Vicinal (3JHH) coupling constants are calculated according to the Altona equation (Tetrahedron 36, 2783-2792) if the protons are attached to two sp3 carbons. This equation takes into account the electronegativity of all adjacent atoms.
For other proton pairs, a simple Karplus equation is used:
J = 8.5 cos2θ - 0.28 if -90 < θ < 90
J = 9.5 cos2θ - 0.28 if 90 < θ or θ < -90
Three alternative equations are provided:
J = 3.56 cos 2θ - cos θ + 4.26
Recommended for general use (Can J Chem (1973) 51 961.)
J = 4.5 - 0.87 cos θ + cos 2θ
(Adv. Carbohydrate Chem. Biochem. (1995) 51, 15-61)
J = 5.8 cos2θ -1.6 cos θ + 0.28 sin 2θ - 0.02 sin θ + 0.52
(Mag. Res. Chem. (1990) 28, 448-457)