Vicinal proton—carbon coupling constants

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... 

What are available of proton and carbon data for compounds containing the 1,2,2,2-tetrafluoroethyl group are given in Scheme 6.7. Note that the vicinal F—H coupling constants for the sulfides are much greater than those of the analogous ethers. 

C(8) and C(5), C(ll) respectively, can be correlated with the one-bond C,H coupling constants via the proton shifts. This assignment of the J(C,H) coupling constants will be confirmed, as shown in Sect. 4, by vicinal C,H coupling constants between the N-methyl carbon and the diastereotopic methylene protons. When the observed J(C,H) values are compared with the reference data for the non-cyclic glycyl peptides it is evident that the rather large value of 150.0 Hz deserves special attention. 

Proton and Carbon NMR Data. Some representative chemical shift and coupling constant data are provided in Scheme 3.48 for alkenes with vicinal fluorines. 

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.

Fig. 10.18. IDR (Inverted Direct Response)—HSQC-TOCSY pulse sequence. The experiment first uses an HSQC sequence to label protons with the chemical shift of their directly bound carbons, followed by an isotropic mixing period that propagates magnetization to vicinal neighbor and more distant protons. The extent to which magnetization is propagated in the experiment is a function of both the size of the intervening vicinal coupling constants and the duration of the mixing period. After isotropic mixing, direct responses are inverted by the experiment and proton detection begins.

The presence of electronegative elements directly attached to the same carbon atom as one of the vicinally coupled protons decreases the magnitude of the coupling constant, while the presence of electropositive elements increases it. This effect is small in chains (which are capable of relatively free rotation) but more pronounced in rigid systems such as alkenes. 

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

In aliphatic and cycloaliphatic compounds, vicinal carbon-proton coupling constants 3Jch are related to the dihedral angle 0, as known from the Karplus-Conroy relation for iJ[iH. The Fermi-contact contribution to 3JCH as a function of the dihedral angle 0 calculated for propane [131] is displayed in Fig. 3.15, and the Karplus relation given by eq. (3.17) can be derived ... 

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