A molecular orbital theory of hydrocarbons

The molecular orbital theory of hydrocarbons developed in the previous two papers [1, 2] has been adapted to the calculation of nuclear spin coupling constants. Using perturbation theory, long range coupling constants are related to those interactions within the molecule which are responsible for electron delocalization between bonds. The theory is applied to the calculation of all nuclear spin coupling constants in ethane, ethylene and acetylene. For nuclei separated by two bonds, the calculations lead to positive constants in all cases, whereas some negative values are found experimentally. However, the observed trend of increasingly positive values with increasing bond angle is well reproduced. For protons separated by three bonds, the theory gives results comparable with those already achieved by valence bond calculations.