首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 343 毫秒
1.
The chemical shifts and multiplicities of the two bridgehead carbons in the 13C NMR spectra of various fused furoxans are snown to provide a general method for assigning structure in these tautomeric systems.  相似文献   

2.
The high field 1H and 13C resonances of obacunone were established by a series of I D and 2D NMR experiments. The 13C-NMR signals of the five tertiary methyl groups and all quatenary carbons were unambiguously assigned. The results confirm the stereochemical assignments at all stereogenic centres.  相似文献   

3.
Natural abundance 13C cross polarized (CP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis of human gall bladder stones collected from patients suffering from malignant and benign gall bladder disease was carried out which revealed different polymorphs of cholesterol in these stones. All gall bladder stones in present study had cholesterol as their main constituent. 13C CP-MAS NMR analysis revealed three forms of cholesterol molecules in these stones, which are anhydrous form, monohydrate crystalline with amorphous form and monohydrate crystalline form. Our study revealed that stones collected from patients associated with chronic cholecystitis (CC) disease have mostly different polymorph of cholesterol than stones collected from patients associated with gall bladder cancer (GBC). Such study will be helpful in understanding the mechanism of formation of gallstones which are associated with different gall bladder diseases. This is the first study by solid state NMR revealing different crystal polymorphism of cholesterol in human gallstones, extending the applicability of 13C CP-MAS NMR technique for the routine study of gallstones.  相似文献   

4.
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.  相似文献   

5.
The proton-decoupled 13C NMR spectra of mixtures of liquid crystals with opposite diamagnetic anisotropies have been studied in the natural abundance of 13C. A new method to assign the spectral lines to specific carbons in the liquid crystalline phase has been developed. For this purpose, the assignments of lines in the isotropic media are required, and they were obtained from two-dimensional hetero-COSY experiments. From the spectra in the “critical” mixtures where both the orientations of the liquid crystal directors, with the alignments along and perpendicular to the direction of the magnetic field, “coexist,” the 13C chemical-shift anisotropies have been determined, assuming uniaxial symmetry.  相似文献   

6.
The 13C NMR solution spectra of 30-crown-10 ether and its tetrahydrate show only one resonance at all accessible temperatures. In contrast, the solid state 13C NMR spectrum of the 30-crown-10.4H2O shows two resonances in the ratio of 4:1, separated by 1.2 ppm. In the case of 30-crown-10 itself, six resolvable 13C resonances in the ratio of 4:1:1:2:1:1 are observed in the solid with an overall chemical shift dispersion of 5 ppm. The remarkably different spectral behavior of these two systems in the solid state is discussed in terms of the torsional environments of the crystallographically unique carbons and the results of GIAO calculations of isotropic 13C shieldings for simpler model compounds. Results of dipolar dephased 13C CPMAS spectra indicate that 30-crown-10 does not undergo a large amplitude molecular motion, in contrast to earlier results for 18-crown-6. Only a small amount of residual intensity is found in the dipolar dephased spectrum of 30-crown-10.4H2O, indicating that it also is relatively rigid in the solid.  相似文献   

7.
To clarify the nature of microscopic structural changes of N(CH3)4CdCl3 at high temperatures, the nuclear magnetic resonance (NMR) spectra of the protons and carbons in N(CH3)4CdCl3 were measured. NMR studies of the 1H and 13C spin–lattice relaxation time, T , in the rotating frame were also performed. No changes in the T of 1H and 13C associated with the N(CH3)4 groups were observed at the high-temperature transition from phase I to phase I′. However, the 14N NMR spectra reflected changes in the structural geometry during the transition to phase I′, indicating that this transition is driven by N(CH3)4.  相似文献   

8.
Residual dipolar coupling between carbons and 14N nuclei in the 13C CPMAS NMR spectrum of solid imidazole is studied. Calculations of expected splittings with a previously reported equation leads to the complete assignment of the solid state carbon chemical shifts. Additionally, information is provided on the location of 14N electric field gradient axes at the N-H site.  相似文献   

9.
An oriented single-crystal NMR experiment was carried out for a cadmium acetate dihydrate crystal having 66% 13C enrichment of the acetate carboxylate groups. The resulting 113Cd-13C direct dipole-dipole interactions give rise to an orientation-dependent modulation of 113Cd NMR linewidths for each of fdur magnetically distinguishable shielding tensors in the cadmium acetate crystal. Comparison of observed cadmium linewidths with those computed as a function of crystal orientation allows for unambiguous assignment of each of the four symmetry-related 113Cd shielding tensors to specific lattice positions in the cadmium acetate crystal. These assignments are in agreement with those made based upon empirical rules, and serve to confirm by means independent of previous assumptions the reliability of the assignment rules used previously. Confirmation of the validity and reliability of such rules is essential for the accurate interpretation of cadmium single-crystal NMR data in terms of shielding tensor-structure correlations to cadmium chemical shifts.  相似文献   

10.
13C cross-polarization/magic-angle spinning (CP/MAS) solid-state NMR spectroscopy has been employed to analyze four vitamin D compounds, namely vitamin D3 (D3), vitamin D2 (D2), and the precursors ergosterol (Erg) and 7-dehydrocholesterol (7DHC). The 13C NMR spectrum of D3 displays a doublet pattern for each of the carbon atoms, while that of Erg contains both singlet and doublet patterns. In the cases of 7DHC and D2, the 13C spectra display various multiplet patterns, viz. singlets, doublets, triplets, and quartets. To overcome the signal overlap between the 13C resonances of protonated and unprotonated carbons, we have subjected these vitamin D compounds to 1D 1H-filtered 13C CP/MAS and {1H}/13C heteronuclear correlation (Hetcor) NMR experiments. As a result, assisted by solution NMR data, all of the 13C resonances have been successfully assigned to the respective carbon atoms of these vitamin D compounds. The 13C multiplets are interpreted due to the presence of s-cis and s-trans configurations in the α- and β-molecular conformers, consistent with computer molecular modeling determined by molecular dynamics and energy minimization calculations. To further characterize the ring conformations in D3, we have successfully extracted chemical shift tensor elements for the 13C doublets. It is demonstrated that 13C solid-state NMR spectroscopy provides a robust and high sensitive means of characterizing molecular conformations in vitamin D compounds.  相似文献   

11.
Recently, a high temperature 10 mm cryoprobe was developed. This probe provides a significant sensitivity enhancement for 13C NMR of polyolefins at a sample temperature of 120–135 °C, as compared to conventional probes. This greatly increases the speed of NMR studies of comonomer content, sequence distribution, stereo- and regioerrors, saturated chain end, unsaturation, and diffusion of polymers. In this contribution, we first compare the 13C NMR sensitivity of this probe with conventional probes. Then, we demonstrate one of the advantages of this probe in its ability to perform 2D Incredible Natural Abundance Double Quantum Transfer Experiment (2D INADEQUATE) in a relatively short period of time. The 2D INADEQUATE has been rarely used for polymer studies because of its inherently very low sensitivity. It becomes even more challenging for studying infrequent polyolefin microstructures, as low probability microstructures represent a small fraction of carbons in the sample. Here, the 2D INADEQUATE experiment was used to assign the 13C NMR peaks of 2,1-insertion regioerrors in a poly(propylene-co-1-octene) copolymer.  相似文献   

12.
To obtain further information about the cause for the rather large splitting of the C4 resonance line into the downfield (C4D) and upfield (C4U) lines in CP/MAS 13C NMR spectra for native cellulose, 13C and 1H spin diffusion measurements have been conducted by using different types of bacterial cellulose samples. In 13C spin diffusion measurements, the C4D resonance line is selectively inverted by the Dante π pulse sequence and the 13C spin diffusion is allowed to proceed from the C4D carbons to other carbons including the C4U carbons with use of the 13C4-enriched bacterial cellulose sample. The analysis based on the simple spin diffusion theory for the process experimentally observed reveals that the C4U carbons may be located at distances less than about 1 nm from the C4D carbons. In 1H spin diffusion measurements, poly(vinyl alcohol) (PVA) films in which ribbon assemblies of bacterial cellulose are dispersed are employed and the 1H spin diffusion process is examined from the water-swollen PVA continuous phase to the dispersed ribbon assemblies by the 13C detection through the 1H–13C CP technique. As a result, it is found that the C4D and C4U carbons are almost equally subjected to the 1H spin diffusion from the PVA phase, indicating that the C4U carbons are not localized in some limited area, e.g. in the surfacial region, but are distributed in the whole area in the microfibrils. These experimental results suggest that the C4U carbons may exist as structural defects probably due to conformational irregularity associated with disordered hydrogen bonding of the CH2OH groups in the microfibrils.  相似文献   

13.
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.  相似文献   

14.
Solid-state NMR is a powerful and non-perturbative method to measure and define chemical composition and architecture in bacterial cell walls, even in the context of whole cells. Most NMR studies on whole cells have used selectively labeled samples. Here, we introduce an NMR sequence relay using frequency-selective REDOR (fsREDOR) and spin diffusion elements to probe a unique amine contribution in uniformly 13C- and 15N-labeled Staphylococcus aureus whole cells that we attribute to the d-alanine of teichoic acid. In addition to the primary peptidoglycan structural scaffold, cell walls can contain significant amounts of teichoic acid that contribute to cell-wall function. When incorporated into teichoic acid, d-alanine is present as an ester, connected via its carbonyl to a ribitol carbon, and thus has a free amine. Teichoic acid d-Ala is removed during cell-wall isolations and can only be detected in the context of whole cells. The sequence presented here begins with fsREDOR and a chemical shift evolution period for 2D data acquisition, followed by DARR spin diffusion and then an additional fsREDOR period. fsREDOR elements were used for 13C observation to avoid complications from 13C–13C couplings due to uniform labeling and for 15N dephasing to achieve selectivity in the nitrogens serving as dephasers. The results show that the selected amine nitrogen of interest is near to teichoic acid ribitol carbons and also the methyl group carbon associated with alanine. In addition, its carbonyl is not significantly dephased by amide nitrogens, consistent with the expected microenvironment around teichoic acid.  相似文献   

15.
Single crystals of a novel compound, bis(ethyltriphenylphosphonium) tetrabromomanganate(II) dihydrate (BTP-Mn) were grown by solution growth-slow evaporation technique from aqueous solution of the compound at ambient temperature. The grown crystals were characterized by elemental analysis, powder X-ray diffraction, thermal analysis, nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infra-red spectroscopy (FTIR) techniques. The chemical composition of the compound was revealed by elemental analysis and its crystallinity was confirmed by powder X-ray diffraction. Thermal analysis confirmed that the compound was stable up to 125°C. The various kinds of protons and carbons present in the compound were confirmed by 1H NMR and 13C NMR technique respectively and the presence of phosphorous was confirmed by 31P NMR spectrum in the compound. The modes of vibration of different molecular groups present in the compound were identified by FTIR spectral analysis. The second harmonic generation behaviour was tested by Nd:YAG laser source.  相似文献   

16.
ABSTRACT

Particulate matter (PM) from atmospheric aerosols contains carbons that are harmful for living organisms and the environment. PM can originate from vehicle emissions, wearing of vehicle components, and dust. Size and composition determine PM transport and penetration depth into the respiratory system. Understanding PM emission characteristics is essential for developing strategies to improve air quality. The number of studies on carbon isotope composition (13C/12C) of PM samples to characterise emission factors has increased. The goal of this review is to integrate and interpret the findings from 13C/12C carbon isotope ratio (δ13C, ‰) analyses for the most common types of emission sources. The review integrates data from 25 studies in 13 countries. The range of δ13C of PM from vehicle emissions was from ?28.3 to ?24.5?‰ and for non-vehicle anthropogenic emissions from ?27.4 to ?23.3?‰. In contrast, PM ranges for δ13C from biomass burning sources differed markedly. For C3 plants, δ13C ranged from ?34.7 to ?25.4?‰ and for C4 plants from ?22.2 to ?13.0?‰. The 13C/12C isotope analysis of PM is valuable for understanding the sources of pollutants and distinguishing vehicle emissions from biomass burning. However, additional markers are needed to further distinguish other anthropogenic sources.  相似文献   

17.
Theoretical calculations of carbon-13 NMR chemical shifts using the gauge including atomic orbitals (GIAO) DFT approach with a moderately large set of basis functions usually yield quite satisfactory results. In the case of chlorinated aromatic carbons, however, abnormally large differences between experimental and calculated values have been noticed. This discrepancy has been proven not to be caused by improper referencing, or the basis set effect, and probably not by neglect of vibrational corrections. One of the possible sources of the chlorine effect could be the impact of relativistic phenomena on electrons moving about the chlorine nucleus. The second, probably more important factor is the influence of electron correlations, ignored in Hartree–Fock SCF and only partially included in DFT calculations. Surprisingly, however, the observed divergence has been significantly larger for DFT than for Hartree–Fock results. In the latter case the observed divergence between theoretical and experimental 13C NMR chemical shifts of chlorine-bonded carbons is systematic but rather small (3.4–4.4ppm).  相似文献   

18.
The 13C NMR spectra of nine 4a-Methyl Octahydrophenanthrols are reported, and their chemical shifts are compared with those of the parent hydrocarbon and correlated against structural parameters. Two new parameters for β interactions are proposed and a α benzylic parameter is analysed. An extension to unsaturated six-membered rings skeletons is also tempted. Predictions for 30 carbons were carried out with an average error of 0.34 ppm.  相似文献   

19.
研究油煤浆热溶产物的成分及含量的变化是控制煤直接液化工艺的重要因素之一. 本文采用固体核磁共振研究了在高压釜中用不同溶剂在不同温度下(250~370 ℃)得到的神华煤及其热溶产物的一些变化规律. 通过对神华煤及其热溶中间产物与抽余煤的13C CP/MAS/TOSS NMR分析可知, 神华煤经热溶处理后, 脱除了一些含氧官能团以及某些脂肪烃结构. 同时, 一些芳香桥碳分子和被取代的芳香碳环发生了断裂, 随着反应温度逐渐升高, 由于不断供氢, 四氢呋喃抽提物的芳香度逐渐降低, 抽提率逐渐增大.   相似文献   

20.
AD-32和DM是蒽环类抗肿瘤抗生素,临床上用于治疗难治性膀胱癌、淋巴瘤、间皮瘤、乳癌、卵巢癌、头颈部癌、小细胞肺癌、睾丸癌、骨髓癌、结肠癌、血管肉瘤、子宫颈癌、肝癌等.本文在进行AD-32和DM的化学合成和工艺研究的基础上,应用  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号