13C NMR chemical shifts of benzophenone azine as a benchmark for configurational assignment of azines with aromatic substituents

An analysis of the ¹³C NMR spectral data and quantum chemical calculations for benzophenone azine shows that the shielding constant of the C _ipso atom of its molecule is stereo-specific. The characteristic difference in chemical shifts of the C _ipso atoms of the phenyl rings in the cis- and trans-positions with respect to the lone pair of the neighboring nitrogen atom is 2–3 ppm. The stereospecificity observed for chemical shifts was proposed to be used for the configurational assignment of azines containing aromatic substituents.     

Determination of configurational isomers in polybutadienes by <Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR spectroscopy

The NMR spectra (600 MHz) of polybutadienes containing different amounts of cis-1,4, trans- 1,4 and 1,2 units are analyzed. The critical consideration of signal assignment and the comparison of ¹H and ¹³C NMR data reveal that there are significant limitations to the quantitative characterization of polybutadienes with the use of NMR spectroscopy.     

<Superscript>13</Superscript>C-<Superscript>13</Superscript>C spin-spin coupling constants in structural studies: XLIV. Carbonyl-containing oximes

¹³C-¹³C spin-spin coupling constants in carbonyl-containing oximes were calculated in terms of the second-order polarization propagator approximation (SOPPA) with account taken of the results of theoretical conformational analysis. Stable conformations of all carbonyl-containing oximes were found to have s-trans orientation of the carbonyl group in the E and Z isomers. The corresponding ¹³C-¹³C coupling constants showed a characteristic dependence upon internal rotation of the carbonyl-containing substituent, which may be used to determine predominant rotational conformations of these compounds. A relation between orientation of the C=O bond and ¹³C-¹³C coupling constants for the adjacent bonds in oximes was revealed; it may be attributed to hyperconjugation between the corresponding carbon-carbon bond and antibonding orbital of the C=O bond.     

<Superscript>13</Superscript>C-<Superscript>13</Superscript>C spin-spin coupling constants in structural studies: XXXIX. Nonempirical calculations of heteroaromatic oximes

The results of high-level nonempirical quantum-chemical calculations of ¹³C-¹³C coupling constants in twelve heteroaromatic ketone oximes are well consistent with the available experimental data. In all the examined compounds, the effect of the unshared electron pair on the oxime nitrogen atom on J _CC strongly predominates over conformational and electronic effects, which makes it possible to unambiguously assign J _CC values to different configurations.     

Experimental and theoretical study of <Superscript>13</Superscript>C shielding tensors in new-style molybdenum complex

¹³C 2D-PASS spectra of two new cis-dioxo catecholatomolybdenum complexes (NH₂CH₂NH₂CHCH₂)₂(H⁺)₃[Mo^vO ₂(C₆H₄O₂)₂] and (NH₂CH₂CH₂CH₂NH₂)₂(H⁺)₃[Mo^(v)O₂ (C₂H₂O₂)₂] have been obtained by solid-state nuclear magnetic resonance (NMR), in which the spinning sidebands were well-separated. The principal components of the ¹³C shielding tensors were extracted by theoretically fitting the intensities of ¹³C spinning sidebands. The effects of counter cations on ¹³C chemical shift isotropy and shielding tensor of cis-dioxo catecholatomolybdenum complex anion [Mo ^(v)O₂(C₆H₄O₂)₂]³⁻ were studied, comparing the ¹³C CSA of those carbon sites in complex anions with that of the counter cations. Based on the known structure of the molybdenum complex crystal, theoretical values of ¹³C shielding tensors were calculated by the ainitio GIAO method, in comparison with the experimental results.     

<Superscript>13</Superscript>C-<Superscript>13</Superscript>C spin-spin coupling constants in structural studies: XLII. Stereochemical study on functionalized 2,5-dihydrofuran-2-imines

The configurations and conformations of twelve 5-imino-2,5-dihydrofuran-3-carboxamides and 5-imino-2,5-dihydrofuran-3-amines were determined by analysis of the experimental ¹³C-¹³C coupling constants and those calculated by nonempirical methods, as well as by quantum-chemical calculations of the total energies and optimization of geometric parameters of preferential conformations at the MP2/6-311G** level. All the examined compounds were found to be formed and to exist in solution as individual Z isomers with orthogonal orientation of the amino and dialkylamino groups and s-trans orientation of the carbamoyl group in the ring with restricted internal rotation about the C(O)-N bond.     

<Superscript>0,2</Superscript>A<Subscript>n</Subscript> cross-ring cleavage as a general diagnostic tool for glycan assignment in glycoconjugate mixtures

In contrast to proteomics significantly less efficient analytical tools are presently available for high throughput glycomics using mass spectrometry. In this article, a strategy to use the (0,2)A(n) ring cleavage ion at the reducing end of free glycans as a diagnostic ion for assignment of free glycans, in presence of glycopeptides containing similar glycosylation patterns, is presented for rapid distinction in complex mixtures by mass spectrometry. The MS to MS/MS automatic switching, already previously introduced for the on-line LC-MS and CE-MS analysis, is shown in this contribution to be highly functional to obtain diagnostic fragmentation patterns of free glycan precursors in rapid screening of highly complex glycoconjugate mixtures obtained from clinical samples, namely from the urine of patients suffering from congenital disorders of glycosylation. Congenital disorders of glycosylation (CDG) are inherited metabolic diseases based on defects in the glycosylation pathways of glycoconjugates. The urine of CDG patients was reported to contain O-glycans and glycosylated amino acids at concentrations two to three orders of magnitude higher in comparison with the healthy control, characterized by a high degree of heterogeneity concerning the type, number, and values of molecular ions. Using the (0,2)A(n) ring cleavage ion approach by tandem MS, it was possible to sort out free glycans and get them assigned.     

Stereospecificity of 1H, 13C and 15N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on 1H chemical shifts

In the ¹³C NMR spectra of methylglyoxal bisdimethylhydrazone, the ¹³C‐5 signal is shifted to higher frequencies, while the ¹³C‐6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the ¹H‐6 chemical shift and ¹J(C‐6,H‐6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the –CH═N– bond does not change. This paradox can be rationalized by the C–H?N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum‐chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ(¹H‐6) and ¹J(C‐6,H‐6) parameters. The effect of the C–H?N hydrogen bond on the ¹H shielding and one‐bond ¹³C–¹H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The ¹H, ¹³C and ¹⁵N chemical shifts of the 2‐ and 8‐(CH₃)₂N groups attached to the –C(CH₃)═N– and –CH═N– moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8‐(CH₃)₂N group conjugate effectively with the π‐framework, and the 2‐(CH₃)₂N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N‐2– and N‐8– nitrogen lone pairs to the π‐framework varies, which affects the ¹H, ¹³C and ¹⁵N shieldings. Copyright © 2012 John Wiley & Sons, Ltd.     

<Superscript>13</Superscript>C-<Superscript>13</Superscript>C spin-spin coupling constants in structural studies: XL. Conformational analysis of <Emphasis Type="Italic">N</Emphasis>-vinylpyrroles

Conformational analysis of ten N-vinylpyrroles was performed on the basis of experimental ¹³C-¹H and ¹³C-¹³C coupling constants and those calculated by high-level quantum-chemical methods, and principal relations between J _CC and J _CH values and stereochemical structure of these compounds were revealed.     

<Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR spectra of some drimanic sesquiterpenoids

One- and two-dimensional homo- and heteronuclear correlation proton, carbon, proton—proton, and proton—carbon NMR spectra of fifteen drimanic sesquiterpenoids: 11,12-dibromodrima-5,8-dien-7-one, drim-8-en-7-one, 11-hydroxydrim-8-en-7-one, 11,12-dihydroxydrim-8-en-7-one, 11-hydroxy-11,12-epoxydrim-8-en-7-one, 11-hydroxy-11,12-epoxydrim-8-en-7-one, 8,9-epoxydriman-7-one, 8,9-epoxydriman-7-ol, 11,12-diacetoxydrim-8-en-7-ol, drimane-7,8,11-triol, 7,8-isopropylidenedioxydriman-11-al, 9, 11-dihydroxydrim-7-en-6-one, drimane-7,8,9-triol, drimane-7,8,11-triol, and drim-8-ene-7,11,12-triol were studied. The proton and carbon chemical shifts were assigned.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2589–2594, December, 2004.     

Spin-spin Coupling Constants <Superscript>13</Superscript>C-<Superscript>13</Superscript>C in Structural Studies: XXXVIII. Nonempirical Calculations: Oximes

Ab initio calculation of spin-spin coupling constants ¹³C-¹³C for a series of ketone oximes was carried out in a framework of the second order polarization propagator approximation (SOPPA). Taking into account the electron correlation effects and the use of special correlation-consistent basic sets including functions allowing for internal correlation and dense functions provided a good agreement between the calculated and experimental values. These results open new possibilities for the configurational assignment of imine nitrogen compounds.     

High-Resolution <Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR of Glycyrrhizic Acid and Its Esters

Resonances for protons and C atoms in the ¹H and ¹³C NMR spectra of glycyrrhizic acid and its esters were assigned using high-resolution ¹H (600 MHz) and ¹³C (150 MHz) NMR methods. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 347–350, July–August, 2005.     

Structure of binuclear platinum(III) acetamidate complexes in solutions as probed by <Superscript>195</Superscript>Pt, <Superscript>13</Superscript>C,and <Superscript>1</Superscript>H NMR spectroscopy

¹⁹⁵Pt, ¹H, and ¹³C NMR spectroscopy was used to study the structure of binuclear platinum(III) acetamidate complexes with 1,10-phenanthroline and 2,2′-bipyridine ligands [Pt₂(phen)₂(acam)₄](NO₃)₂ (1) and [Pt₂(bipy)₂(acam)₄](NO₃)₂ (2) in aqueous solutions. The ¹⁹⁵Pt NMR spectra of solutions of complexes 1 and 2 in D₂O exhibit two signals with satellites due to the ¹⁹⁵Pt–¹⁹⁵Pt spin-spin coupling (¹ J(Pt–Pt) ≈ 6345 Hz), whereas their ¹H and ¹³C NMR spectra contain four sets of signals for the protons and the carbon atoms of the heterocyclic and acetamidate ligands. The signals were assigned using the COSY, NOESY, and HSQC/ HMBC experiments and comparing the coordination shifts of the signals for the protons of heterocycles. These data allowed us to draw a conclusion that binuclear complexes 1 and 2 in solution have a head-to-head structure with nonequivalent platinum(III) atoms (coordination cores PtN₅ and PtN₃O₂), the axial-equatorial coordination of the bidentate heterocyclic molecules, and two bridging and two terminal acetamidate ligands.     

Synthesis,scope, <Superscript>1</Superscript>H and <Superscript>13</Superscript>C spectral assignments of isomeric dibenzofuran carboxaldehydes

Two isomeric dibenzofuran carboxaldehydes, namely 2-methoxydibenzo[b,d]furan-1-carbaldehyde (4) and 2-methoxydibenzo[b,d]furan-3-carbaldehyde (5), were synthesized. Formylation of 2-methoxydibenzo[b,d]furan (3) with α,α-dichloromethyl methyl ether and tin(IV) chloride gave a mixture of aldehydes 4 and 5 in 95 % yield and in a 35:65 ratio. Their ¹H and ¹³C NMR spectral signals were not sufficiently resolved in CDCl₃ solution to achieve their complete assignment, but this was possible in DMSO-d ₆ with the help of 2D-NMR techniques: NOESY for ¹H–¹H interactions and HSQC and HMQC experiments for ¹H–¹³C correlations. These aldehydes were used in the synthesis of novel β-phenylethylamines and NBOMe derivatives, which are undergoing biological evaluation.     

<Superscript>13</Superscript>C NMR and mass spectrometry of soil organic matter

Liquid state, high resolution ¹³C NMR spectroscopy and mass spectrometry were used to study the composition and structure of soil organic matter (SOM) using soil extracts from two long-term experiments at the Rothamsted Experimental Station. Both one- and two-dimensional NMR techniques were applied. ¹³C NMR sub-spectra of the CH _n (n=0...3) groups, obtained by the Distortionless Enhancement by Polarisation Transfer (DEPT) technique, were used for the elucidation of the qualitative and quantitative composition of humic and fulvic acids in the soils. The chemical structure of SOM was further analysed at the molecular level through Fast Atom Bombardment Mass Spectrometry (FABMS) and Gas Chromatography-Mass Spectrometry (GC/MS). Humic and fulvic extract results were not only compared to each other, but also to the solid state ¹³C NMR results for the complete soil sample.     

Mechanism of proton conductivity in polyvinyl alcohol-phenolsulfonic acid membranes from <Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR data

With line narrowing during magic angle spinning in solid-state NMR, molecular mobility and hydration in composite membranes based on polyvinyl alcohol (PVA) and phenol-2,4-disulfonic acid (PSA) were studied as functions of the ratio of the acidic and polymeric components, the degree of cross-linking in the polymeric matrix, and the moisture content. It is shown that at high relative humidity proton transport takes place by means of the network of hydrogen bonds, which are formed by the H⁺ counterions, sulfonate groups, and water molecules. At low moisture content, the hydroxyl groups in PVA play an active role in proton transport.     

<Superscript>234</Superscript>U/<Superscript>235</Superscript>U activity ratios as a probe for the <Superscript>238</Superscript>U/<Superscript>235</Superscript>U half-life ratio

High-resolution alpha-particle spectrometry was performed on three uranium materials enriched in ²³⁵U. Besides the ²³⁵U peaks, separate peaks belonging to impurity traces of ²³⁴U could be quantified. Relying on the isotopic composition of the uranium, as determined by mass spectrometry, the ratio of the half-lives of ²³⁸U and ²³⁵U was determined via the activity ratio of ²³⁴U and ²³⁵U in the materials. As an intermediate link, the ²³⁴U/²³⁸U half-life ratio was taken from published mass spectrometric analyses of ‘secular equilibrium’ uranium material. The resulting half-life ratio T _1/2(²³⁸U)/T _1/2(²³⁵U) = 6.351±0.031 is in agreement with the commonly adopted half-life values determined by Jaffey et al.     

<Superscript>13</Superscript>C NMR analysis of cellulose samples from different preparation methods

Alpha-cellulose is a part of the wooden material that preserves isotopic composition during tree-growth, and therefore provides important indirect data for paleoclimatological studies. For this reason, it is exceptionally important to extract the alpha-cellulose component from plants, e.g. from tree rings of wood. Since the cell wall of plant cells consists of multicomponent polysaccharides, the extraction of cellulose from wood is not an obvious task. In this paper, we describe, evaluate and compare nine methods, based on the literature and experimental observations, for obtaining cellulose from tree rings of wood. We show that the distortionless enhancement by polarization transfer (DEPT-135) variant of liquid-state ¹³C NMR spectroscopy is a powerful analytical method for monitoring the preparation process. Trifluoroacetic acid was applied as solvent for the NMR analysis. We proved that all the preparation methods give pure cellulose samples without hemicellulose and lignin content, and we propose methods resulting in non-fragmented cellulose. ¹³C and ¹⁸O isotope ratio measurements have shown that all the applied extraction methods result in similar isotope ratios, thus they are suitable for paleoclimatological studies.