Vicinal proton-carbon

Vicinal proton-carbon coupling constants in a series of conformationally rigid monosaccharide derivatives showed a satisfactory dependence of upon the torsion angles (2). The... 

Vicinal Proton-Carbon Coupling Constants for Glycosides... 

I. Tvaroska, M. Hricovmi, and E. Petrakova, An attempt to derive a new Karplus-type equation of vicinal proton-carbon coupling constants for C-O-C-H segments of bonded atoms, Carbohydr. Res., 189 (1989) 359-362. 

Vicinal proton-carbon coupling constants (3.7h65,C4 and J is.v.ci) also provide valuable information about the hydroxymethyl conformation. Theoretical calculations were applied to establish Karplus-type equations that correlate these couplings with lu torsion angle and then these were used to estimate C5—C6 rotamer... 

A conformational analysis of naproxen anion and monensin sodium has been performed by the use of the /hc couplings. Especially extensive use of these couplings has been made by the authors in the case of the latter compound. They concluded that the conformation of monoensin in solution is very close to that in crystals. Further examples of the application of vicinal proton-carbon couplings in the conformational analysis of compounds include the NMR and quantum-chemical study of the stereochemistry of spiroepoxides obtained by oxidation of (Z)-3-arylidene-l-thioflavan-4-ones performed by Toth et and the elucidation of the structure of isomalyngamides A and B isolated from a collection of the cyanobacterium Lyngbya majuscula from Hawaiian waters by Kan et... 

A recently developed approach based inter alia on molecular modelling and on analysis of NMR data including vicinal proton-carbon couplings has been tested by Tanaka et alP They used one of the synthetic chiral dihydrofuro-angelicines, 4-methyl-8(2- -phenylethenyl)-8,9-dihydro-2H-furo[2.3-/i]-l-ben-zopyran-2-one, as a model compound and extended the method to establish the absolute stereochemistry of six new dihydrofuroangelicins bearing C-8 substituted double bonds. 

The redundant set of experimental vicinal proton-carbon, /hc, and proton-proton, /hh, couplings involving the OH protons of a and anomers of a o-glucopyranoside derivative has been applied by Kover et al. in order to elucidate the OH-rotamer equilibrium in polyalcohols. The populations of the lowest energy conformers obtained on the basis of the experimental data were similar to those derived by the use of the quantum-mechanical approach. Good agreement has been also observed between the experimental and calculated J values. 

A tautomeric equilibrium in substituted purines at low temperature has been studied by Seckafova et al. Characteristic C, N chemical shifts and vicinal proton-carbon coupling values have allowed the authors to assign particular sets of the signals to the corresponding tautomers and to establish... 

Karplus-type dependence of vicinal proton-carbon couplings combined with ab initio molecular orbital (MO) calculations has been applied by Sasanuma and Touge in order to derive conformational characteristics of poly(L-lactide) and poly(DL-lactide) /hc couplings have been measured in a variety of solvents for C-enriched (S)-methyl 2-aceto g -propanoate used as the model compound. Another paper published by Sasanuma et has been devoted to conformational analysis of... 

Vicinal proton-carbon and proton-proton couplings have provided useful information on the structure of the antibiotic strepturidin isolated by Pesic et from the microorganism Streptomyces albus DSM 40763. 

COSY and HETCOR experiments are extremely useful in the structure elucidation of complex organic molecules. The geminal and vicinal protons and their one-bond C-H connectivities are first identified from the HETCOR spectrum, and then the geminal couplings are eliminated from the COSYspectrum, leaving vicinal connectivities. By careful interpretation of the COSY and the one-bond HETCOR spectra, it is then possible to obtain information about the carbon-carbon connectivities of the protonated carbons ( pseudo-INADEQUATE information). In this way the carbon-carbon connectivity information of protonated carbons is obtainable through a combination of COSY and HETCOR experiments. 

Figure 19 Schematic effect of the STAR operator on 2JCH and 3,/CH couplings. The vicinal component of magnetization in the long-range response that is two-bond coupled to a protonated carbon experiences modulation, which serves as a pseudo-evolution for this coupling. In contrast, the vicinal component of magnetization in the long-range response that is three-bond coupled to a protonated carbon does not exhibit a F, skew. Homonuclear modulation during the evolution period f, is still present, as the full experiment is not a constant-time experiment.

The coupling constant between two vicinal protons which are attached to adjacent carbon atoms (or other atoms) depends upon the torsion angle. 

The magnitude of carbon-fluorine coupling attenuates with coupling distance in alkyl and cycloalkyl fluorides, as shown in Tables 4.20(b) and 4.21. Analogously to vicinal proton-proton, carbon-proton and carbon-carbon coupling (Sections 3.2.2.4, 3.2.4.2), 37cf depends on the dihedral angle enclosed by the C—C—C —F connection, and Karplus cosine relations such as eq. (4.7) have been derived empirically [266] and theoretically [267],... 

The magnetic coupling that causes spin-spin splitting takes place primarily through the bonds of the molecule. Most examples of spin-spin splitting involve coupling between protons that are separated by three bonds, so they are bonded to adjacent carbon atoms (vicinal protons). 

A special situation applies to compound 30. The exo-endo structure was identified from the two allyl vicinal proton coupling constants33. Only one stereoisomer could be detected. With increasing temperature above 210 K the latter two coupling constants average, the result of presumably sequential rotations around both allyl carbon-carbon... 

---