糖基）-3-正丁基咪唑盐（化合物2）,它是葡萄糖衍生的手性N-杂环卡宾前体.化合物2的结构中存在多个手性碳原子,导致它的核磁共振（NMR）谱较为复杂.在此,用元素分析、红外吸收光谱（IR）以及液相色谱-高分辨质谱（LC-HRMS）对它的组成进行了分析,然后利用1D和2D NMR谱（包括¹H NMR、¹³C NMR、DEPT135、DEPT90、DEPT45、COSY、¹H-¹³C HSQC、¹H-¹³C HMBC）对化合物2的¹H和¹³C NMR信号进行了归属.     

Ultra-low field NMR measurements of liquids and gases with short relaxation times

Interest in nuclear magnetic resonance measurements at ultra-low magnetic fields (ULF, approximately microT fields) has been motivated by various benefits and novel applications including narrow NMR peak-width, negligible susceptibility artifacts, imaging of samples inside metal containers, and possibility of directly imaging neuronal currents. ULF NMR/MRI is also compatible with simultaneous measurements of biomagnetic signals. However the most widely used technique in ULF NMR-prepolarization at high field and measurement at lower field-results in large transient signals which distort the free induction decay signal. This is especially severe for the measurement of signals from samples and materials with short T1 time. We have devised an approach that largely cancels the transient signals. The technique was successfully used to measure NMR signals from liquids and gases with T1 in the range 1-4 ms.     

新型连翘脂素-布洛芬酯合物的波谱学数据解析

以连翘脂素和布洛芬为原料,通过Schotten-Baumann酯化合成了酯合物——连翘脂素-布洛芬酯.对其紫外吸收光谱（UV）、红外吸收光谱（IR）、基质辅助激光解吸/电离飞行时间质谱（MALDI-TOF-MS）、1D和2D核磁共振（NMR）波谱（包括¹H NMR、¹³C NMR、DEPT、¹H-¹H COSY、¹H-¹H NOESY、¹H-¹³C HSQC和¹H-¹³C HMBC）进行了解析,对其¹H和¹³C NMR谱峰进行了全归属.     

A 13C solid-state NMR investigation of the alkynyl carbon chemical shift tensors for 2-butyne-1,4-diol

The alkynyl carbon chemical shift (CS) tensors for 2-butyne-1,4-diol are reported, based on analyses of the carbon-13 NMR spectra of stationary-powder and slow magic-angle spinning (MAS) samples for which the alkynyl carbon nuclei are enriched in 13C. NMR spectra of slow MAS samples exhibit spinning-frequency-dependent fine structure typical of crystallographically equivalent but magnetically distinct nuclei. Simulated spectra of slow MAS samples of this two-spin system are particularly sensitive to the relative orientations of the CS tensors. In addition, the value of 1J(13C, 13C), +175 +/- 10 Hz, is determined by examination of the total NMR lineshape of slow MAS samples. The CS tensors are almost axially symmetric, delta11 = 158.9 +/- 1.0 ppm and delta22 = 155.7 +/- 1.0 ppm; the direction of greatest shielding is approximately along the alkynyl C-C bond, delta33 = -57.8 +/- 2.0 ppm. Both the magnitudes of the principal components of the CS tensors and their orientations are in agreement with those predicted from first-principles calculations at the HF and MP2 levels of theory. This study demonstrates the importance of examining the NMR spectra of homonuclear two-spin systems with and without MAS under a variety of conditions (e.g., two or more applied magnetic fields and slow MAS).     

Magnetic Resonance Study of Detonation Nanodiamonds with Surface Chemically Modified by Transition Metal Ions

We report on electron magnetic resonance (EMR) and nuclear magnetic resonance (NMR) study of detonation nanodiamonds (DND) with the surface modified by copper and cobalt ions. The EMR spectrum of the pure DND sample shows an intense singlet originating from broken carbon bonds, while the spectra of copper- and cobalt-modified samples reveal additional signals with g > 2 and pronounced hyperfine structure (for copper). Increase in the Cu/Co concentration causes an increase of the corresponding EMR signals and broadening of the intense carbon-inherited singlet line. Subsequent annealing of the copper-modified samples in a hydrogen gas stream at 550 and 900°C causes narrowing of the singlet line and reduction of the Cu²⁺-related components. Applying the same annealing process to the cobalt-modified samples leads to broadening of the singlet line, reduction of Co²⁺ component and appearance of new intense low-field signals. NMR data correlate well with the EMR findings and yield information on interactions and locations of transition metal ions. ¹³C nuclear spin–lattice relaxation rate R ₁ in pure DND is driven by the interaction of nuclear spins with unpaired electron spins of broken bonds. Chemical modification of the DND surface by Cu and Co causes an increase in the relaxation rate, revealing appearance of paramagnetic Cu²⁺ and Co²⁺ complexes at the DND surface and their interaction with the carbon nuclear spins, both directly and via a coupling of Cu²⁺ and Co²⁺ electrons with those of the broken bonds. The aforementioned annealing of the Cu- and Co-DND results in an inverse process, i.e., a reduction of the relaxation rate, indicating that these complexes are destroyed and metal ions presumably join each other forming copper and cobalt nanoclusters. In the case of Co the nanoclusters are ferromagnetic, which results in the noticeable broadening of the ¹³C NMR lines.     

Separation of aromatic-carbon 13C NMR signals from di-oxygenated alkyl bands by a chemical-shift-anisotropy filter

Selection of alkyl-carbon and suppression of aromatic-carbon 13C nuclear magnetic resonance (NMR) signals has been achieved by exploiting the symmetry-based, systematic difference in their 13C chemical-shift anisotropies (CSAs). Simple three- or five-pulse CSA-recoupling sequences with "gamma-integral" cleanly suppress the signals of all sp2- and sp-hybridized carbons. The chemical-shift-anisotropy-based dephasing is particularly useful for distinguishing the signals of di-oxygenated alkyl (O-C-O) carbons, found for instance as anomeric carbons in carbohydrates, from bands of aromatic carbons with similar 13C isotropic chemical shifts. The alkyl signals are detected with an efficiency of > 60%, with little differential dephasing. Combined with C-H dipolar dephasing, the CSA filter can identify ketal (unprotonated O-C-O) carbons unambiguously for the first time. Conversely, after short cross polarization and the CSA filter, O-CH-O (acetal) carbon signals are observed selectively. The methods are demonstrated on various model compounds and applied to a humic acid.     

Electron spin density distribution in the polymer phase of CsC60: assignment of the NMR spectrum

We present high resolution 133Cs-13C double resonance NMR data and 13C-13C NMR correlation spectra of 13C enriched samples of the polymeric phase of CsC60. These data lead to a partial assignment of the lines in the 13C NMR spectrum of CsC60 to the carbon positions on the C60 molecule. A plausible completion of the assignment can be made on the basis of an ab initio calculation. The data support the view that the conduction electron density is concentrated at the C60 "equator," away from the interfullerene bonds.     

High-Temperature Dynamic Nuclear Polarization Enhanced Magic-Angle-Spinning NMR

Dynamic nuclear polarization (DNP) transfers electron spin-polarization to nuclear spins in close proximity, increasing sensitivity by two-to-three orders of magnitude. This enables nuclear magnetic resonance (NMR) experiments on samples with low concentrations of analyte. The requirement of using cryogenic temperatures in DNP-enhanced solid-state NMR (ssNMR) experiments may impair the resolution and hence limit its broad application to biological systems. In this work, we introduce a “High-Temperature DNP” approach, which aims at increasing spectral resolution by performing experiments at temperatures of around 180?K instead of?~100?K. By utilizing the extraordinary enhancements obtained on deuterated proteins, still sufficiently large DNP enhancements of 11–18 are obtained for proton and carbon, respectively. We recorded high sensitivity 2D ¹³C–¹³C spectra in?~9?min with higher resolution than at 100?K, which has similar resolution to the one obtained at room temperature for some favorable residues.     

Solid-state spectroscopic analysis of lignins from several Austral hardwoods

In order to gain information about lignin molecular characteristics with a direct bearing on the remarkable susceptibility of some Austral hardwoods to biological delignification, milled-wood lignins were isolated and analyzed by spectroscopic techniques in the solid state. Cross polarization and magic-angle spinning 13C nuclear magnetic resonance (13C CPMAS NMR) and Fourier-transform infrared (FTIR) spectra of the lignin preparations were obtained. The most diagnostic peaks were assigned and quantified as percentages of the total spectral area, and the differences observed discussed in terms of lignin composition. The spectral patterns obtained revealed that the woods from Gevuina avellana, Eucryphia cordifolia and Nothofagus dombeyii have lignin with high syringyl/guaiacyl ratio, as evidenced by relative areas of 13C NMR signals at 153 and 148 ppm, and FTIR bands at 1,335 and 1,275 cm-1. The presence of syringyl-rich lignins, characterized by lower redox potential and condensation degree than guaiacyl-rich lignins, could be a structural factor contributing to the ease of extensive delignification of these woods by white-rot fungi.     

Triple resonance experiments for aligned sample solid-state NMR of (13)C and (15)N labeled proteins

Initial steps in the development of a suite of triple-resonance (1)H/(13)C/(15)N solid-state NMR experiments applicable to aligned samples of (13)C and (15)N labeled proteins are described. The experiments take advantage of the opportunities for (13)C detection without the need for homonuclear (13)C/(13)C decoupling presented by samples with two different patterns of isotopic labeling. In one type of sample, the proteins are approximately 20% randomly labeled with (13)C in all backbone and side chain carbon sites and approximately 100% uniformly (15)N labeled in all nitrogen sites; in the second type of sample, the peptides and proteins are (13)C labeled at only the alpha-carbon and (15)N labeled at the amide nitrogen of a few residues. The requirement for homonuclear (13)C/(13)C decoupling while detecting (13)C signals is avoided in the first case because of the low probability of any two (13)C nuclei being bonded to each other; in the second case, the labeled (13)C(alpha) sites are separated by at least three bonds in the polypeptide chain. The experiments enable the measurement of the (13)C chemical shift and (1)H-(13)C and (15)N-(13)C heteronuclear dipolar coupling frequencies associated with the (13)C(alpha) and (13)C' backbone sites, which provide orientation constraints complementary to those derived from the (15)N labeled amide backbone sites. (13)C/(13)C spin-exchange experiments identify proximate carbon sites. The ability to measure (13)C-(15)N dipolar coupling frequencies and correlate (13)C and (15)N resonances provides a mechanism for making backbone resonance assignments. Three-dimensional combinations of these experiments ensure that the resolution, assignment, and measurement of orientationally dependent frequencies can be extended to larger proteins. Moreover, measurements of the (13)C chemical shift and (1)H-(13)C heteronuclear dipolar coupling frequencies for nearly all side chain sites enable the complete three-dimensional structures of proteins to be determined with this approach.     

<Superscript>15</Superscript>N–<Superscript>13</Superscript>C Dipole Couplings in Smectic Mesophase of a Thermotropic Ionic Liquid

Unique combination of ionic conductivity and anisotropic physical properties in ionic liquid crystals leads to new dynamic properties exploited in modern technological applications. Structural and dynamics information at atomic level for molecules and ions in mesophases can be obtained by nuclear magnetic resonance (NMR) spectroscopy through the measurements of dipole–dipole spin couplings. While ¹³C–¹H and ¹⁵N–¹H dipolar NMR spectra can be routinely acquired in samples with natural isotopic abundance, recording ¹⁵N–¹³C dipolar NMR spectra is challenging because of the unfavourable combination of two rare isotopes. In the present study, an approach to measure ¹⁵N–¹³C dipole-dipole NMR spectra in static liquid crystalline samples with natural abundance is introduced. We demonstrate that well-resolved spectra can be recorded within 10 h of experimental time using a conventional NMR probe and a moderately strong magnetic field. The technique is applied to a thermotropic smectic mesophase formed by an ionic liquid with imidazolium-based organic cation.     

Nuclear magnetic resonance study of ultrananocrystalline diamonds

We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique. Analysis of the ¹³C and ¹H NMR spectra, spin-spin and spin-lattice relaxation times in purified UNCD samples is presented. Our measurements show that UNCD particles consist of a diamond core that is partially covered by a sp ²-carbon fullerene-like shell. The uncovered part of outer diamond surface comprises a number of hydrocarbon groups that saturate the dangling bonds. Our findings are discussed along with recent calculations of the UNCD structure. Significant increase in the spin-lattice relaxation rate (in comparison with that of natural diamond), as well as stretched exponential character of the magnetization recovery, are attributed to the interaction of nuclear spins with paramagnetic centers which are likely fabrication-driven dangling bonds with unpaired electrons. We show that these centers are located mainly at the interface between the diamond core and shell.     

13C NMR chemical shift of single-wall carbon nanotubes

We compute the magnetic shielding tensor within the London approximation and estimate the Knight shift of single-wall carbon nanotubes. Our results indicate that high resolution 13C NMR should be able to separate the metallic and insulator character of the nanotubes since a 11 ppm splitting is predicted from the respective resonances. As a model for disorder, bending, and defects in these structures, we investigate the magnetic response of nanotubes with finite size. We get a small line broadening coming from an intrinsic length dependent resonance effect. The nanotube packing is also studied and leads to a 20 ppm broadening which disappears under experimental high-resolution conditions.     

Electrophysical studies of nanoporous carbon materials prepared of silicon carbide powders

Measurements of the electrophysical properties of nanoporous carbon (NPC) samples (conductivity and Hall effect), as well as studies of the same samples using electron spin resonance (ESR), are summarized. Free holes are shown to play the major part in charge transport in such materials, thus permitting identification of the ESR signal with free holes. An analysis of the shape of the resonance lines, as well as of their dependence on temperature and the technology employed in sample preparation, established the ESR signals to consist of two superimposed resonance lines associated with carriers, free or localized to various extents, whose magnetic susceptibility obeys Pauli and Curie-Weiss laws, respectively. The temperature dependences of the ESR signal parameters were studied, and the experimental relations were compared with theory to determine the model-parameters. An analysis of the temperature behavior of these parameters suggests the conclusion that NPC samples are heterophase porous systems whose properties are dominated by structural characteristics.     

固体核磁共振技术在锂/钠离子电池碳负极中的应用及研究进展

碳负极材料作为锂/钠离子电池的传统负极材料一直获得广泛的推广和应用,但其仍存在充电时间长、库伦效率低等问题,研究碳负极材料充放电机理是解决这些问题的关键.固体核磁共振（NMR）技术是一种研究固体材料中目标原子所处化学环境以及材料内部结构变化的有效手段.通过测定锂/钠离子电池中⁶Li、⁷Li和²³Na高速魔角旋转（MAS）条件下的固体NMR谱图,能够清晰获得锂/钠离子电池碳负极脱/嵌过程中的结构变化,以及碳原子与Li/Na的配位情况,从而为碳负极材料的设计及其电化学性能的提升提供充分的理论依据.本文综述了近年来固体NMR技术在锂/钠离子电池碳负极材料研究中的应用以及相关研究进展.     

Accurate and sensitive measurements of magnetic susceptibility using echo planar imaging

Susceptibility differences are common causes for artifacts in magnetic resonance (MR); therefore, it is important to choose phantom materials in a way that these artifacts are kept at a minimum. In this study, a previously proposed MR imaging (MRI) method [Beuf O, Briguet A, Lissac M, Davis R. Magnetic resonance imaging for the determination of magnetic susceptibility of materials. J Magn Reson 1996; Series B(112):111-118] was improved to facilitate sensitive in-house measurements of different phantom materials so that such artifacts can more easily be minimized. Using standard MRI protocols and distilled water as reference, we measured magnetic volume susceptibility differences with a clinical MR system. Two imaging techniques, echo planar imaging (EPI) and spin echo, were compared using liquid samples whose susceptibilities were verified by MR spectroscopy. The EPI sequence has a very narrow bandwidth in the phase-encoding direction, which gives an increased sensitivity to magnetic field inhomogeneities. All MRI measurements were evaluated in two ways: (1) manual image analysis and (2) model fitting. The narrow bandwidth of the EPI made it possible to detect very small susceptibility differences (equivalent susceptibility difference, Deltachi(e)> or =0.02 ppm), and even plastics could be measured. Model fitting yielded high accuracy and high sensitivity and was less sensitive to other image artifacts as compared with manual image analysis.     

依卡倍特钠的波谱学数据解析

对胃粘膜保护剂依卡倍特钠的紫外吸收光谱（UV）、红外吸收光谱（IR）、高分辨-质谱（HR-MS）、核磁共振（NMR）波谱（包括¹H NMR、¹³C NMR、DEPT、¹H-¹H COSY、¹H-¹³C HSQC和¹H-¹³C HMBC）数据进行了解析,对其所有的¹H和¹³C NMR谱信号进行了归属,通过多种谱学技术确证了依卡倍特钠的结构.     

硼替佐米的波谱学数据解析

对抗肿瘤药硼替佐米的紫外吸收光谱（UV）、红外吸收光谱（IR）、电喷雾离子源-质谱（ESI-MS）、核磁共振（NMR）波谱（包括¹H NMR、¹³C NMR、DEPT、¹H-¹H COSY、¹H-¹³C HSQC和¹H-¹³C HMBC）数据进行了解析,对其¹H和¹³C NMR谱峰进行了全归属,通过多种谱学技术确证了硼替佐米的结构.