13C NMR study of ortho-, meta- and para- substituted phenyldiphenylamines: Substituent effect correlations

The ¹³C chemical shifts of 11 substituted triphenylamines have been determined and the assignment of these resonances made using intensities, ¹H and ¹⁹F couplings and predictions from bond additivity relationships. ¹³C chemical shifts at carbons bearing the substituent and at carbons ortho to the substituent correlated reasonably well with the Q parameter. A multiple regression analysis of chemical shifts with the field and resonance parameters of Swain and Lupton and the Q parameter produced significantly better correlations than those obtained when Q was omitted for these positions. ¹³C chemical shift correlations for carbons meta and para to the substituent were not significantly better than when Q was omitted. Significant correlations were obtained between field and resonance parameters and ¹³C chemical shifts of C-o and C-p, and C-i, C-o, C-m and C-p of the non-substituent bearing phenyl rings in ortho- and para-substituted phenyldiphenylamines, respectively.     

Stereospecificity of <Superscript>13</Superscript>C 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.     

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.     

The structure of thiophene sesquioxide: Syn,endo-3a,4,7,7a-tetrahydro-4,7-epithiobenzo[b] thiophene 1,1,8-trioxide

Thiophene sesquioxide is shown to be syn, endo-3a,4,7,7a-tetrahydro-4,7-epithiobenzo[b]-thiophene 1,1,8-trioxide (I) by ¹ H and ¹³ C nmr evidence. Assignment of the ¹³ C spectrum was facilitated by a cross-ring long-range ¹³ C-¹ H coupling. The mass spectrum of I is dominated by an unusual break-down to give benzene radical cation.     

Naphthopyrones,part II Carbon-13 magnetic resonance spectral studies of some methylnaphthopyran-2-one and methylnaphthopyran-4-one derivatives

Assignments of¹³C chemical shifts and¹³C-¹H coupling constants are presented for four methylnaphthopyrone derivatives. Differentiation between the benzocoumarin and benzochromone series is best performed by consideration of the carbonyl chemical shift and coupling pattern.

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Naphthopyrone, 2. Mitt.:¹³ C-NMR einiger Methylnaphthopyran-2-one und Methylnaphthopyran-4-one

Zusammenfassung Es werden die Zuordnungen der chemischen Verschiebungen der¹³C-Resonanzen und der¹³C-¹H-Kopplungen für vier Methylnaphthopyrone diskutiert. Die Unterscheidung zwischen der Benzocumarin- und der Benzochromon-Reihe erfolgt am besten mittels chemischer Verschiebungen und dem Kopplungsmuster.

     

Conformational and Configurational Determination in 1,1,4,4-Tetrafluoro-2-Propylidene-3-Methyl-6-Tert-Butyl-1,4-Disila-5-Cyclohexene

A complete assignment of the ¹H, ¹³C and ¹⁹F NMR spectra of 1,1,4,4-tetrafluoro-2-propylidene-3-methyl-6-tert-butyl-1,4-disila-5-cyclohexene 3, the product of the cycloaddition reaction between tetrafluorodisilacyclobutene 1 and cis, trans-2,4-hexadiene mediated by (n⁵-C₅H₅)Mn(CO)₃, has been made by means of various 2D NMR chemical shift correlation methods and selective heteronuclear decoupling techniques. An analysis of the three bond ¹³C-¹H coupling constants and two bond ¹³C-¹⁹F coupling constants and the molecular modeling results enabled the determination of the conformation and configuration of compound 3. The conformation of 3 provides insight into mechanistic details of the Mn-mediated cycloaddition.     

13C-13C spin-spin coupling constants in the spectra of monosubstituted pyridines

The ¹³C- {¹H} NMR spectra of pyridine and a number of monosubstituted pyridines for compounds with the natural percentage of the ¹³C isotope were analyzed. The direct, geminal, and vicinal ¹³C-¹³C spin-spin coupling constants (SSCC) were determined. Linear relationships that link the ¹³C-¹³C SSCC in the spectra of monosubstituted pyridines and benzenes were obtained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1383–1385, October, 1985.     

Reducing the computational cost of NMR crystallography of organic powders at natural isotopic abundance with the help of 13C-13C dipolar couplings

Structure determination of functional organic compounds remains a formidable challenge when the sample exists as a powder. Nuclear magnetic resonance crystallography approaches based on the comparison of experimental and Density Functional Theory (DFT)-computed ¹H chemical shifts have already demonstrated great potential for structure determination of organic powders, but limitations still persist. In this study, we discuss the possibility of using ¹³C-¹³C dipolar couplings quantified on powdered theophylline at natural isotopic abundance with the help of dynamic nuclear polarization, to realize a DFT-free, rapid screening of a pool of structures predicted by ab initio random structure search. We show that although ¹³C-¹³C dipolar couplings can identify structures possessing long range structural motifs and unit cell parameters close to those of the true structure, it must be complemented with other data to recover information about the presence and the chemical nature of the supramolecular interactions.     

Triterpenoids fromAbies species 18. NMR spectra of abieslactone and its dehydro derivative

The contradictions in the interpretations of the¹³C NMR spectrum of a natural lanostanoid, abieslactone, and of the 1H NMR spectrum a its 9,11-dehydro derivative reported in the literature were resolved by two-dimensional¹H-¹H and¹³C-¹H (COSY) NMR spectroscopy.Translated fromIzvestiya Akademi Nauk. Seriya Khimicheskaya, No. 11, pp.2780–2784, November,1996     

Synthesis and nmr analysis of cyclo-[85% 13C-asp]-pro: 13C-13C vicinal coupling constants and conformation

The dipeptide cyclo(Asp-Pro) where the aspartic acid residue was 85% ¹³C enriched, was synthesized with the aim of analyzing its conformation in solution by using ¹C-¹H, ¹³H-¹H and ¹³C-¹³C coupling constant parameters. The values of these couplings agree well with each other and show that the side chain of the aspartic acid residue adopts privileged conformations the proportions of which vary somewhat with pH, and more weakly, with a change in solvent. The ¹³C-¹³C interresidue coupling constants ³J_Cl'–_C2^β and ³J_Cl'–_C2^γ obtained after long accumulation of the signals of unenriched carbons, have different values; they show puckering in the pyrrolidine ring similar to that found in cyclo(Leu-Pro) in the solid state. It was concluded that ¹³C-¹³C coupling constants represent an excellent means of determining the side chain conformation (whenever the incorporation of an enriched amino acid into the peptide is possible) that will find applications particularly in the case of peptides with complicated proton spectra.     

Assignment of the 13C-NMR spectrum of phenanthro[4,3-A]dibenzothiophene through the utilization of two-dimensional relayed coherence transfer and double quantum coherence spectroscopy

Total assignment of the ¹³C-nmr spectrum of the helical molecule phenanthro[4,3-a]dibenzothiophene at 125.762 MHz is reported. Assignments were made by the combined application of ¹³C-¹³C autocorrelated double quantum coherence and heteronuclear relayed coherence transfer (RELAY) experiments.     

Orientational ordering in the nematic phase of 4-alkyl-4'-cyanobicyclohexanes

The orientational ordering of three liquid crystals (trans,trans-4-n-alkyl-4'-cyanobicyclohexanes) has been investigated using ¹³C NMR. 2D proton-encoded local field spectroscopy was used to determine the ¹³C-¹H dipolar coupling constants, from which the order parameters were calculated. Semi-empirical linear relations between the obtained order parameters and anisotropic ¹³C chemical shifts were established. It was found that the order parameters of the major axis of the cyclohexane rings are smaller than those of liquid crystals with phenyl rings, and the order parameters of the C-H bonds in the aliphatic chains also behave differently.     

13C-13C Spin-Spin Coupling Constants in Structural Studies: XXXI. Bicyclo[2.1.0]pentane Heteroanalogs

¹³C-¹³C spin-spin coupling constants and orbital hybridization of the bridging bond in bicyclo[2.1.0]pentane heteroanalogs and their unsaturated derivatives were calculated in terms of the self-consistent finite perturbation theory (SCPT INDO). The results showed that the bridging CÄC bond in all the examined compounds has an anomaloulsy low s character.     

The structural dependence of vicinal 13C?13C coupling constants in s-cis-butane and cis-butane

The dependence of three-bond ¹³C-¹³C couplings of cis-butane and cis-butene on the valence angle, the torsional angle of the methyl groups and methyl and methylene substituents is discussed on the basis of INDO-SCPT calculations. The results support the interpretation of the experimental couplings between the bridgehead carbons of bicyclic hydrocarbons based on a multiple-path mechanism.     

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. Original Russian Text ? K.A. Chernyshev, L.B. Krivdin, G.N. Rozentsveig, I.V. Ushakova, I.B. Rozentsveig, G.G. Levkovskaya, 2008, published in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 1, pp. 82–91. Dedicated to Full Member of the Russian Academy of Sciences G.A. Tolstikov on his 75th anniversary     

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 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.     

<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.     

Aromatic imine stereochemistry as studied by 13C and 1H NMR of 15N-enriched materials

A series of thirteen ¹⁵N-enriched N-aryl imines derived from benzaldehydes and acetophenones have been prepared and examined by ¹³C and ¹H NMR. Preferred conformations of the C-arvl and N-aryl rings have been deduced from ¹³C chemical shifts and ¹³C-¹⁵N couplings. Evidence for steric inhibition of resonance in O-alkylated materials is presented. Relative signs of ¹J(CN) and ²J(CCN) have been determined in some compounds. An E: Z interconversion barrier of 21.6 kcal mol^?1 has been obtained from ¹H NMR coalescence data.     

13C NMR of methyl N-methylthiobenzimidates. A novel approach to the interpretation of the relative 13C-E/Z shifts in amides and related compounds

The ¹³C NMR shifts of the E/Z mixtures of twelve, mainly ortho substituted, methyl N-methylthio-benzimidates have been completely assigned. The angles of twist about the aryl-imino-carbon bond were determined by means of the C-1 aryl shifts. Depending on the bulk of the ortho substituents the values vary from 18° in methyl 2-fluoro-N-methylthiobenzibenzimidate (Z) to 73° in methyl 4-bromo-N,2,6-trimethylthio-benzimidate (E). An explanation for the ¹³C-E/Z-N-alkyl and -S-alkyl resonances is facilitated by a new model which is initially based on the influence of electric fields. As this model is derived from amides, thioamides etc. it is possible to interpret the ¹³C-E/Z shifts in all related classes of compounds. The role of steric compression does not seem to be as important as was previously assumed.