Karplus equation vicinal coupling constants

Vicinal coupling constants Jhh indicate very clearly the relative configuration of the coupling protons. Their contribution depends, according to the Karplus-Conroy equation... 

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 . 

In this context, Pachler and Wessels explore the dependency of these vicinal coupling constants with temperature and solvent dielectric constant ( ). This is based on a proposed modified Karplus equation,... 

Vicinal coupling constants Jhh for HCCH and HCNH groups have been approximated by the Karplus equation, one form of which is... 

The small interaction between the two C hybrid orbitals, as well as the other orbital interactions, can be related to the corresponding overlap integrals and these depend on the geometry of the HC CH fragment. In particular, the dihedral angle 0 between the H(1)CC and CCH(2) planes is an important factor. As a consequence, vicinal coupling constants are of extreme value in conformational studies. The relationship (the Karplus equation) is of the following type ... 

Vicinal coupling constants strongly depend on the dihedral angle, <]) (Karplus equation) ... 

A modified Karplus equation applied to vicinal coupling constants in olefins predicts a larger trans coupling than cis coupling. This is observed to be true vicinal protons on olefinic bonds in a trans disposition (f) = 180°) afford J values of 12-18 Hz while cis protons (0 = 0°) afford J values of 7-10 Hz in acyclic systems (Figure 2.49). 

The coupling constant, J, between vicinal protons varies with dihedral angle, 0. The relationship between J and 0 is given by the Karplus equation . 

For each molecule (isomer A and isomer B), obtain dihedral angles for the following pairs of vicinal hydrogens Hg-Hy, H Hg, Hy-Hn, and Hg-Hyaxiai- Use the Karplus equation to estimate coupling constants for each pair, and then compare your predictions to the experimental coupling constants (see above). Which molecule is artemisin acetate and which is 6-epiartemisin acetate ... 

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