Carbon-proton coupling constants conformational dependence

V. Conformational Dependence of Carbon-Proton Coupling Constants. 29... 

M. Hricovini and I. Tvaroska, Conformational dependence of the one-bond carbon-proton coupling constants in oligosaccharides, Magn. Reson. Chem., 28 (1990) 862-866. 

I. Tvaroska, Dependence on saccharide conformation of the one-bond and three-bond carbon-proton coupling constants, Carbohydr. Res., 206 (1990) 55-64. 

I. Tvaroska and J. Gajdos, Angular dependence of vicinal carbon-proton coupling constants for conformational studies of the hydroxymethyl group in carbohydrates, Carbohydr. Res., 271 (1995) 151-162. 

The conformations of flexible chain molecules incorporated in a nematic environment are investigated. Proton-proton and carbon-carbon dipolar coupling constant measurements are attempted for 1,2-dimethoxyethane and 1,2-diphenoxyethane, in addition to 2H NMR observations of quadrupolar splittings. These conformation-dependent properties are analyzed according to the RIS scheme. Studies are further extended to a mixture of 1,2-diphenoxyethane with a nematic liquid crystal, 4,4 -azoxyanisole. 

As an independent parameter J(C,H) can be used to confirm the conformational conclusions which will be drawn from the one-bond coupling constants. The sarcosyl peptides are unique in this respect since a vicinal coupling between the N-methyl carbon and the two diastereotopic Ca,H protons can be obtained from the non-decoupled spectrum of the N-methyl group. This C,H coupling constant will depend on the tor-... 

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

The teirperature dependencies of the chemical shift values for both Cl and C4 were determined in four different solvents (water, dimethyl sulfoxide, methanol and dioxane) and are shown in Figures 8 and 9. The resonance for Cl at 298 C varied from 101.6 ppm in D2O to 104.0 ppm in methanol. The resonance for C4 at the same temperature varied from 75.3 ppm in dimethyl sulfoxide to 78.3 ppm in methanol. The most pronounced tenperature dependence is observed in water and dioxane, where Cl and C4 signals varied from 101.4 ppm to 101.9 ppm (Cl, water, 278-358 K) and from 75.7 ppm to 76.5 ppm (C4, dioxane, 288-360 K), respectively. Thus, both tenperature and solvent dependence of C shifts indicate different conformational behavior of the molecule at various physico-chemico conditions. This feature is manifested even more clearly by the dependencies of the three-bond proton-carbon J and J coupling constants (< ) - Hl -Cl -04-C4 and f = H4-C4-04-C1 ) which are plotted against tenperature in Figures 10 and 11. 

In solution, the method of choice to study the three-dimensional structure of saccharides is NMR, through the parameters represented by chemical shifts, coupling constants, nuclear Overhauser effects (nOe), and also relaxation time measurements. Although the conformational dependence of the carbon chemical shifts is far from imderstood, coupling constants can be used to evaluate the magnitude of the torsion angles, and nOe measurements can provide estimations of distances between protons located in rather close proximity. In addition, relaxation time measurements give information on the mobility and the behavior of molecules in solution. 

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