This program calculates 3JHH values according to the Karplus equation
J = A cos2(d) - B cos(d) + C,(where A = 7.76, B = 1.1, C = 1.4)*
the coefficients are taken from Haasnoot et al. 1980 Tetrahedron paper.
If more precision is needed, the paper offers
a variation of the Karplus equation, which
takes into account electronegativities:
"The relationship between proton-proton nmr coupling constants and substituent electronegativities-I"
C.A.G. Haasnoot, F.A.A.M. DeLeeuw and C. Altona
Tetrahedron 1980, 36, 2783-2792