3JH,H calculationThe magnitude of three-bond 1H-1H coupling constants is determined mainly by the torsion angle between the protons but substituents, especially electronegative ones, do also affect the coupling. They influence the coupling in two ways; they cause a change in overall magnitude of the Karplus equation, and they cause a shift of the maxima and minima of the curve. Several attempts have been made to account for these effects.
The program here calculates 3JH,H values according to C.A.G. Haasnoot, F.A.A.M. DeLeeuw and C. Altona; Tetrahedron 36 (1980) 2783-2792. The actual equation is rather complex and requires the electronegativities of all substituents to be entered.
Since the equation requires the relative orientation of the substituents a molecular model will be of help.
Turn the model so that the proton on the near side is pointing up then use the schemes below to determine the numbering of the substituents (e.g. S1, S2 etc.) and select the appropriate elements from the menus.
The paper (above) contains set of different equations (A-E) for use under different circumstances. The program will automatically select an equation (C, D, or E) depending on the number of attached hydrogens. (Equations A and B are 'general' and give slightly higher deviations - however the difference between the equations is not great)
It is also possible to calculate the torsion angles that correspond to a particular 3JH,H-value. There can be one to four angles depending on the size of the coupling constant.