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 FT-NMR spectra of alkyl substituted furans. A study of the influence of steric interactions

The ¹³C FT-NMR spectra of thirteen furans, monodi- or trisubstituted with methyl and/or t-butyl groups, were studied in detail. Substituent effects of methyl and t-butyl groups on the chemical shifts of ring carbon atoms are additive in nonsterically hindered furans. Steric shifts for the ring carbon atoms are found in furans with bulky neighbouring substituents, but the hybridisation of the carbon atoms in these hindered furans is not changed. The chemical shifts of the substituents are calculated according to the Grant-Cheney formula. No simple relationship between steric shift and steric hindrance can be ascertained.     

Carbon-13 NMR studies of 1-aryl-3-phenyl-2-thioxo-4-imidazolidinones. Conformational isomerism and substituent effects

Characteristic ^13C chemical shift ranges and substituent shifts of heterocyclic ring carbon atoms have been identified for a number of 1-aryl-3-phenyl-2-thioxo-4-imidazolidinones. ¹³CNMR spectra may be used to detect slow internal rotation about the aryl C? N-1 bond in compounds with diastereomeric rotational isomers; many corresponding carbon atoms in the rotamers have distinctly different chemical shifts. The δ-effects originating from aryl ortho substituents are both electronic and steric in origin.     

X-ray photoelectron spectra and structure of 2-(2-phenylhydrazono) acetoacetanilide

2-(2-Phenylhydrazono)acetoacetanilide, itsN-methyl derivatives, and model compounds were studied by X-ray photoelectron spectroscopy. The chemical shifts were obtained from the¹³C NMR spectra. A correlation between the calculated charges, the binding energies on N atoms, and the¹³C NMR chemical shifts was found. The analysis of the XPS data and the¹³C NMR chemical shifts led to the conclusion that crystalline 2-(2-phenylhydrazono)acetoacetanilide exists mainly in the oxo hydrazone form. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 488–491, March, 1999.     

Zur Kenntnis des Chinoiden Zustandes—XX: 13CMR-Untersuchungen an Cyclohexadienonen

The ¹³C-NMR-chemical shifts of 19 para- and 5 ortho-cyclohexadienones are determined by ¹³C-Fourier-transformation spectroscopy and assigned. The effect of substituents on the chemical shift of the ring carbon atoms is discussed. The mutual dependence of the shifts of the olefinic ring carbons and the allylic carbon atom in the para-quinolid ring system is shown by computing regression lines. A frequently observed correlation between ¹³C-NMR and ¹H-NMR is examined in the case of cyclohexadienones.     

Solid-state NMR studies of methyl celluloses. Part 1: regioselectively substituted celluloses as standards for establishing an NMR data basis

Three methyl celluloses with completely uniform substitution pattern, 2-O-methyl cellulose (1), 3-O-methyl cellulose (2) and 6-O-methyl cellulose (3), were prepared according to the cationic ring opening polymerization approaches starting from substituted 1,2,4-orthopivalate derivatives of d-glucose. These samples allowed for the first time to sort out the methyl substitution effects on solid-state NMR chemical shifts and relaxation. Dipolar dephasing experiments allowed the detection and assignment (¹H, ¹³C) of the methyl groups. In 1 and 2, these resonances overlapped with those of C-6, whereas in 3, the methyl signal experienced a low-field shift into the region of C-2,3,5. ¹³C T₁ experiments were used to verify different relaxation behavior of the carbon sites, particularly the short relaxation time of at the carbon substitution site next to the methyl groups. This effect was used to unambiguously identify the ¹³C chemical shifts of the carbons carrying the methoxyl substituent, although they overlap with all resonances in the C-2,3,5 region. The data obtained for the standard samples with uniform substitution will now be used as the basis for determining methylation patterns and substitution degree in commercial methyl celluloses.     

Observed and calculated 1H and 13C chemical shifts induced by the in situ oxidation of model sulfides to sulfoxides and sulfones

A series of model sulfides was oxidized in the NMR sample tube to sulfoxides and sulfones by the stepwise addition of meta‐chloroperbenzoic acid in deuterochloroform. Various methods of quantum chemical calculations have been tested to reproduce the observed ¹H and ¹³C chemical shifts of the starting sulfides and their oxidation products. It has been shown that the determination of the energy‐minimized conformation is a very important condition for obtaining realistic data in the subsequent calculation of the NMR chemical shifts. The correlation between calculated and observed chemical shifts is very good for carbon atoms (even for the ‘cheap’ DFT B3LYP/6‐31G* method) and somewhat less satisfactory for hydrogen atoms. The calculated chemical shifts induced by oxidation (the Δδ values) agree even better with the experimental values and can also be used to determine the oxidation state of the sulfur atom (? S? , ? SO? , ? SO₂? ). Copyright © 2010 John Wiley & Sons, Ltd.     

Cellulose organic solutions: A nuclear magnetic resonance investigation

Four solvents of cellulose have been studied by using ¹³C-NMR spectroscopy. All these solvents, N-methyl morpholine-N-oxide, methylamine, hydrazine, and paraformaldehyde (PF), contained dimethyl sulfoxide (DMSO) as a cosolvent. Oligomers of cellulose of DP = 10 soluble in hot DMSO have been used as model compounds. ¹³C chemical shifts and line shapes show that three of the mentioned solvents are “true solvents” of cellulose. On the other hand, dissolution of cellulose in DMSO-PF system occurs by the formation of a statistical derivative of cellulose. Enriched ¹³C bacterial cellulose on C-1 and C-6 positions have been used to identify the ¹³C positions mainly in DMSO-N-methyl morpholine-N-oxide system. This solvent has been found to be degradative for the macromolecule when the solution is kept at 100°C over a long period. Viscosity measurements show a reduction of the molecular weight in these conditions. Polarimetry indicates that no glucose is present in solution and hence there is a statistical break of the chain. Enriched cellulose solution in DMSO–N-methyl morpholine-N-oxide has been also used for relaxation time (T₁) determination both of the solvent and of the enriched carbons of the polymer. Nuclear Overhauser enhancement (NOE) was found to be 1.8 for C-1 and 2.1 for C-6 showing that relaxation phenomenon is not purely dipolar. T₁ values of 97 and 65 msec are found for C-1 and C-6 of cellulose, in good agreement with the values known for polysaccharides. Determination of T₁ for the different carbon atoms of the solvent DMSO-N-methyl morpholine-N-oxide with and without cellulose shows a large reduction of T₁ for N-methyl morpholine-N-oxide molecule. This denotes a slower molecular motion of this molecule and a preferential interaction with the cellulose macromolecule.     

Konformationsuntersuchungen mit Hilfe der 13C-NMR-Spektroskopie. III. 13C-NMR-spektroskopische Verschiebungen durch sterisch gehinderte Methylgruppen

The ¹³C NMR chemical shifts of some cyclohexane derivatives containing 1,3-diaxial methyl groups are assigned. The resonance signals of the methyl carbon atoms 1 and 3 in these compounds are shifted on average by 4.5 ppm to lower field (δ-effect). The ring carbon atoms 1 and 3 also show shifts to lower field, averaging 0.7 ppm (γ-effect). In open-chain hydrocarbons, analogous shift effects are observed when the investigated compounds have the geometry of the g^Pg^M conformer of n-pentane.     

Charge calculations in molecular mechanics. III: Amino acids and peptides

Atomic charges obtained with a previously published charge scheme are given for amino acids and peptides. In order to do this, a method of handling charged species with the basic scheme^2,3 has been developed. The charges obtained for alkylammonium ions and carboxylate ions with the scheme are presented and compared with CNDO and ab initio values. The calculated experimental dipole moments of the zwitterionic forms of glycine, alanine and β-alanine are then discussed. Finally, the atomic charges obtained for the naturally occurring amino acids are given, both in the form of the N-acetyl-N′-methyl amino acid amides, used as models for the amino acid residues in enzymes, and as the free zwitterionic amino acids. The charges obtained show a good correlation with n. m. r. chemical shifts of both carbon and hydrogen atoms.     

The structure of α-(1,2-dithiole-3-ylidene) ketones and α-(1,2-dithiole-3-ylidene) aldehydes studied by means of n.m.r. spectroscopy

1H and ¹³C n.m.r. studies of a series of twelve 1,2-dithiole-3-ylidene ketones and aldehydes have shown that the geometry of the carbon backbone is the same as found in 1,6,6aλ⁴-trithiapentalenes. No evidence has been found which favours a bicyclic structure for the system. A linear correlation of observed ¹³C chemical shifts with calculated charge densities is found to be valid. The observations are in agreement with a structure which is a hybrid between a true ketonic structure and a true mesoionic structure. By using the difference in the ¹³C chemical shifts of ortho and meta carbon atoms in substituent phenyl groups it is possible to qualify the degree of coplanarity of the phenyl groups with the backbone of the molecule.     

1H and 13C NMR study of 8-hydroxyquinoline and some of its 5-substituted analogues

¹H and ¹³C NMR spectra of 8-hydroxyquinoline (oxine) and its 5-Me, 5-F, 5-Cl, 5-Br and 5-NO₂ derivatives have been studied in DMSO-d₆ solution. The ¹H and ¹³C chemical shifts and proton–proton, proton–fluorine, carbon–proton and carbon–fluorine coupling constants have been determined. The ¹H and ¹³C chemical shifts have been correlated with the charge densities on the hydrogen and carbon atoms calculated by the CNDO/2 method. The correlation of the ¹H and ¹³C chemical shifts with the total charge densities on the carbon atoms is approximately linear (r_H² = 0.85, r_C² = 0.84). The proton in peri position to the nitro group in 5-NO₂-oxine is an exception.     

Carbon-13 nuclear magnetic resonance spectra of some mesoionic xanthine analogs

Natural abundance ¹³C NMR chemical shifts have been experimentally determined for a series of mesoionic thiazolo[3,2-a]pyrimidine-5,7-diones. The spectral data are compared with those of related mesoionic dihydrothiazolo[3,2-a]pyrimidine-5,7-diones and mesoionic 1,3,4-thiadiazolo[3,2-a]pyrimidine-5,7-diones. Resonable correlation between the observed ¹³C NMR chemical shifts and CNDO/2 total charge densities have been obtained for the different carbon atoms of 8-methylthiazolo[3,2-a]pyrimidine-5,7-dione.     

Conformation et Spectrometrie de RMN. III. Spectres 13C de Diméthylamino Cyclanols et Dérivés

The ¹³C NMR spectra of 2-dimethylaminocyclohexanols and the four trans-3-dimethylamino-2-decahydronaphtols are described. The gauche interactions allow precise estimation of chemical shifts for each carbon atom; thus, band attribution can be resolved without mistake. In the case of the cis and the trans diequatorial compounds, the conformational perturbations which we have suggested before are verified by our present measurements on the substituted carbon atoms. We also show effects on adjacent atoms, which were unobservable with the usual IR and ¹H NMR techniques.     

Carbon-13 NMR and CNDO/2 studies of phenoxachalcogenins

The ¹³C chemical shifts of the carbon atoms in dibenzodioxin, phenoxathiin, phenoxaselenin and phenoxatellurin were determined in CDCl₃ solutions and assigned. The total (σ and π) charge densitites on the carbon atoms calculated by the CNDO/2 method without consideration of d-orbitals correlated well with the experimentally determined shifts. Rather good agreement was also found between experimental shifts and shifts calculated from ¹³C data for phenyl methyl chalcogenides on the assumption that a phenoxachalcogenin molecule can be assembled from C₆H₅O and C₆H₅X groups. Only the shifts of the carbon atoms bonded to the heavier chalcogen atoms show an upfield trend in the sequence O, S, Se, Te. All other shifts exhibit a downfield trend. These trends are rationalized in terms of the electronegativities, abilities to participate in π-interactions, and anisotropy effects of the chalcogen atoms.     

RMN du 13C de Composés Méthylés Soufrés

A ¹³C NMR study of a series of methyl sulphur compounds is described. The results are discussed in terms of the deshielding effects on the methyl carbon exerted by –SH, –SMe, –SSMe, –SSEt, –SSMe, –SC(O)Me, –SC(S)Me, –SC(S)SMe. The ¹³C NMR chemical shifts of a series of S-methyl thioesters and dithioesters are compared with corresponding esters and connected with chemical properties.     

Characterization of 2‐aryl‐1,3,4‐oxadiazoles by 15N and 13C NMR spectroscopy

¹⁵N NMR chemical shifts of 2‐aryl‐1,3,4‐oxadiazoles were assigned on the basis of the ¹H–¹⁵N HMBC experiment. Chemical shifts of the nitrogen and carbon atoms in the oxadiazole ring correlate with the Hammett σ‐constants of substituents in the aryl ring (r² ≥ 0.966 for N atoms). ¹⁵N NMR data are a suitable and sensitive means for characterizing long‐range electronic substituent effects. Additionally, ¹³C NMR data for these compounds are presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Nuclear magnetic resonance studies of boron compounds : XVI. Carbon-13 studies of organoboranes: Phenylboranes and boron-substituted aromatic heterocycles

¹³C and ¹¹B NMR data of 29 phenylboranes and 9 boron-substituted aromatic heterocycles (thiophene, N-methylpyrrole and furan) are discussed. The observed ¹³C chemical shifts of the para-carbon atoms in phenylboranes and the corresponding carbon atoms in the aromatic heterocycles are consistent with mesomeric interactions of the boryl group with the aromatic system. The trend of δ(¹³C(para)) in phenylboranes corresponds to that observed for isoelectronic phenylcarbocations. Low temperature ¹³C NMR and/or ¹³C {¹¹B, ¹H} heterocuclear triple resonance experiments were employed to obtain the ¹³C chemical shifts of the boron-bonded carbon atoms.     

Conformational and long-range low field steric deuterium isotope effects on carbon chemical shifts

Comparison of the room and low temperature (203 K) ¹³C NMR spectra of 3-cis-bicyclo[4.4.0]decanone and its 2,2,4,4-tetradeuterio isotopomer provides information about the deuterium isotope effect on the conformational equilibrium in this compound. Four-bond low field deuterium isotope effects on some carbon chemical shifts are observed. These effects can be used for unambiguous assignment of the ¹³C lines to carbon atoms in alicyclic compounds.     

Carbon-13 nuclear magnetic resonance spectroscopy—VII:para-substituted fluorobenzenes

C-13 and F-19 NMR spectra of seventeen para-substituted fluorobenzenes were measured and the chemical shifts as well as coupling constants with respect to substituents were analysed. The chemical shifts of the fluorine, the C₁ and the C₂ atoms were found to depend on the total electron densities. In the case of the C₃ atom, the chemical shifts seem to depend on π-electron densities rather than the total electron densities. The present calculations also indicate that the chemical shift of the C₄ atom depends mainly on σ-electron densities due to the inductive effects of substituents. The strongest factor influencing the coupling constant, ⁿJ(C? F), is also considered to be the π-electron densities on the carbon atoms. In the case of the direct couplings, ¹J(C? F), the π-bond orders are important.