Spin–spin coupling constants in ethylene: equilibrium values

The nuclear magnetic resonance spin–spin coupling constants for the ethylene have been calculated at the multiconfigurational self-consistent field (MCSCF) method using the restricted (RAS) and complete (CAS) active space approximations. An analysis of the calculated values and of the effects of different factors (active spaces, valence and core–electron correlation) is presented. The effect of increase the number of active orbitals and that of triple and quadruple excitations is important and opposite to the effect of core–electron correlation. The best calculated values for 1JCC (68.83 Hz), 1JCH (151.56), 2JCH (−1.56 Hz), 2JHH (1.07 Hz) 3JHH(cis) (11.47 Hz), and 3JHH(trans) (17.78 Hz) are in good agreement with the best calculated and experimental values.