Efficient identification of different types of carbons in organic solids by 2D solid-state NMR spectroscopy

An efficient method for identifying different types of carbon groups (CH(3), CH(2), CH, and quaternary carbons) in organic solids is proposed by utilizing the combination of a two-dimensional (2D) (13)C-(1)H polarization inversion spin exchange at magic angle (PISEMA) NMR experiment and numerical simulation results of simple isolated (13)C-(1)H dipolar coupling models. Our results reveal that there is a unique line shape of the (13)C-(1)H dipolar splitting pattern and a corresponding characteristic splitting value for each carbon group, based on which different carbon types can be distinguished unambiguously. In particular, by using this method, the discrimination and assignment of overlapped signals from different types of carbons can be achieved easily. The efficacy of this method is demonstrated on typical solid small molecules, polymers, and biomacromolecules.     

二萜生物碱的~(13)C核磁共振谱

本文通过对186个二萜生物碱及其衍生物的~(13)C NMR谱数据的分析比较,就以下几方面作了归纳总结:①信号归属的方法;②常见取代基OCH_3、NCH_3、、NCH_2CH_3、OCOCH_3、的化学位移范围;③季碳和某些特定碳的化学位移规律。某些特定结构,如C_(20)二萜生物碱中的噁唑烷环以及C_(20)差向异构体等的~(13)C NMR谱特征;④C_(19)二萜生物碱中不同位置上取代基(H→OH、H→OMe、OH→OAc、OH→OMe、OH→C=O)效应和立体化学效应。这些归纳总结有助于此类化合物结构的阐明。     

NMR detection of single-walled carbon nanotubes in solution

The detection of nanotube carbons in solution by (13)C NMR is reported. The highly soluble sample was from the functionalization of (13)C-enriched single-walled carbon nanotubes (SWNTs) with diamine-terminated oligomeric poly(ethylene glycol) (PEG(1500N)). The ferromagnetic impurities due to the residual metal catalysts were removed from the sample via repeated magnetic separation. The nanotube carbon signals are broad but partially resolved into two overlapping peaks, which are tentatively assigned to nanotube carbons on semiconducting (upfield) and metallic (downfield) SWNTs. The solid-state NMR signals of the same sample are similarly resolved. Mechanistic and practical implications of the results are discussed.     

Quantitative analysis of deuterium using the isotopic effect on quaternary C NMR chemical shifts

Quantitative analysis of specifically deuterated compounds can be achieved by a number of conventional methods, such as mass spectroscopy, or by quantifying the residual ¹H NMR signals compared to signals from internal standards. However, site specific quantification using these methods becomes challenging when dealing with non-specifically or randomly deuterated compounds that are produced by metal catalyzed hydrothermal reactions in D₂O, one of the most convenient deuteration methods. In this study, deuterium-induced NMR isotope shifts of quaternary ¹³C resonances neighboring deuterated sites have been utilized to quantify the degree of isotope labeling of molecular sites in non-specifically deuterated molecules. By probing ¹³C NMR signals while decoupling both proton and deuterium nuclei, it is possible to resolve ¹³C resonances of the different isotopologues based on the isotopic shifts and the degree of deuteration of the carbon atoms. We demonstrate that in different isotopologues, the same quaternary carbon, neighboring partially deuterated carbon atoms, are affected to an equal extent by relaxation. Decoupling both nuclei (¹H, ²H) resolves closely separated quaternary ¹³C signals of the different isotopologues, and allows their accurate integration and quantification under short relaxation delays (D1 = 1 s) and hence fast accumulative spectral acquisition. We have performed a number of approaches to quantify the deuterium content at different specific sites to demonstrate a convenient and generic analysis method for use in randomly deuterated molecules, or in cases of specifically deuterated molecules where back-exchange processes may take place during work up.     

13C-NMR分析混合酸处理脱灰对永兴褐煤结构的影响

运用煤的结构化学理论和固态 ¹³C-NMR的研究方法,对比分析了永兴褐煤在混合酸处理脱灰前后碳组成的变化。研究表明,混合酸处理脱灰对永兴褐煤中脂肪碳和芳香碳两者比例分配的影响并不大,但对季碳、环内氧接脂碳、芳香桥碳和羧基碳的破坏作用比较明显;在芳香桥碳锐减的同时,芳甲基的比例非但没有减少反而略有增加,表明永兴褐煤中的双环及多环结构被破坏为单环结构,芳香簇的尺寸变小了,这是混合酸处理脱灰对永兴褐煤结构最重要的影响。     

Strategy for the study of paramagnetic proteins with slow electronic relaxation rates by nmr spectroscopy: application to oxidized human [2Fe-2S] ferredoxin

NMR studies of paramagnetic proteins are hampered by the rapid relaxation of nuclei near the paramagnetic center, which prevents the application of conventional methods to investigations of the most interesting regions of such molecules. This problem is particularly acute in systems with slow electronic relaxation rates. We present a strategy that can be used with a protein with slow electronic relaxation to identify and assign resonances from nuclei near the paramagnetic center. Oxidized human [2Fe-2S] ferredoxin (adrenodoxin) was used to test the approach. The strategy involves six steps: (1) NMR signals from (1)H, (13)C, and (15)N nuclei unaffected or minimally affected by paramagnetic effects are assigned by standard multinuclear two- and three-dimensional (2D and 3D) spectroscopic methods with protein samples labeled uniformly with (13)C and (15)N. (2) The very broad, hyperfine-shifted signals from carbons in the residues that ligate the metal center are classified by amino acid and atom type by selective (13)C labeling and one-dimensional (1D) (13)C NMR spectroscopy. (3) Spin systems involving carbons near the paramagnetic center that are broadened but not hyperfine-shifted are elucidated by (13)C[(13)C] constant time correlation spectroscopy (CT-COSY). (4) Signals from amide nitrogens affected by the paramagnetic center are assigned to amino acid type by selective (15)N labeling and 1D (15)N NMR spectroscopy. (5) Sequence-specific assignments of these carbon and nitrogen signals are determined by 1D (13)C[(15)N] difference decoupling experiments. (6) Signals from (1)H nuclei in these spin systems are assigned by paramagnetic-optimized 2D and 3D (1)H[(13)C] experiments. For oxidized human ferredoxin, this strategy led to assignments (to amino acid and atom type) for 88% of the carbons in the [2Fe-2S] cluster-binding loops (residues 43-58 and 89-94). These included complete carbon spin-system assignments for eight of the 22 residues and partial assignments for each of the others. Sequence-specific assignments were determined for the backbone (15)N signals from nine of the 22 residues and ambiguous assignments for five of the others.     

13C RMN: Non-équivalences d'ammoniums quaternaires diastéréoisoméres

¹³C magnetic resonance spectra of six diastereomeric mixtures of quaternary ammonium salts were investigated. Assignment of the signals of the non-aromatic carbons and measurement of the non-equivalences allowed the determination of the ratio of isomers in the mixtures. A deshielding β effect and a shielding γ effect for the quaternisation were pointed out.     

1H, 13C and 77Se NMR study of tetraselenafulvalene derivatives supported by novel H(Se)C and H(C)Se triple-resonance experiments at natural abundance

A series of substituted tetraselenafulvalenes (TSeFs) was studied in solution using 1H, 13C and 77Se NMR spectroscopy. Chemical shifts and heteronuclear coupling constant values were determined and assigned. Novel two-dimensional H(Se)C and H(C)Se triple-resonance correlation experiments were applied at natural abundance in order to accomplish 13C and 77Se signal assignments. Using this approach, all the signals were unambiguously assigned and atom connectivity was established in the studied TSeF derivatives. These experiments, allowing signal assignments of quaternary carbons, may find wide application in the study of substituted TSeF and other organoselenium compounds. To the best of our knowledge, triple-resonance experiments with proton detection have been applied to organoselenium compounds for the first time.     

Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

A series of amino acids and carboxylic acids were determined by 13C NMR spectroscopy.The results showed that addition of 3M MgCl2 led to the 13C NMR integral area of samples being well proportional to number of carbon atoms that produce the particular signal with reliability over 95%. Measurements of 13C spin-lattice relaxation times (T1's) are reported for a number of amino acids. T1's of all the carbons in amino acids generally tend to decrease with the increase of the concentration of electrolytes, and the presence of magnesium slats is of significant. Carboxylic carbons in amino acids are the most sensitive "acceptor" of the 13C spin-lattice relaxation accelerating effects in electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes is Mg(ClO4)2 ＞MgCl2 ＞CaCl2 ＞NaCl ＞KCl ＞LiClO4 ＞NaOH. In general, T1's of C1 carbons in nonpolar a-amino acids are higher than those in polar and basic a-amino acids both in aqueous and 3M MgCl2 medium. In aliphatic straight-chain amino acids, a-, a-, a-, ai- and a- amino acids, T1's of C1 carbons tend to reduce with the increase of inserted carbon numbers between amino and carboxylic groups compared with Gly. T1's can be decreased even more when amino acids are mixed in 3M MgCl2, but T1's of carbons in amino acids decrease slightly with increase of the concentration of amino acids in 3M MgCl2. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction and paramagnetic impurity in electrolytes, large contributions of intermolecular interaction which is enhanced in electrolytes concentrate on the incoming "unsaturation" of the primary solvation shell of cations with the increase of electrolytes concentration and complexes formation of amino acids with metal ions. In electrolytes, amino acids are "anchored" to cations and molecule tumbling is slowed down, molecular rigidity is increased and molecular size is "enlarged", all of these are helpful to accelerate the 13C spin-lattice relaxation. Atlast, MgCl2 is proposed as an efficient relaxation agent for analysis of amino acids and some carboxylic acids.Samples were dissolved with the aid of supersonic which has the effect of degassing, and they were degassed again with supersonic for 30 seconds right before determination. All of the 13C NMR was obtained with a Bruker DPX-300 NMR instrument, using NOE-suppressed inverse gated decoupling with a recycle delay 8.00s and a sweep width 30120.48Hz, experiment temperature is integral of the carbon with the smallest chemical shift is calibrated as 10.00. Spin-lattice relaxation times (T1's) were determined by using inversion recovery according to Bruker avance user's guide.The pulse sequence is (T-90.°-T-180°-o-90t°) n.     

Gradient‐enhanced one‐dimensional pulse schemes for quaternary and methylene carbon detection

Gradient‐enhanced pulse schemes are presented for the detection of quaternary and methylene carbons. The new pulse schemes permit the detection of quaternary and methylene (—CH₂) carbons or CH₂ carbons alone from a single experiment. Efficient suppression of CH and CH₃ carbons in all the pulse schemes is achieved by creating their antiphase magnetization and then dephasing using gradients. In the pulse schemes for detection of only CH₂ carbons, the quaternary carbons are also suppressed either by dephasing using gradients or by rendering them unobservable by retaining in longitudinal order. The experimental results of the gradient pulse schemes are demonstrated on cholesteryl acetate and cyclosporin A. These one‐dimensional techniques, because of their simplicity and the ease of performing the experiments, can be important for routine chemical applications. Copyright © 2003 John Wiley & Sons, Ltd.     

Identification of halogen substituents in natural products by measurement of 13c spin-lattice relaxation times and integrated intensities

A method is proposed for differentiating brominated carbons from chlorinated carbons by means of natural-abundance ¹³C NMR spectroscopy. The basis of the method is that the spin-lattice relaxation behaviour of brominated carbons is influenced by carbon-bromine scalar interactions, which can lead to shortened ¹³C spin-lattice relaxation times and reduced values of the nuclear Overhauser enhancement. C-Cl scalar interactions make a negligible contribution to the spin-lattice relaxation of chlorinated carbons. These effects are illustrated by measurement of the ¹³C spin-lattice relaxation times and integrated intensities of chloro-, bromo and iodobenzene and chloro-, bromo- and iodocyclohexane. The method is then tested on four polyhalogenated marine natural products. The results indicate that ¹³C relaxation measurements can be used to distinguish brominated carbons from chlorinated carbons in the case of halogenated quaternary carbons, sp² hydridized methine carbons and some sp³ hydridized methine carbons, but not in the case of halogenated methylene carbons or gem-dihalo substituted methine carbons.     

13C nuclear magnetic resonance studies of cellulose acetate

¹³C-NMR spectra of ring carbons and O-acetyl carbonyl carbons of cellulose acetate (CA) in dimethyl sulfoxide-d₆ were analyzed. The CA samples with the degree of substitution (DS) ranging from 0.84 and 1.91 were prepared by homogeneous acetylation of cellulose with acetic anhydride in a 10% LiCl/dimethyl acetamide solvent. It was found that the use of these low DS samples permitted easier assignments not only of the ring carbon but also of the O-acetyl carbonyl carbon signals. The assignments were confirmed by comparing with the ¹H-NMR spectra of the samples obtained by complete acetylation of the corresponding CA samples with acetyl-d₃ chloride. Two methods for determining the distribution of O-acetyl groups of CA, i.e., the relative DS at the three different types of hydroxyl groups, were developed. One is based on the measurements of the relative intensities of the signals for the ring carbons and the other is based on the measurements of the relative intensities of the signals for the O-acetyl carbonyl carbons.     

An atomic electronegative distance vector and carbon-13 nuclear magnetic resonance chemical shifts of alcohols and alkanes

A novel atomic electronegative distance vector (AEDV) has been developed to express the chemical environment of various chemically equivalent carbon atoms in alcohols and alkanes. Combining AEDV and Y parameter, four five-parameter linear relationship equations of chemical shift for four types of carbon atoms are created by using multiple linear regression. Correlation coefficients are R = 0.9887, 0.9972, 0.9978 and 0.9968 and roots of mean square error are RMS = 0.906, 0. 821, 1.091 and 1.091 of four types of carbons, i.e., type 1, 2, 3, and 4 for primary, secondary, tertiary, and quaternary carbons, respectively. The stability and prediction capacity for external samples of four models have been tested by cross-validation.     

Arsenic containing heterocycles. II. Confirmation of the structure of benzo[a]-9-chlorophenarsazine 12-chloride by 13C-NMR spectroscopy and spin-lattice relaxation time studies: Possible evidence for the contribution of an 75as-13C mechanism to the relaxation of the arsenic bearing quarternary carbons in the phenarsazine ring system

The product of the reaction of (3-chlorophenyl)-2-naphthylamine with arsenic trichloride has been confirmed to be benzo(a)-9-chlorophenarsazine 12-chloride by ¹³C-nmr spectroscopy and spin-lattice (T₁) relaxation time measurements. A considerable shortening of the spin-lattice relaxation times of the arsenic bearing quaternary carbons, relative to the calculated relaxation times, was observed. This discrepancy has been interpreted in terms of an ⁷⁵As-¹³C dipolar contribution to quaternary carbon relaxation.     

NMR spectral properties of the tetramantanes – nanometer‐sized diamondoids

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their ¹H and ¹³C NMR spectra because it translates into repeating structural features, such as diamond‐cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C₂₂H₂₈ isomers, diamond‐lattice structures result in long‐range ⁴J_HH, W‐coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven ⁴J_HH). Isobutyl‐cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their ¹³C spectra (<29.5 ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W‐couplings between these methines and methylenes clarify spin‐network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid ¹³C NMR chemical shifts are largely determined by α and β effects, however γ‐shielding effects are important in [123]tetramantane. ¹H NMR chemical shifts generally correlate with numbers of 1,3‐diaxial H–H interactions. Tight van der Waals contacts within [123]tetramantane's molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47 ppm for geminal hydrogens bonded to C‐3,21 . These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural characterization of vulcanized natural rubber by latex state 13C NMR spectroscopy

Structural characterization of vulcanized natural rubber was performed by high‐resolution latex‐state ¹³C NMR spectroscopy. The vulcanized natural rubber latex was prepared by vulcanization of high ammonia natural rubber latex with sulfur and sodium di‐n‐butyldithiocarbamate as vulcanizing agents. High resolution was attained for latex‐state ¹³C NMR spectroscopy even after vulcanization of the rubber latex, as is evident from no background in spectrum and narrow half width of signals, which were independent of vulcanization time. Small signals at 44 and 58 ppm in the carbon region were assigned by measurements of both distortionless enhancement by polarization transfer (DEPT) and attached proton test (APT) to secondary, tertiary, and quaternary carbons of crosslinking points. The assignment was proved by high‐resolution solution‐state NMR spectroscopy of vulcanized liquid cis‐1,4‐polyisoprene as a model, in which DEPT, APT, 2‐dimensional ¹H‐¹³C correlation (HETCOR), and 2‐dimensional heteronuclear multiple bond correlation (HMBC) measurements were applied. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1003–1009, 2007     

Heteronuclear two-bond correlation: suppressing heteronuclear three-bond or higher NMR correlations while enhancing two-bond correlations even for vanishing 2J(CH)

A novel two-dimensional NMR pulse sequence, H2BC, for long-range correlation of 1H and 13C nuclei is presented. The experiment has several attractive features compared to the widely used HMBC experiment, for example, (a) typically strong enhancement of correlations over two bonds while suppressing those over more bonds, that is, resolving ambiguities in standard HMBC spectra and showing two-bond correlations not present in HMBC spectra, (b) independence of long-range 1H-13C coupling constants, (c) full homo- and heteronuclear decoupling in the indirect dimension and heteronuclear decoupling in the acquisition dimension, (d) pure 2D absorption peak shapes, and (e) a pulse sequence duration significantly shorter than that of HMBC. The experiment is quite complementary to HMBC and does not effect correlations to quaternary carbons that must be obtained by HMBC.     

Chemical Modification of Activated Carbons

Mesoporous activated carbons and silica gel (Kieselgel 100, Merck) were modified as follows: (i) treatment with vinyltrimethoxysilane; or (ii)chlorination with CCl₄ followed by the reaction with a Grignard reagent. Modification of silica gel was proved by methods FTIR, NMR-¹³C and element analysis on carbon. The chemical modification of coals by alkans and olefins was supported by comparison of results of thermogravimetry for modified coals and modified silica gels. Polymerization of vinyl groups on carbon surface is shown by methods thermogravimetric analysis and differential scanning calorimetry. This revised version was published online in August 2006 with corrections to the Cover Date.     

Initial formation of an olefin-copper(I) π-complex in conjugate addition of lithium dimethylcuprate to t-butyl cinnamate

The ¹³C NMR spectrum observed upon mixing lithium dimethylcuprate and t-butyl cinnamate in toluene-d₈/THF at ?70°C shows the signals of the olefinic carbons shifted to lower δ values. These are attributed as due to a π-complex formed by coordination of copper to the carbon—carbon double bond of the cinnamate.     

Synthesis and NMR elucidation of novel pentacyclo-undecane diamine ligands

The synthesis and NMR elucidation of eight novel pentacyclo-undecane (PCU) diamine compounds are reported. These ligands are potential anti-inflammatory agents to be used against rheumatoid arthritis (RA). One-dimensional NMR techniques (¹H and ¹³C spectra) show major overlapping of methine resonances of the “cage” (PCU) thereby making it extremely difficult to assign all NMR signals. This overlapping occurs as a result of the substitutions made at the quaternary carbons (C-8/C-11) of the cage. Two-dimensional NMR techniques proved to be a useful tool in overcoming this problem.