Coupling constants, and dihedral angles

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 . 

Fig. 59. Plot of three-bond 3/(19F-Si-N-13C) coupling constants against dihedral angles in 36g (circles) and 37g (, interpolated from dihedral angles).50 Reproduced with permission from the...

FIGURE 5.2 Calculated variation of vicinal and allylic proton-proton coupling constants with dihedral angle between the C—H bonds shown. 

K. G. R. Pachler, The dependence of vicinal proton-proton coupling constants on dihedral angle and substituents, J. Chem. Soc. Perkin II (1972) 1936-1940. 

Molecular configurations are often deduced from the conformations, which in turn are often deduced from the Karplus curve (see Table XI) relating spin-spin coupling constants to dihedral angles, H-C-C-H. The limitations of the Karplus equation have been emphasized by Karplus and others although not precise, it is useful. 

Equatorial-equatorial and equatorial-axial coupling constants (60° dihedral angles) are on the order of 2-4 Hz. Thus it is often a relatively simple matter to determine stereochemical relationships on a six-membered ring using NMR spectroscopy. 

From a comparison of proton-fluorine coupling constants and bond angles in a series of cyclic organic molecules, it has been found that V( H, F) generally decreases as the H-C-C-F bond angle increases. In addition, the dihedral angle dependence of V( H, F) is very similar to the dependence of V( H, H), i.e. ( H, F) has a maximum value when the dihedral angle 0 between H and F is 0° and 180°,... 

In all cases, a minimum in the vicinal coupling constant versus dihedral-angle plot is found around 90°. Semiempirical MO (Giessner-Prettre and Pullman, 1974 Albright, 1976) and ab initio MO (Giessner-Prettre and Pullman, 1978) calculations have approximately reproduced the torsional-angle dependence to the PC and PH three-bond coupling constants. 

Si-N Bond Lengths (A), Bond and Dihedral Angles (deg.), and Corresponding Two-and Three-Bond Coupling Constants in Hexacoordinate Fluoro-Complexes50... 

Figure 12.11. Illustration of the Karplus relationship between three-bond scalar coupling constants and the dihedral angle of the intervening bond. The relationship is indicated for the torsion angle of the H2 and H3 protons within the rigid core of taxol and related derivatives. See Fig. 12.6 for the structure of taxol.

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