The geometry dependence of the spin–spin coupling constants in ethane: a theoretical study

The intramolecular surfaces of the spin–spin coupling constants in ethane have been analysed in ab initio calculations at the MCSCF level. The Karplus equation has been modified by including the influence of the C–C and C–H bond lengths, and the CCH bond angle. The C–C bond length affects the vicinal H–H coupling constant in a wide range of torsion angles, while the influence of the other two geometric parameters is largest for the torsion angles near 180°. The one-bond coupling constants are sensitive to bond stretching, while the change of the CCH angle mostly influences the C–C and geminal H–H coupling constant. The C–H coupling constants exhibit differential sensitivity to the changes of C–H bond length.