13C-13C Coupling Constants Research Articles

Dual-optimized 1Jcc-edited 1,n-adequate: A novel NMR structure elucidation technique

Establishing the carbon skeleton of a molecule is an integral aspect of structure elucidation. Recent advances in ADEQUATE experiments have highlighted their potential; 1,1-ADEQUATE, provides direct 13C-13C correlation via 1 J CC; and 1,n-ADEQUATE yields both long-range (typically 3 J CC) and direct 1 J CC correlation. Direct and long-range correlations are unfortunately indistinguishable in 1,n-ADEQUATE. A modified pulse sequence was developed in 2012 that inverted 1 J CC correlations in 1,n-ADEQUATE spectra. This change provided a significant improvement, allowing 1 J CC and n J CC correlations to be differentiated in the same spectrum. Careful matching of the amplitude transfer function with 1 J CC coupling constants was required with that version of the experiment to achieve near uniform inversion. Some 1 J CC correlations could still have positive intensity due to the oscillatory nature of the transfer function. Those shortcomings significantly limited application of the method. We now report a dual-optimized 1 J CC-edited 1,n-ADEQUATE experiment, that provides uniform inversion of 1 J CC correlations across the range of 29 – 82 Hz. Fine-tuning for a molecule's 1 J CC coupling constant values is not required. Beneficially, the dual-optimized version provides up to two-fold improvement in signal-to-noise for some long-range correlations. Using modern, cryogenically-cooled probes, the experiment can be successfully applied to samples as small as 1 mg. In addition to strychnine, examples of applications of the method to staurosporine and coniothyrione are also presented.

Stereospecificity of 13C shielding constants in acetone azine as a tool for configurational assignment of ketone azines

According to the 13C NMR data, the chemical shift of the methyl carbon atom in acetone azine in the trans position with respect to the lone electron pair on the neighboring nitrogen atom is lower by 7 ppm than that of the methyl carbon atom in the cis position. The corresponding direct 13C-13C coupling constant for the trans-methyl groups is lower by 10 Hz as compared to the cis-methyl groups. The experimental spectral data are reproduced well by nonempirical quantum-chemical calculations. The observed stereospecificity of 13C chemical shifts and 13C-13C coupling constants may be used as an effective tool in configurational analysis of various ketone azines.

Theoretical Modeling of Molecular Spectra Parameters of Disubstituted Diacetylenes

Symmetrically disubstituted diacetylenes, X-C≡C-C≡C-X, were studied computationally by using the DFT B3LYP/aug-cc-pVDZ method. For more than 35 substituents the bond lengths, charge density and Laplacian in bond critical points, C≡C stretching vibrational frequencies, (13)C NMR chemical shifts and spin-spin CC coupling constants through diacetylene moiety were calculated and examined by using the substituent sEDA and pEDA descriptors. It is demonstrated that in disubstituted diacetylenes the triple bond length increases with the electron donating and decreases with the electron withdrawing properties of the substituents. The σ-electron repulsion is likely to be responsible for this phenomenon. The electron density of the C-X bond critical point decreases linearly with an increase of the σ-electron donating properties, whereas the Laplacian of electron density in the C≡C bond critical point increases as the sEDA descriptor i.e., σ-electron donating properties of the substituent, are increased. Thus, ρ(C≡C) is locally reduced with an increase of sEDA. The ν(as)(C≡C) and ν(s)(C≡C) mode frequencies decrease with the electron donating and increase with the electron withdrawing properties of the substituents. The calculated chemical shift of the C1-atom, to which the substituent is attached, does not correlate with the substituent descriptors, whereas the δ(C2) deshielding increases when the σ-electron donating properties and C≡C distance are increased. The calculated (1)J(CC) coupling constants decrease with an increase of the triple bond length, the sEDA parameter, and ρ(C-X), whereas they decrease with Laplacian in the C≡C BCP.

13C-13C 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.

13C-13C spin-spin coupling constants in structural studies: XLIII. Stereochemical study on functionalized 3-iminopyrrolizines

Three 1-ethylsulfanyl-3-imino-3H-pyrrolizine-2-carboxamides were synthesized by intramolecular cyclization of substituted (2Z)-2-cyano-3-ethylsulfanyl-3-(1H-pyrrol-2-yl)prop-2-enamides. The products were assigned syn configuration at the C=N bond and preferential s-cis orientation of the carbamoyl group on the basis of the experimental 13C-13C coupling constants and high-level nonempirical quantum-chemical calculations.

13C-13C 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.

13C-13C spin-spin coupling constants in structural studies: XLI. Stereochemical study on N-(polychloroethylidene)arenesulfonamides and N’-arylsulfonylformimidamides

The second-order polarization propagator approach (SOPPA) was used to calculate 13C-1H, 13C-13C, and 15N-1H coupling constants for a series of N-(polychloroethylidene)arenesulfonamides and N’-arylsulfonylformimidamides, and their configuration with respect to the C=N bond was determined by comparing the calculated data with the experimental values. All the examined compounds were found to exist in solution exclusively as the corresponding E isomers. The most favorable conformations and relative energies of the E and Z isomers in the gas phase were determined in terms of the second-order perturbation theory (MP2/6-311G**). N’-Arylsulfonylformimidamides are characterized by restricted internal rotation of the dialkylamino group about the C-N bond having an increased order.

13C-13C spin-spin coupling constants in structural studies: XL. Conformational analysis of N-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.

13C-13C 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 JCC strongly predominates over conformational and electronic effects, which makes it possible to unambiguously assign JCC values to different configurations.

1H-detected double- J-modulated INEPT-INADEQUATE for simultaneous determination of one-bond and long-range carbon–carbon connectivities and the measurement of all carbon–carbon coupling constants

We describe a new NMR experiment, 1H-detected double- J-modulated (DJM)-INEPT-INADEQUATE, for tracing out the carbon skeleton of molecules. This experiment allows simultaneous correlation of directly bonded carbon atoms and those separated by multiple bonds, while at the same time also providing the values of all J CC coupling constants. This is achieved by replacing both fixed carbon–carbon coupling evolution intervals of the INEPT-INADEQUATE experiment by variable time intervals, which are incremented in concert with the DQ evolution period ( t 1). We show that the analysis of the fine structure of cross-peaks in DJM-INEPT-INADEQUATE spectra leads to accurate values of coupling constants and give guidelines for the proper usage of the method. The proposed experiment is two times less sensitive that the original INEPT-INADEQUATE experiment. We show that, using a 600-MHz cryoprobe and 20 mg of a monosaccharide, spectra that are suitable for the analysis of coupling constants as small as 2 Hz can be obtained within 24 h. Instead of performing multiple experiments, a single DJM-INEPT-INADEQUATE experiment can thus provide a wealth of information for the structural analysis of small molecules.

13C-13C Spin-Spin Coupling Constants in Structural Studies: XXXVII. Rotational Conformations of Hydroxy Groups in Pyranose, Furanose, and Septanose Rings

Rotational surfaces for 13C-13C coupling constants with respect to two dihedral angles at C1 and C2 in model structures of aldopyranoses, aldofuranoses, and aldoseptanoses of the D-series were calculated in terms of the self-consistent finite perturbation theory. Internal rotation of the hydroxy groups exerts an appreciable effect (within 2.5 Hz) on the 13C-13C coupling constants of all cyclic forms of monosaccharides, which provides the possibility for performing conformational analysis of carbohydrates and their metabolites containing pyranose, furanose, and septanose fragments.

13C-13C Spin-Spin Coupling Constants in Structural Studies: XXXVI. Stereochemical Study of the Septanose Ring

13C-13C Spin-spin coupling constants of aldoseptanoses of the Dseries were calculated in terms of the self-consistent finite perturbation theory. General relations holding in the stereochemical behavior of the 13C-13C coupling constants in the septanose ring were determined, which made it possible to perform conformational analysis and assign configuration of the anomeric centers in carbohydrates and products of their metabolism.

13C-13C Coupling Constants in Structural Studies: XXXII. Additivity of Spin-Spin Couplings in Sterically Strained Heterocycles

Calculations in terms of the self-consistent finite perturbation theory (SCPT) and analysis of contributions of localized molecular orbitals in terms of the polarization propagator theory (CLOPPA) indicate additivity of 13C-13C coupling constants in saturated sterically strained heterocycles. Their fused derivatives, especially those containing 3rd Period elements, show considerable deviations of the calculated coupling constants from the additive values.

13C-13C coupling constants in structural studies: XXXIII. Stereochemical study of the pyranose ring

13C-13C coupling constants for all aldopyranoses of the D-series were calculated in terms of the self-consistent finite perturbation theory. General relations holding in the stereochemical behavior of the 13C-13C coupling constants for the pyranose ring were found. The results obtained make it possible to perform conformational analysis and assign configuration of the anomeric center in molecules of carbohydrates and products of their metabolism, containing a pyranose fragment.

Refinement of d(GCGAAGC) hairpin structure using one- and two-bond residual dipolar couplings.

The structure of the 13C,15N-labeled d(GCGAAGC) hairpin, as determined by NMR spectroscopy and refined using molecular dynamics with NOE-derived distances, torsion angles, and residual dipolar couplings (RDCs), is presented. Although the studied molecule is of small size, it is demonstrated that the incorporation of diminutive RDCs can significantly improve local structure determination of regions undefined by the conventional restraints. Very good correlation between the experimental and back-calculated small one- and two-bond 1H-13C, 1H-15N, 13C-13C and 13C-15N coupling constants has been attained. The final structures clearly show typical features of the miniloop architecture. The structure is discussed in context of the extraordinary stability of the d(GCGAAGC) hairpin, which originates from a complex interplay between the aromatic base stacking and hydrogen bonding interactions.

New strategy for the conformational analysis of carbohydrates based on NOE and 13C NMR coupling constants. Application to the flexible polysaccharide of Streptococcus mitis J22.

For complex oligosaccharides, which are relatively rigid with modest excursions from a single minimum energy conformation, it is straightforward to build conformational models from NOE data. Other oligosaccharides are more flexible with transitions between distinct minima separated by substantial energy barriers. We show that modeling based on scalar coupling data is superior to NOE-based modeling for the latter case. Long range 13C-13C and 13C-1H coupling constants measured for the heptasaccharide repeating subunit of the cell wall polysaccharide from Streptococcus mitis J22 are correlated with individual glycosidic dihedral angles, effectively uncoupling the degrees of freedom of the oligosaccharide and allowing a search for combinations of dihedral angles which are energetically reasonable, i.e., with no bad van der Waals contacts, and which can be combined to satisfy all the measured J values. Allowed values of the individual angles can then be combined to search for overall oligosaccharide conformations which contribute to the ensemble. We show that while the polysaccharide from S. mitis J22 is flexible, requiring multiple conformations, most of the flexibility is localized to a few bonds and only a rather small number of conformations is required to reproduce the experimental NOE and scalar coupling data.

New conformational constraints in isotopically (13C) enriched oligosaccharides.

Multidimensional heteronuclear NMR studies have been applied to the resonance assignment and conformational analysis of 13C-enriched Neu5Acalpha2-3Galbeta1-4Glc. It is demonstrated that three-dimensional ROESY-HSQC experiments provide through-space distance restraints which cannot be observed with conventional homonuclear 1H techniques due to resonance overlap. In particular, connectivities demonstrating the existence of the "anti" conformation about the Galbeta1-4Glc glycosidic linkage are unambiguously observed. It is shown that 13C isotopic enrichment of the trisaccharide at a level >95% enables straightforward measurement of trans-glycosidic 1H-13C and 13C-13C coupling constants and a Karplus-type relation is derived for the latter. In total 15 conformational restraints were obtained for the trisaccharide in aqueous solution, all of which were in excellent agreement with theoretical parameters computed from a 5 ns molecular dynamics simulation of the glycan.

Retrobiosynthetic NMR Studies with 13C-Labeled Glucose: FORMATION OF GALLIC ACID IN PLANTS AND FUNGI

The biosynthesis of gallic acid was studied in cultures of the fungus Phycomyces blakesleeanus and in leaves of the tree Rhus typhina. Fungal cultures were grown with [1-13C]glucose or with a mixture of unlabeled glucose and [U-13C6]glucose. Young leaves of R. typhina were kept in an incubation chamber and were supplied with a solution containing a mixture of unlabeled glucose and [U-13C6]glucose via the leaf stem. Isotope distributions in isolated gallic acid and aromatic amino acids were analyzed by one-dimensional 1H and 13C NMR spectroscopy. A quantitative analysis of the complex isotopomer composition of metabolites was obtained by deconvolution of the 13C13C coupling multiplets using numerical simulation methods. This approach required the accurate analysis of heavy isotope chemical shift effects in a variety of different isotopomers and the analysis of long range 13C13C coupling constants. The resulting isotopomer patterns were interpreted using a retrobiosynthetic approach based on a comparison between the isotopomer patterns of gallic acid and tyrosine. The data show that both in the fungus and in the plant all carbon atoms of gallic acid are biosynthetically equivalent to carbon atoms of shikimate. Notably, the carboxylic group of gallic acid is derived from the carboxylic group of an early intermediate of the shikimate pathway and not from the side chain of phenylalanine or tyrosine. It follows that the committed precursor of gallic acid is an intermediate of the shikimate pathway prior to prephenate or arogenate, most probably 5-dehydroshikimate. A formation of gallic acid via phenylalanine, the lignin precursor, caffeic acid, or 3,4, 5-trihydroxycinnamic acid can be ruled out as major pathways in the fungus and in young leaves of R. typhina. The incorporation of uniformly 13C-labeled glucose followed by quantitative NMR analysis of isotopomer patterns is suggested as a general method for biosynthetic studies. As shown by the plant experiment, this approach is also applicable to systems with low incorporation rates.

Boronic acids as fructose sensors. Structure determination of the complexes involved using 1 J CC coupling constants

Complex formation between p-tolylboronic acid and D-fructose was investigated by 13C NMR spectroscopy both in neutral nonaqueous and alkaline aqueous solutions to evaluate the possibilities of boronic acid based fructose sensors. Under both conditions a mixture of complexes was observed. The structures of the complexes in solution were assigned on the basis of 1JCC coupling constants which provided the information of the binding sites for the formed cyclic boronic esters. In alkaline aqueous solution seven different complexes were observed. At a 1:1 boronic acid: fructose ratio the major complex (82%) was β-D-fructofuranose 2,3,6-tri-O-(p-tolylorthoboronate)1. The minor complex was β-D-fructofuranose 2,3-(p-tolylhydroxyboronate) present in two diastereomeric forms 2a and 2b. At higher boronic acid:fructose ratios four additional complexes were observed namely the four possible diastereomeric forms of β-D-fructopyranose 2,3:4,5-bis(p-tolylhydroxyboronate) 3a–d. Under nonaqueous conditions signals arising from 13 different compounds were observed. Five of these complexes were structurally assigned. At a 1:1 boronic acid : fructose ratio the major complex was β-D-fructofuranose 2,3(p-tolylboronate) 5 present in 46%. At a ratio of 4:1 the major species was β-D-fructopyranose 2,3:4,5-bis(p-tolylboronate) 7 present in 67%. Two additional monoesterified pyranoses and one furanose were observed. They were assigned to the β-D-fructopyranose 1,2-(p-tolylboronate) 8, the α-D-fructopyranose 1,3-(ptolylboronate) 9 and the β-D-fructofuranose 1,3-(p-tolylboronate) 6. It is concluded that results of earlier studies on binding constants bear no relation to the actual formed complexes.

Neutral hyperconjugation challenged: No support from one‐bond CC coupling constants

AbstractA significant decrease in 1J(Cipso, CH2) coupling constants from a value typical of C(sp2)C(sp3) bonds (43.5 Hz or smaller) to the unexpectedly low value of 36 Hz typical of C(sp3)C(sp3) bonds was reported by Lambert and Singer on passing from XC6H4CH2Sn(CH3)3 to XC6H4CH2C(CH3)3 (X = H, CH3, OCH3, NO2). This trend was invoked by these authors as the first evidence for neutral hyperconjugation in terms of coupling constants. However, the present measurements reveal that also in C6H5CH2C(CH3)3 and p‐O2NC6H4CH2C(CH3)3 the 1J(Cipso, CH2) coupling constant bears a value typical of this type of bonding (43.8 and 42.6 Hz, respectively). Thus, at variance with Lambert and Singer's observation, a slight increase in 1J(CC) coupling constants occurs on passing from Sn to C derivatives. This contradicts not only their experimental data but also conclusions concerning hyperconjugation in the neutral ground state.