Assignment of the 13C nuclear magnetic resonance spectra of three 27-hydroxyfriedelanes

¹³C NMR spectra of 27-hydroxyfriedelane, kokoonol (27-hydroxyfriedelan-3-one) and kokoononol (27-hydroxyfriedelan-3,21-dione) have been recorded and signals assigned using the off-resonance decoupling, inversion recovery and lanthanide induced shift techniques.     

13C nuclear magnetic resonance spectra of some naturally occurring friedelanones

¹³C NMR spectra of friedelan, friedelan-21-one, friedelan-6-one, friedelane-321-dione, friedelane-3,6-dione and friedelane-3,6,21-trione have been recorded and signals assigned using off-resonance decoupling, inversion recovery and lanthanide induced shift techniques.     

A 13C NMR study of mesomorphic solutions of poly(p-phenylene terephthalamide)

Solutions of poly(p-phenylene terephthalamide) in fuming sulfuric acid were characterized by ¹³C NMR spectroscopy and solution viscosity measurements over the 2–28% w/w concentration range. The spectra showed the presence of two distinct amide carbonyl resonances at low concentration, tentatively assigned to cis and trans conformations. As the concentration increased, additional carbonyl lines were observed along with significant broadening. Peak area measurements showed that only the polymer molecules in the isotropic environments contributed to the ¹³C NMR spectra and a considerable amount of the polymer remained in the isotropic phase at concentrations previously considered to consist of polymer in highly anisotropic regions. Spin-lattice relaxation times were measured at six concentrations using the inversion recovery method. The aromatic carbons relaxed at a much faster rate (ca. 0.10 s) than the carbonyls (ca. 0.45 s), but the relaxation rates for both carbons were essentially constant over the concentration range, indicating that the observed isotropic phase is not affected by changes in the macroscopic solution behavior so as to alter spin-lattice relaxation mechanisms.     

Structural characterization of monomeric and oligomeric arylamides by solution and solid‐state NMR spectroscopy

An extensive study of both liquid‐ and solid‐state NMR spectroscopy was undertaken in order to elucidate the structural features of a phenyleneterephthalamide oligomer (OPTA) and of some related diarylamides. 1D‐ and 2D‐COSY measurements allowed us to assign completely the proton signals of the title compounds in solution, while 1D‐, 2D‐HETCOR and 2D‐COLOC measurements were used to assign ¹³C resonances. Solid‐state ¹³C NMR experiments, by conventional cross‐polarization (CP) at different contact times and with the dipolar dephased CP technique, were used to characterize these molecules in the solid state. Such techniques allowed us to differentiate among different carbon atoms; in the resulting spectra it was then possible to observe the selective appearance of signals from protonated and quaternary carbon atoms. It was also ascertained that the limited structural mobility of the insoluble OPTA, existing as a single monophasic species, can be explained in terms of hydrogen‐type bonds present in the solid state. Copyright © 2002 John Wiley & Sons, Ltd.     

Solid state 15N NMR study of 15N enriched melamine-formaldehyde resins

Molecular dynamics and structure of uncured and cured melamine-formaldehyde resins isotopically ¹⁵N enriched at amine sites were studied by solid-state ¹⁵N nuclear magnetic resonance (NMR). Spectra recorded with direct (DP) and cross-polarization (CP) pulse sequences reflect two motionally different regions arising from similar chemical structures. DP spectra of uncured resins at higher temperatures have narrow lines and the detection of slightly different structural units is possible. With increasing crosslinking resonances broaden and overlap and the direct detection of individual signals in cured resins is not possible. On the basis of variable contact time, variable spinning speed, and interrupted decoupling experiments three protonated and one nonprotonated group of signals are identified in the CP spectra for all samples. Short polarization-transfer rates, T_NH, for nonprotonated nitrogen in uncured and lightly cured samples reveal more effective hydrogen bonding in viscous and rubber-like resins compared to the highly cured rigid resins. The rigid portions of the resins exhibit longer T₁ and short T_1ρ relaxation times, while the shorter T₁ times and longer T_1ρ times are associated with the more mobile portion of samples. ©1995 John Wiley & Sons, Inc.     

Background‐free solution boron NMR spectroscopy

The appearance of background signals arising from the NMR probe and tube is a well‐known problem of boron NMR spectroscopy. Background suppression may be achieved by using DEPTH, which increases the signal‐to‐background (S/B) ratio. Although, the quality of such spectra is often adequate, but in the case of rapid relaxation broadened resonances (T₁ < 1 ms), the residual background signals may still hamper the interpretation of the spectra. It was observed that the background signals are practically invisible in solution ¹⁰B NMR. The unusual isotopic effect on the (S/B) ratio was interpreted as an inherent consequence of the integer versus half‐integer spin of ¹⁰B and ¹¹B, respectively. The practicability of ^10/11B NMR was compared for a selected set of boron compounds covering the typical range of (S/B) ratio. The application of ¹¹B is more favourable than ¹⁰B as long as it is possible to achieve the desired spectral quality by using DEPTH. Otherwise, the ‘background‐free’ appearance of ¹⁰B NMR spectra makes ¹⁰B a reasonable alternative of ¹¹B DEPTH. This was found typical for compounds having relaxation broadened resonances. The variable temperature (VT) NMR study of an adduct formation process was also presented here as an example of the advantage of ¹⁰B over ¹¹B. Copyright © 2012 John Wiley & Sons, Ltd.     

The temperature dependence of the 13C nuclear magnetic resonance spectra of porphyrins

The high temperature ¹³C NMR spectra of the methyl esters of deuteroporphyrin-IX and protoporphyrin-IX, and octaethylporphyrin show sharp discrete signals for the inner macrocycle carbon resonances. The temperature dependence of these shifts is discussed.     

Studies on comblike polymers. X. Characterization of comblike polyesters by 13C NMR spectroscopy

The ¹³C NMR spectra of aromatic comblike polyesters obtained from 1,2-icosanediol and isomeric phthalic acids are reported and discussed. The analysis shows that the most informative spectral regions are those of the carbonyl and quaternary carbon resonances, whose peak multiplicities have been interpreted in terms of dyad and triad sequences. The results indicate that the extent of structural characterization possible for the polymers examined is dependent on the type of isomerism of the aromatic acid involved.     

1H and 13C NMR of PMMA macromonomers and oligomers - end groups and tacticity

¹H NMR and ¹³C NMR have been used to study the end groups and tacticity in PMMA macromonomers and oligomers. These macromonomers are terminated almost exclusively in one vinylidene group per chain. The end group signals from the macromonomers are identified in both the ¹³C and ¹H NMR spectra. The spectra of the purified oligomers (n = 1-4) were used to aid in assignment. The macromonomers are predominantly syndiotactic, and the tacticity measured is consistent with Bernoullian statistics. The tetramer is a mixture of r and m isomers in a 4:1 ratio. It is shown that T₁ experiments can provide a useful method of distinguishing resonances due to low molecular weight impurities from those due to stereochemical or isomeric effects in macromonomers. The absence of internal double bonds was confirmed by isomerizing the vinylidene group of several oligomers and of the macromonomer, and verifying the absence of the isomerized signals in the NMR spectra of the original materials.     

Structural analysis of alanine tripeptide with antiparallel and parallel beta-sheet structures in relation to the analysis of mixed beta-sheet structures in Samia cynthia ricini silk protein fiber using solid-state NMR spectroscopy

The structural analysis of natural protein fibers with mixed parallel and antiparallel beta-sheet structures by solid-state NMR is reported. To obtain NMR parameters that can characterize these beta-sheet structures, (13)C solid-state NMR experiments were performed on two alanine tripeptide samples: one with 100% parallel beta-sheet structure and the other with 100% antiparallel beta-sheet structure. All (13)C resonances of the tripeptides could be assigned by a comparison of the methyl (13)C resonances of Ala(3) with different [3-(13)C]Ala labeling schemes and also by a series of RFDR (radio frequency driven recoupling) spectra observed by changing mixing times. Two (13)C resonances observed for each Ala residue could be assigned to two nonequivalent molecules per unit cell. Differences in the (13)C chemical shifts and (13)C spin-lattice relaxation times (T(1)) were observed between the two beta-sheet structures. Especially, about 3 times longer T(1) values were obtained for parallel beta-sheet structure as compared to those of antiparallel beta-sheet structure, which could be explicable by the difference in the hydrogen-bond networks of both structures. This very large difference in T(1) becomes a good measure to differentiate between parallel or antiparallel beta-sheet structures. These differences in the NMR parameters found for the tripeptides may be applied to assign the parallel and antiparallel beta-sheet (13)C resonances in the asymmetric and broad methyl spectra of [3-(13)C]Ala silk protein fiber of a wild silkworm, Samia cynthia ricini.     

Nuclear magnetic resonance studies on microstructure of ethylene copolymers. VI. Carbon-13 spectra of ethylene–vinyl acetate copolymers

The 22.6-MHz Fourier-transform noise-decoupled ¹³C (carbon-13) NMR spectra of several ethylene–vinyl acetate (E–VA) copolymers were obtained. We found that triad information on monomer placement can be deduced from carbonyl resonances, triad and pentad information can be deduced from methine carbon resonances, and triad information is available from the methylene carbon resonances. The random comonomer distributions in E–VA polymerizations were demonstrated up to pentad placements. In addition, the use of model-compound data in the analysis of copolymer spectra was shown.     

Synthese und Eigenschaften von 1,1,3,3-Tetrakis(pentafluorphenyl)-cyclo-di(silthian) und verwandten Verbindungen

Summary The synthesis of tetrakis(pentafluorophenyl)-cyclo-di(silthiane) and related compounds by reacting organosubstituted silanes with sulfur or hexamethyldisilthiane is reported The compounds are characterized by analysis, by relative molecular mass, by¹H,¹³C,¹⁹F, and²⁹Si NMR spectroscopy, and by mass and IR spectra, respectively. Two²⁹Si NMR signals of different intensity found for compounds with two different organic substituents on silicon are explained by the existence of diastereomers.

     

Synthesis and NMR elucidation of pentacyclo-undecane diamine derivatives as potential anti-tuberculosis drugs

The complete structural elucidation of six novel pentacycloundecane (PCU) derivatives is reported. The target molecules are potential anti-tuberculosis agents. The addition of side arms to the PCU cage skeleton at position C-8/C-11 results in major overlapping of the methine resonances of the ¹H NMR spectrum. The use of 2D NMR techniques proved to be a very useful tool in overcoming the difficulties encountered in the elucidation of cage compounds using ¹H and ¹³C spectra only. All compounds reported are meso compounds thereby simplifying the complexity of the NMR spectra. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Raman and 29Si MAS NMR spectroscopy of framework materials containing three-membered rings

Raman and ²⁹Si MAS NMR spectroscopies are evaluated for the identification of three-membered rings (3MR) in framework oxide materials. Raman and ²⁹Si MAS NMR spectra from the 3MR-containing materials euclase, phenakite, clinohedrite, willemite, lovdarite, VPI-7, ZSM-18 and dipotassium zinc tetrasilicate are presented. The Raman spectra from these materials do not exhibit common bands representing vibrational modes assignable to individual 3MR. The dense beryllosilicate and zincosilicate minerals exhibit ²⁹Si MAS NMR resonances indicative of silicon positioned in 3MR while the molecular sieves lovdarite and VPI-7 give ²⁹Si MAS NMR resonances that can be assigned to silicons located at the center of “spiro-5” units that are constructed from two 3MR. Silicon atoms located in isolated 3MR in the molecular sieves ZSM-18 and dipotassium zinc tetrasilicate do not exhibit ²⁹Si MAS NMR resonances that can be distinguished from those assigned to silicons residing in 4MR and larger.The ²⁹Si MAS NMR spectra from the new materials VPI-8, VPI-9 and VPI-10 do not exhibit ²⁹Si MAS NMR resonances indicative of “spiro-5” units. The presence of isolated 3MR in these materials cannot be ruled out from the ²⁹Si MAS NMR spectroscopic results.     

Targeted projection NMR spectroscopy for unambiguous metabolic profiling of complex mixtures

Unambiguous identification of individual metabolites present in complex mixtures such as biofluids constitutes a crucial prerequisite for quantitative metabolomics, toward better understanding of biochemical processes in living systems. Increasing the dimensionality of a given NMR correlation experiment is the natural solution for resolving spectral overlap. However, in the context of metabolites, natural abundance acquisition of ¹H and ¹³C NMR data virtually excludes the use of higher dimensional NMR experiments (3D, 4D, etc.) that would require unrealistically long acquisition times. Here, we introduce projection NMR techniques for studies of complex mixtures, and we show how discrete sets of projection spectra from higher dimensional NMR experiments are obtained in a reasonable time frame, in order to capture essential information necessary to resolve assignment ambiguities caused by signal overlap in conventional 2D NMR spectra. We determine optimal projection angles where given metabolite resonances will have the least overlap, to obtain distinct metabolite assignment in complex mixtures. The method is demonstrated for a model mixture composition made of ornithine, putrescine and arginine for which acquisition of a single 2D projection of a 3D ¹H–¹³C TOCSY‐HSQC spectrum allows to disentangle the metabolite signals and to access to complete profiling of this model mixture in the targeted 2D projection plane. Copyright © 2010 John Wiley & Sons, Ltd.     

NMR study of some mono- and di-tert-butylnaphthalenes

The proton decoupled ¹³C FT—NMR spectra and ¹H FT—NMR spectra of some mono- and di-tert-butylnaphthalenes have been studied. Some interesting features observed in these spectra have been found to be of diagnostic value in distinguishing the closely related isomers of these compounds. On the basis of the assignment done by earlier workers in case of mono-substituted naphthalenes following the additivity approach, it has been shown that all the ¹³C—NMR signals observed for the present di-substituted naphthalenes can also be assigned; and this procedure thus obviates the use of tedious and some times experimentally inaccessible techniques.     

Two-Dimensional NMR Studies of Polyene Systems. I. All-trans-Retinal

Two-dimensional (2-D) NMR results are presented for all-trans-retinal. 2-D J-resolved ¹H-NMR separated the multiplets of the olefinic protons and accurately determined their chemical shifts. 2-D shift-correlated ¹H-NMR gave the connectivities between scalar coupled protons. From the observed H,H long-range couplings the assignment of the methyl resonances was possible. 2-D J-resolved ¹³C-NMR separated overlapping C,H-multiplets and allowed analysis of the C,H long-range couplings, 2-D shift-correlated ¹³C-NMR related each directly bonded C,H-pair in this molecule. The potential of 2-D NMR in resolving and identifying individual resonances in polyene spectra is discussed.     

A novel approach to the rapid assignment of 13C NMR spectra of major components of vegetable oils such as avocado,mango kernel and macadamia nut oils

Assignment of ¹³C nuclear magnetic resonance (NMR) spectra of major fatty acid components of South African produced vegetable oils was attempted using a method in which the vegetable oil was spiked with a standard triacylglycerol. This proved to be inadequate and therefore a new rapid and potentially generic graphical linear correlation method is proposed for assignment of the ¹³C NMR spectra of major fatty acid components of apricot kernel, avocado pear, grapeseed, macadamia nut, mango kernel and marula vegetable oils. In this graphical correlation method, chemical shifts of fatty acids present in a known standard triacylglycerol is plotted against the corresponding chemical shifts of fatty acids present in the vegetable oils. This new approach (under carefully defined conditions and concentrations) was found especially useful for spectrally crowded regions where significant peak overlap occurs and was validated with the well‐known ¹³C NMR spectrum of olive oil which has been extensively reported in the literature. In this way, a full assignment of the ¹³C{1H} NMR spectra of the vegetable oils, as well as tripalmitolein was readily achieved and the resonances belonging to the palmitoleic acid component of the triacylglycerols in the case of macadamia nut and avocado pear oil resonances were also assigned for the first time in the ¹³C NMR spectra of these oils. Copyright © 2009 John Wiley & Sons, Ltd.     

One- and two-dimensional NMR studies of methyl acrylate/vinyl acetate/N-vinyl carbazole terpolymers

Terpolymers of methyl acrylate/vinyl acetate/N-vinyl carbazole (M/A/C) with different compositions were synthesized by solution polymerization using AIBN as an initiator. Composition of terpolymers was determined from quantitative ¹³C{¹H} NMR spectrum. Two-dimensional heteronuclear single quantum correlation (HSQC) and total correlated spectroscopy (TOCSY) were used to assign the methylene and methine carbon resonances by analyzing two and three bond order couplings. Various resonance signals were assigned to different compositional and configurational sequences with the help of one- and two-dimensional NMR spectra. Three and four bond order coupling between carbonyl carbon and other neighboring protons have been investigated with the help of 2D heteronuclear multiple bond correlation (HMBC) spectra. The complex and overlapped ¹H NMR spectrum of terpolymer was analyzed completely with the help of 2D HSQC and TOCSY spectra.