Vicinal proton-proton coupling constants. Basis set dependence in SCF ab initio calculations

An SCF ab initio study of the angular dependence and substituent effects upon the vicinal coupling constants has been carried out for the molecules CH3CH3, CH2FCH3 and CHF2CH3. The four contributions to 3JHH (JFC, JSD, JOD and JOP) have been computed using the STO-3G, 6–31G, 6–31G* and 6–31G** basis sets. The major contributions arise from the FC term. The magnitude of the SD contributions is very small and near independent of the size of the basis set. The magnitude of the orbital contributions OR (=OD+OP) decreases as the size of the basis set increases. The FC term slightly overestimates both the individual and the interaction substituent effects for basis sets larger than the STO-3G one. For this basis such effects are underestimated.