Carbon-13 chemical shielding tensors in acetic acid

High resolution double resonance methods have been employed to study the anisotropic ¹³C chemical shifts in a single crystal of acetic acid. The pr     

Carbon-13 NMR assignments in 1,3-cyclohexadiene

Unambiguous ¹³C-NMR assignments for the olefinic carbons of 1,3-cyclohexadiene (in CDCl₃: δ_C(1) = 126.02 ppm and δ_C(2) = 124.30 ppm downfield from TMS) have been made by comparison with regioselectively synthesized 2-deuterio-1,3-cyclohexadiene (in CDCl₃: δ_C(1) = 125.88, δ_C(3) = 124.22 and δ_C(4) = 126.02 downfield from TMS). ¹H-NMR chemical shifts and splittings are shown for both compounds.     

An experimental and theoretical study of the 13C and 31P chemical shielding tensors in solid O-phosphorylated amino acids

A series of l and dl forms of O-phosphorylated amino acids (serine, threonine, tyrosine) have been studied by using solid-state multinuclear NMR spectroscopy and ab initio calculations. Principal elements of the (13)C and (31)P chemical shielding tensors have been measured and discussed in relation to zwitterionic structures and intermolecular contacts. DFT calculations have been compared with experimental data showing their ability to reproduce experimentally obtained tensor values in this challenging class of compounds. The changes of orientation of (31)P chemical shielding tensor with respect to the molecular frame in the presence of hydrogen bonds have been revealed and discussed on the ground of theoretical calculations. The measurements of internuclear P...P distances, based on Zeeman magnetization exchange between (31)P spins with differing chemical shielding tensor orientations, were exploited for a clear distinction between enantiomers and racemates.     

Carbon-13 NMR of 1,2-polybutadienes: Configurational sequencing

Carbon-13 Fourier-transform spectra (67.88 MHz) of 1,2-polybutadienes (PBDs) (I–IV) were obtained. The triad and pentad intensities were calculated for the olefinic methylene carbon (C-1). Bernoullian and first-order Markov-chain propagation statistics were tested to fit the experimental data. The pentad placements for the olefinic methine carbon (C-2) were also assigned using the data calculated for the C-1 carbon. Fine-structure splitting due to placements higher than pentads was observed for both C-1 and C-2 carbons in PBD samples III and IV.     

Carbon-13 NMR relaxation and transitions in polymers

High-resolution proton-decoupled carbon-13 nuclear magnetic resonance relaxation parameters have been obtained as a function of temperature for a set of completely amorphous polymers, semicrystalline polymers, and a series of ethylene–vinyl acetate copolymers. With these samples the nature of the glass temperature, other postulated amorphous transitions, and the β transition were investigated. For the completely amorphous polymers, the average correlation times depend on temperature according to the Williams–Landel–Ferry relation. Spectral collapse occurs at temperatures whose ratio to T_g is in the range 1.2–1.4 and corresponds to a correlation time of about 10^?7s. The loss of resolvable spectra is demonstrated to be a consequence of experimental methods and is not due to the occurrence of another amorphous transition. Both the methylene and methine carbons can be resolved for the ethylenevinyl acetate copolymers. Although the correlation time for the methylene carbon is continuous and resolvable through the β transition region, the methine branch-point resonance is lost. The implication of these results to the molecular nature of the β transition is discussed.     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline amino acids

We have presented a systematic experimental investigation of carboxyl oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in crystalline amino acids. Three 17O-enriched amino acids were prepared: L-aspartic acid, L-threonine, and L-tyrosine. Analysis of two-dimensional 17O multiple-quantum magic-angle spinning (MQMAS), MAS, and stationary NMR spectra yields the 17O CS, EFG tensors and the relative orientations between the two tensors for the amino acids. The values of quadrupolar coupling constants (CQ) are found to be in the range of 6.70-7.60 MHz. The values of deltaiso lie in the range of 268-292 ppm, while those of the delta11 and delta22 components vary from 428 to 502 ppm, and from 303 to 338 ppm, respectively. There is a significant correlation between the magnitudes of delta22 components and C--O bond lengths. Since C--O bond length may be related to hydrogen-bonding environments, solid-state 17O NMR has significant potential to provide insights into important aspects of hydrogen bonds in biological systems.     

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.     

Carbon-13 NMR of polyethylenes: Correlation of the crystalline component T1 with structure

The carbon-13 spin-lattice relaxation times T₁ of the crystalline portion of a set of polyethylenes have been studied. Chain structure and crystallization conditions have been varied over the widest possible extremes so that large differences are developed in the level of crystallinity, the supermolecular structure, and the crystallite thickness. Concomitantly, the observed crystalline T₁ values cover the extraordinarily wide range of about 40–4500 s. They bear a one-to-one relation with the crystallite thickness, which is found to be the key structural variable determining this property. A correlation with the temperature for the α-transition can be established, which implies a similar type of segmental motions for the two phenomena. Major changes in the interfacial structure can also have a drastic influence on the value for the crystalline T₁. Analysis of the magnetization decay curve also allows for a quantitative determination of the degree of crystallinity, which is found to be in excellent agreement with the corresponding value found from Raman spectroscopy.     

Carbon-13 NMR spectra of taxane-type diterpenes: Oxiranes and oxetanes

The ¹³C NMR spectra of a series of nine taxane derivatives are reported. A detailed analysis of the assignments is presented. Observed long-range substituent effects are explained on the basis of the proximity of rings A and C in this type of molecules.     

Carbon-13 NMR substituent effects in para-substituted benzophenones

The carbon-13 NMR spectra for a series of benzophenones was obtained and analyzed, using the dual substituent parameter (DSP) equation. The DSP analysis indicates that transmission of the substituent effect to the substituted ring is primarily resonance controlled while the transmission to the second phenyl ring involves a π-bond polarization mechanism. The DSP analysis of the C'₄ chemical shifts (para carbon in ring B) was found to compare very favorably with the fluorine “tag” data for analogous systems. Data for some para-para' benzophenones is also given.     

Carbon-13 NMR investigation of the structure of hydroxy-azoloazines with a bridgehead nitrogen

Carbon-13 nmr data has been obtained on four hydroxy-azoloazines containing a bridgehead nitrogen atom, on the corresponding methoxide derivatives, and on selected anionic and cationic derivatives. These results have been interpreted in terms of the site of protonation of the parent hydroxy-compounds. A comparison of the anionic derivatives with those of the parent compounds demonstrate that the neutral parent hydroxy-species exist predominately in the “hydroxy form” rather than as the ionized species. The chemical shift data also provides information on the conformation of the hydroxy- and methoxy-groups in 8-hydroxy-6-methyl-s-triazolo[4,3-6]-pyridazine and 8-rnethoxy-6-methyl-s-triazolo[4,3-6]pyridazine.     

Carbon-13 NMR spectra and the method for their calculation for long-chain polybranched carboxylic acids and their derivatives

Carbon-13 NMR spectra of 56 long-chain polybranched carboxylic acids their esters, including chloroand acetoxy-substituted derivatives, have been studied. An empirical linear two-parameter equation, relating carbon chemical shifts in α-branched carboxylic acids and their derivatives to the chemical shifts of the corresponding carbon atoms in analogous hydrocarbons, and to the degree of branching at the α-carbon atom, is suggested for the assignment of the signals, where δ_X is the shift in the acid (ester), δ_H the shift in its hydrocarbon counterpart and n the degree of branching. The regression coefficients, A, B and C, are given for X=COOH, COOMe and COOEt. It is shown that with polysubstituted derivatives, the equation may be applied sequentially. The secondary coefficients for the α-substituent then depend on the nature of the substituent X entering the primary equation. The equation may, if necessary, be applied in combination with the additive and structural increments of halogen atoms.     

Carbon-13 NMR Studies: 82. Carbon-13 spectra of several 10-methyl-cis-decalins. Conformational preferences of some cis-decalin derivatives

The ¹³C spectra of a series of 21 cis-decalins, 17 of which bear 10-methyl substituents, have been recorded to examine geometrical and conformational effects on the ¹³C shieldings. Although the data for a few of the simpler examples have been reported, these compounds were examined to obtain results for a common medium and temperature. All of the derivatives are conformationally mobile and their spectra were examined as a function of temperature to obtain shieldings representative of the slow exchange limit. The examples include compounds which populate both available chair-chair conformations equally, some which exhibit a preference for one of these conformations and some which exist essentially in a single conformation. The data for the latter group combined with the observed temperature dependencies and results for other decalin systems led to specific assignments for each carbon. The internal consistency of the entire body of data offers strong support for these assignments. The conformational preferences observed for a number of decalones and Δ³-10-methyldecalin derivatives are discussed.     

Carbon-13 NMR studies of flavonoids—I : Flavones and flavonols

¹³C-NMR spectra of hydroxylated flavones and flavonols are presented for the first time. Analyses of the spectra are derived by consideration of a series of acetophenones, cinnamic acids, flavones and flavonols of increasing oxygenation pattern. The accepted substitution additivity rules have been shown to hold for these compounds except in cases where structural modifications involve the C-3,4 and 5 positions.     

A fast MAS 1H NMR study of amino acids and proteins

Fast sample spinning (up to 32 kHz) in tandem with delayed acquisition enabled resolved ¹H NMR spectra of solid amino acids to be recorded. The resulting spectra are, however, significantly dependent on sample crystallinity and on sample preparation conditions, e.g. sample drying. Sample heating leads to a marked increase in signal to noise ratio and enables groups with different dynamic properties to be identified. In addition, the observation of peak shifts as a function of heating allows the identification of hydrogen bonded sites. Spectral assignment of the ¹H MAS spectra is proposed for some examples based on relaxation properties, study of deuteriated samples and 2D NMR. The effects of molecular weight and sample complexity on the ¹H MAS spectra were investigated using tetraglycine, polyglycine and two proteins: a small protein (lysozyme) and a much larger protein (a cereal storage protein named high molecular weight subunit 1Dx5). Moderate spectral resolution was achieved for the peptides and lysozyme, but for 1Dx5, significant resolution enhancement was obtained enabling the identification of resonances in all regions of the spectra including the alpha region, the aromatic region and the NH backbone region.     

Combined first-principles computational and experimental multinuclear solid-state NMR investigation of amino acids

13C, 14N, 15N, 17O, and 35Cl NMR parameters, including chemical shift tensors and quadrupolar tensors for 14N, 17O, and 35Cl, are calculated for the crystalline forms of various amino acids under periodic boundary conditions and complemented by experiment where necessary. The 13C shift tensors and 14N electric field gradient (EFG) tensors are in excellent agreement with experiment. Similarly, static 17O NMR spectra could be precisely simulated using the calculation of the full chemical shift (CS) tensors and their relative orientation with the EFG tensors. This study allows correlations to be found between hydrogen bonding in the crystal structures and the 17O NMR shielding parameters and the 35Cl quadrupolar parameters, respectively. Calculations using the two experimental structures for L-alanine have shown that, while the calculated isotropic chemical shift values of 13C and 15N are relatively insensitive to small differences in the experimental structure, the 17O shift is markedly affected.