Carbon-proton coupling constants three-bond couplings

Table 3.10. One-, Two-, Three-, and Four-Bond Carbon-Proton Coupling Constants (Hz) of Benzene (a), Pyridine (b), Pyrimidine (c), Monosubstituted Benzenes [128] (d) and Five-Membered Heteroaromatics [130] (e).

The results of the carbon-proton coupling constants of some of these monosubstituted cyclopropanes are given in Table 7. Long-range carbon-proton coupling constants in monosubstituted cyclopropanes are such that they may be construed either as two bond or three bond coupling in some cases the latter have been found to be larger. Cyclopropane itself shows Jaim — 2 55 Hz data for a number of monosubstituted derivatives are shown in Table 8. 

Proton NMR spectra in organic molecules can be interpreted without regard to the structural carbon framework because the predominant 12C has no nuclear spin. However, 13C has a spin of V2, which not only permits its direct observation but also provides features in the H spectrum from the 13C that is present at a natural abundance of 1.1%. As we saw in Chapter 5, J(13C-H) is normally 100—200 Hz, whereas two- and three-bond coupling constants often run 5-10 Hz. Hence a resonance line from a proton attached to a 12C atom is accompanied by weak 13C satellites separated by 1J(13C-H) and placed almost symmetrically about the main line. (The departure from precisely symmetrical disposition arises from the 13C/12C isotope effect on the 1H chemical shift, as described in Section 4.8.) For example, the proton resonance of chloroform in Fig. 6.14a shows 13C satellites. 

Three-Bond Coupling The magnitude of the coupling constant between protons on adjacent carbons, is related to the torsional angle between the protons [403] by the Karplus... 

Previously, it was reported that the chemical shift of the proton atoms of the C-12 of the fatty acid methyl ester can be found at 3.503 ppm (50). In the HMBC-NMR spectrum, there is a cross peak between this proton of C-12 and the carbon atom with a chemical shift of 95.23 ppm. This is a tertiary carbon atom. The discrimination between quaternary and tertiary C signals has been performed by using an attached proton test (APT) pulse sequence, one each second. The cross peak refers to the three-bond coupling of the H-12 protons with the anomeric center C-1 of lactose. The H-data of C-1 were found with the one-bond coupling constant, which is 150 MHz. With a COSY-NMR spectrum and its corresponding HMBC spectrum, the H and C NMR values of C-2, C-3, could be determined. 

Figure 3.16. Typical two- and three-bond coupling constants for nonidentical vicinal protons (H and H ). Notice that the nonidentical (geminal) protons on the same carbon couple with a relatively small coupling constant compared to those on adjacent carbons.

Carbon and Proton NMR Data. Some typical carbon and proton chemical shift and coupling constant data for fluorothiophenes are given in Scheme 3.70. Note that the two-, three-, and four-bond F—C... 

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.

Conformational analysis of oligosaccharides in solution by NMR spectroscopy is based on the study of chemical shifts, n.O.e. s, and three-bond, proton-carbon coupling constants. Generally, the experimental NMR parameters P. . , (such as n.O.e. s or... 

Until now, the determination of three-dimensional structures of oligosaccharides in solution was based primarily on proton-proton distance information obtained from n.O.e. data. Here, we discuss the application of three-bond proton-carbon coupling constants. 

Three-Bond Proton-Carbon Coupling Constants. Routine applications of the three-bond C-O-C-H proton-carbon coupling constants ( Jpu) have been coiqplicated by experimental difficulties involved in tneir measurement using classical H-coupled NMR spectroscopy and a limited knowledge of the angular dependence of for the C-O-C-H... 

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. 

Organophosphorus compounds rarely have more than one P atom in the molecule, giving rise to a doublet for any C atom within three bonds (assuming the proton-carbon coupling has been removed) and so these compounds show clearly the connectivity of carbon atoms close to the P atom. The size of the coupling constant is dependent on the number of bonds and the phosphorus oxidation state, such that Vis the largest at about 45-150 Hz, while V and V are of the order of 10-15 Hz. 

Table 4.69. Two- and Three-Bond Carbon-Proton Coupling Constants of Representative Heterocyclic Compounds (in Hz) [115, 130, 416-464, 466, 467].

Three-Bond Carbon-Proton Coupling Constants... 

Based on three-bond carbon-hydrogen coupling constants, specific selective proton decoupling experiments and investigations of specifically deuterated compounds, the resonances of C-6 and C-8 in 5,7-dihydroxyflavonoids appear in the range of 90 to 100 ppm and C-8 is always more shielded compared to C-6. The chemical shift differences found are small for flavanones ( 1 ppm) and larger for flavones and flavonols ( 5 ppm). 

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