Vicinal proton–proton coupling constants: MCSCF ab initio calculations of ethane

Ab initio Hartree–Fock self consistent field and multiconfigurational self consistent field calculations have been carried out to study the dihedral angle dependence of the vicinal proton–proton coupling constant in an ethane molecule. The four contributions to 3JHH, Fermi contact, spin dipolar, orbital paramagnetic and orbital diamagnetic, have been computed with five different basis sets, three of them rebuilt specifically to calculate coupling constants. The importance of the noncontact contributions is small and the correlation effect on these is practically null. The calculated Karplus equation reproduces the experimental equation for 3JHH with a maximum deviation of 1.0 Hz when the proton–proton torsional angle is 180°. Our best average 〈3JHH〉 coupling constant calculated using the experimental geometry (7.77 Hz) is in good agreement with the experimental value (8.02 Hz).