Vicinal couplings, dihedral

Smith and Cox S7) found the geminal H—H coupling constants in a series of rigid bicycloheptenes to vary with solvent, but report that both the cis and trans vicinal couplings (dihedral angle = 0° and 120° respectively) are solvent invariant. 

It is difficult to decide whether the discrepancy between the calculated and experimental data is due to a different conformational preference of the thietane dioxides in the liquid and the solid phase, or to the crude approximations included in the Karplus-Barfield equation. However, the relationship between vicinal coupling constants and dihedral angles appears qualitatively valid in thietane oxides and dioxides, particularly if trends instead of exact values are discussed . At any rate thietane dioxides, 1,3-dithietane dioxides and tetroxides maintain either planarity or a slightly distorted average vibrating conformation with a low barrier to ring planarity . 

Elucidation of the stereostructure - configuration and conformation - is the next step in structural analysis. Three main parameters are used to elucidate the stereochemistry. Scalar coupling constants (mainly vicinal couplings) provide informa-hon about dihedral bond angles within a structure. Another way to obtain this information is the use of cross-correlated relaxation (CCR), but this is rarely used for drug or drug-like molecules. 

Most characteristic for the overall conformation of the cinchonan carbamates in solution is the dihedral angle of the torsion H8-C8-C9-H9, which can be inferred from the vicinal coupling constant of the NMR proton signal of H9. The experimentally measured coupling constant /hsh9 represents actually the average over the populations P(i) of the different conformers in solution according to [100]... 

The vicinal coupling (protons on adjacent carbon atoms) h-c-c-h have values 0 - 16 Hz depending mainly on the dihedral angle (f). 

Vicinal coupling constants in HC—CH fragments are related to the dihedral angle by the relationship... 

Instead, we see a strong mutual NOE. Apparently, the H3-C3-C4-H4 dihedral angle is very near the minimum in the Karplus curve (90° angle) so that the vicinal coupling constant is very small. 

Laurie s studies of the heteronuclear NMR parameters of sugars and other molecules did much to extend the experimental generality of the Karplus dependence of vicinal coupling constants on the dihedral angles and other properties of the coupled nuclei. 

Using Lambert s28 equation and the experimentally measured vicinal coupling constants, the N(3)-C(4)-C(5)-0(6) dihedral angle is estimated to be 54°. A value... 

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