Enhanced 13C resolution in semi-selective HMbC: a band-selective, constant-time HMBC for complex organic structure elucidation by NMR

A semi-selective 2D HMBC experiment is described which yields high-resolution in the indirect carbon-13 dimension by suppressing homonuclear proton coupling modulations and so provides an NMR technique suitable for the structure elucidation of organic compounds which exhibit particularly crowded carbon-13 spectra.     

Substituent effects in heterocyclic systems by carbon-13 nuclear magnetic resonance. Isoxazoles

The carbon-13 nmr spectra of a series of 3-aryl-5-phenylisoxazoles (I) and 3-phenyl-5-arylisoxazoles (II) have been recorded and the signals assigned. Carbon-13 data for series I show little effect of substituent on the chemical shift of the isoxazole ring carbons. However, a plot of the carbon-13 chemical shift of carbon-5 in the isoxazole system I versus the chemical shift of carbon-3 in the 3-(4′-aryl)-1-phenylpropenones gives a straight line (r = .989) with a slope of 0.35. In series II, the chemical shifts of both carbon-4 and -5 are relatively sensitive to substituent effects. Fair correlations between Hammett sigma values and the chemical shifts of these two carbons are found; dual substituent parameter treatment improves the correlations. The results obtained from correlations with carbon-4 in series II are similar to those obtained from β-carbons of a number of styrene systems. The data show that carbon-4 in series II is approximately 20% less sensitive to substituent effects than the previously reported data for carbon-3 of 2-arylfurans. Transmission of substituent effects in the isoxazole system compare well with those of the benzothiazole system.     

Synthesis of Quadruply Carbon-13 Labeled Tetramethyltetrathiaful Valene

A short synthesis of tetramethyltetrathiafulvalene quadruply labeled with carbon-13 is described.     

Carbon-13 n.m.r. analysis of selected strychnos alkaloids

The carbon-13 chemical shifts of strychnine and ten closely related analogues have been assigned and confirmed in several cases using selective proton decoupling techniques. Where additivity relationships are used the importance of having data on an array of structural analogues is emphasized. Quaternization at N-19 exerts long range polarization effects throughout the molecular framework. The effects of N-oxide formation on the carbon-13 shifts of 5 to 9-membered ring heterocycles and strychnine are discussed as aids to structural assignment. A Zwitterionic structure for strychnic acid is deduced.     

Aromatic solvent-induced shifts (ASIS) in carbon-13 NMR spectroscopy

A new method is proposed for the estimation of aromatic solvent-induced shifts (ASIS) in carbon-13 NMR spectroscopy. The observed C-13 ASIS are shown to be useful for studing the structure elucidation.     

A carbon-13 Nmr assignment study of 3,5-diarylisoxazoles

The assignment of the carbon-13 nmr resonances of 3-(aryl)-5-phenylisoxazoles (I) and 3-phenyl-5-(aryl)-isoxazoles (II) in dimethylsulfoxide-d₆ has been made. The assignments were made by using model compounds, chemical shift arguments, coupling with fluorine substituents, and shifts resulting from quaternization of the isoxazole ring. A carbon-13 nmr method for distinguishing between I and II which can be produced simultaneously by certain synthetic methods is presented.     

Carbon-13 NMR spectroscopic studies on chemically modified and unmodified synthetic and natural humic acids

Carbon-13 cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy ((13)C-CP/MAS-NMR) was applied to study the chemical modification process of humic acids (HA) with diazomethane and the subsequent alkaline hydrolysis of the methylated HA. This modification process results in HA with selectively blocked phenolic OH groups, which can be used for metal ion binding studies with humic substances. Different chemically modified and unmodified natural and synthetic HA with carbon-13 of natural abundance were investigated. In addition, carbon-13 labeled modified synthetic HA, that were synthesized with [(13)C]diazomethane as methylation reagent, were studied to confirm the assumed modification process and to determine the type of functional groups that have the highest affinity for methylation with diazomethane. The results of the NMR studies with carbon-13 labeled modified HA show that predominantly carboxyl and phenolic OH groups are methylated with diazomethane resulting in methyl ester and methyl ether groups, respectively. Due to the alkaline treatment of the methylated HA, the methyl esters of carboxyl groups are hydrolyzed, whereas methyl ethers of phenolic OH groups remain unchanged, which results in modified HA with blocked phenolic OH groups. From the spectra of the modified and unmodified HA with carbon-13 of natural abundance it can be concluded that the applied preparative modification procedure causes only the desired structural changes in HA.     

Investigation of carbon-13 nuclear magnetic resonance spectra-structure correlation based on novel atomic distance-edge (ADE) vector

A set of novel graph-theoretical parameters,called the atomic distance-edge (ADE) vector,was developed.Based on the connecting C-C bond number between central carbon atom and the other ones,various carbon atoms of alkanes were classified as four types,i.e.,type 1,2,3 and 4 for primary,secondary,ternary and quaternary carbon,respectively; and then four regression equations were obtained to link carbon-13 chemical shift (CS) of each type of atoms.Furthermore,these regression models were used to predict the carbon-13 nuclear magnetic resonance spectra of alkanes and it was found that the estimated CS were in agreement with the experimental results.     

反BIRD在NMR谱编辑中的应用

建议了一个新的双线性旋转去偶序列 ,称为反BIRD (inverse -BIRD) ,其去偶作用与碳连质子的数目有关。将反BIRD结合到INEPT实验中 ,得到了INEPT -iBIRD实验技术 ,它可用于核磁共振碳谱的编辑与简化     

Structure determination of some quinazolone lactones

A series of quinazolone lactones were prepared and their structures analyzed by 360 MHz proton and carbon-13 nmr. It was found that the compound capable of lactonization to yield either a six or seven-membered ring forms only the six-membered system.     

The carbon-13 and proton NMR spectra of nicotine in aqueous media

The carbon-13 and proton nmr spectra of nicotine in aqueous solution have been ivnestigated. The changes resulting from alteration of pD level are detailed. In the case of the carbon-13 spectrum, these changes are essentially independent of the acid present. Changes in previously published assignments for carbons (2) and (6) and protons (2) and (6) are presented.     

Determination of chemical purity and isotopic composition of natural and carbon-13-labeled arsenobetaine bromide standards by quantitative1H-NMR

In this study, we report the characterization of three arsenobetaine-certified reference materials by quantitative NMR. We have synthesized an arsenobetaine bromide high-purity standard of natural isotopic composition (ABET-1) and two carbon-13-labeled isotopic standards (BBET-1 and CBET-1). Assignments of the chemical purity and isotopic composition are not trivial in the case of arsenobetaine, and in this study we utilized quantitative¹H-NMR techniques for the determination of the mass fractions (chemical purity). The isotopic purity of all three standards was also assessed by NMR from the carbon-13 satellite signals. The standards are non-hygroscopic, high-purity (ca. 0.99 g/g), and the carbon-13 enrichment for both isotopic standards is x(¹³C)≈0.99. These standards are designed for use as primary calibrators for mass spectrometric determination of arsenobetaine in environmental samples.     

Palladium-catalyzed double carbonylation using near stoichiometric carbon monoxide: expedient access to substituted 13C2-labeled phenethylamines

A novel and general approach for (13)C(2)- and (2)H-labeled phenethylamine derivatives has been developed, based on a highly convergent single-step assembly of the carbon skeleton. The efficient incorporation of two carbon-13 isotopes into phenethylamines was accomplished using a palladium-catalyzed double carbonylation of aryl iodides with near stoichiometric carbon monoxide.     

Resolving overlap in two-dimensional NMR spectra: nuclear Overhauser effects in a polysaccharide

Overlapping resonances in the two-dimensional nuclear Overhauser (NOESY) spectrum of the O-antigenic polysaccharide from Escherichia coli O147 have been resolved by recording a tilted projection of a three-dimensional NOESY-HSQC spectrum, where the carbon-13 and proton evolutions are linked together. Through the introduction of small contributions from the appropriate carbon-13 shifts, a cluster of five unresolved proton peaks is separated into its components without the need to perform the full three-dimensional measurement, a time saving of at least an order of magnitude.     

An investigation of contributions to carbon-13 spin-lattice relaxtion in amino acids and peptide hormones

Carbon-13 spin-lattice relaxation times have been measured in glycine and the tripeptide pro-leu-gly-NH₂. These times are compared with those measured in the same compounds where the glycine α-carbon has been deuterated. In this manner evidence is obtained which indicates that mechanisms other than dipolar interactions with covalently bonded protons may contribute to carbon-13 spin-lattice relaxation. The effect of these additional mechanisms is found to be non-negligible for the carbonyl carbon of glycine and the glycine α-carbon of the tripeptide. The implication of these findings for deducing motional information from carbon-13 relaxation measurements is briefly discussed.     

13C, 13C coupling constants in phenyl substituted ethylene,naphthalene, phenanthrene and cyclopentadienone. Dependence on dihedral angles. Determination of signs by the symmetrical double labelling (SDL) method. IV

Carbon-13, carbon-13 coupling constants and carbon-13 chemical shifts have been measured in a series of phenyl substituted ethylenes and aromatics all doubly labelled with ¹³C at the olefinic positions (α,β-) or at neighbouring aromatic positions, tetraphenylcyclopentadienone labelled at the 3,4-positions, and dichlorodiphenylmethane labelled at the α-carbon. Signs of coupling constants were determined by the symmetrical double labelling (SDL) method. Coupling constants over as many as five bonds are reported. Two-bond couplings between carbons in the aromatic skeleton belong to different classes according to the nature of the coupling path. The magnitudes of three-bond coupling constants between such carbons correlate linearly with π-bond orders and a separation of the δ- and π-contributions is evident. The three-bond couplings between the 2-position in a phenyl substituent and the olefinic β-position or a corresponding aromatic position depend on the out-of-plane twist of the phenyl ring and may lead to information about the twist angle. Contrary to findings with aromatic carbonyl compounds two- and three-bond couplings to the α-carbon in the present compounds are fairly constant. The reported data suggest that the signs of coupling constants over more than two bonds alternate in aromatic systems. Carbon-13, carbon-13 coupling constants in naphthalene have been calculated by the INDO-SOS method.     

Comprehensive parameter set for the prediction of the 13C-NMR chemical shifts of sp3-hybridized carbon atoms in organic compounds

A parameter set is described for prediction of the carbon-13 chemical shifts of sp³-hybridized carbon atoms based on a simple linear additivity relationship. It was tested against 88 692 known chemical shift values. Many of the previously reported parameters were amended and new parameters added. With this parameter set, the chemical shifts in 99.7% of the 88 692 cases can be estimated with an overall standard deviation of 5.6 ppm for the predicted values relative the the experimental ones. The additivity parameters reported here can be used per se or in connection with a recent computer program for the estimation of carbon-13 chemical shifts, which automatically selects and applies additivity rules appropriate for the individual carbon atoms of the chemical constitution entered.     

Determination of the orientational ordering of 4′-cyanophenyl-4-alkylbenzoates by 13C NMR

Abstract

The orientational ordering of three 4′-cyanophenyl-4-alkylbenzoates (with number of carbons in the alkyl chain, n = 6,7 and 8; hereafter abbreviated as n-CPBs) has been investigated by ¹³C NMR. The order parameters of different molecular segments in the nematic phase of the n-CPBs were determined by the two-dimensional technique of separated local field (SLF) spectroscopy combined with off-magic-axis, variable-angle spinning (VAS) of the sample. The carbon-13 chemical shifts for each carbon nucleus in these compounds were determined by slowly spinning the sample parallel to the applied magnetic field. The order parameters obtained from SLF/VAS studies are linearly related to the corresponding anisotropic carbon-13 chemical shifts. These results provide a convenient way to obtain the order parameters for other homologous members of this liquid crystal series by direct measurement of only their carbon-13 chemical shifts in conjunction with the observed linear relationship between order parameters and chemical shifts.     

Carbon-13 magnetic resonance study of spiro compounds

A large series of cyclohexane and cyclopentane spiro compounds has been studied by carbon-13 NMR. Increments have been derived in terms of α, β, γ spiro substituent effects, one ring being considered as the substituent of the other. As usual α and β effects are paramagnetic and γ effects are diamagnetic. They are compared with the corresponding effects associated with the introduction of a gem dimethyl group into a cyclohexane or cyclopentane ring. The size of the two rings involved in the spirane structure has a marked influence especially on the chemical shift of the spirane carbon atom. Comparison between monospiro and dispiro derivatives gives information about the so-called 1,5 or δ effect which is paramagnetic. From an analytical point of view, carbon-13 NMR appears to be a powerful method of detecting the spirane structure, apparently including the relative size of the rings.     

Neighboring group participation by the pyrrole nucleus

Using proton and carbon-13 NMR spectroscopy, the transformation of 2-hydroxyethylpyrroles (5,15) into 2-haloethylpyrroles by treatment with thionyl chloride or carbon tetrabromide/triphenylphosphine is shown to proceed by randomization of the two side-chain carbon atoms. A spirocyclopropylpyrrolium ion (19) is postulated as an intermediate in this unexpected process.