Coupling constants Karplus equation

J is almost always positive and its magnitude often exceeds that of T. It always depends in a predictable way on the dihedral angle ( ) between the outer two of the tluee bonds in die coupling patliway. Karplus first showed theoretically that T varies to a good approximation as A cos ( ) + B cos ( ), where A and B are constants, and also that A S>B [17]. Flis equation has received wide-ranging... 

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

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 ... 

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 . 

Haasnoot, C. A. G., De Leeuw, F. A. A. M., De Leeuw, H. P. M., Altona, C. Relationship between proton-proton NMR coupling constants and substituent electronegativities. III. Conformational analysis of proline rings in solution using a generalized Karplus equation. Biopolymers 1981, 20,1211-1245. 

Theoretical coupling constants for individual rotamers were obtained directly from MacroModel employing an empirical generalization of the Karplus Equation ( ). Calculated average couplings came from the expression ... 

G.K. Hamer That is certainly true. I think the problem is not that we can t measure the coupling constants. I think the accuracy with which we can measure the coupling constants is pretty good because it turns out that the simulations are extremely sensitive to small changes in chemical shifts, and the changes in chemical shifts that are important are in the proton spectrum. They are minute changes. The real problem is in the Karplus equation and in parameterising the curve. I think one... 

A study on the conjugate acids of dihydro-6//-dibenz[c,e]azepines has demonstrated the validity of the Karplus equation for determining H—N—C—H coupling constants in secondary and tertiary amines of known rigid conformation (73CJC2433). 

An increased preference for a trans location of the vicinal oxygens in the 2-methoxyethoxy fragment of l-(2,-methoxyethoxy)silatrane as compared with tetrakis(2-methoxyethoxy)silane was found237 by analysis of their vicinal 1H-1 H coupling constants using a modified Karplus equation. Enhanced electrostatic repulsion between these heteroatoms reflects the electron donation from nitrogen via silicon to the axial ligand in the former compound. 

The lycopene stereoisomers were identified by using a Bruker AMX 600 spectrometer and, as a control unit, a Bruker peak sampling unit (BPSU 12). In general, the assignments of the Z/Z-isomers are available, adapted from the Karplus equation, by taking the differences of the coupling constants into consideration. 

You may guess that, when the dihedral angle is 90°, the coupling constant is zero. What happens in between these extremes was deduced by Karplus in the 1960s and the relationship is usually known as the Karplus equation. It is easiest to understand from a graph of j against dihedral angle. 

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