Finite perturbation theory-density functional theory calculation of the spin-dipolar contribution to NMR spin-spin coupling constants

In this work an implementation of the FPT-DFT approach for calculating the spin-dipolar contribution to NMR spin-spin coupling constants is presented. This method was tested in a set of small molecules, giving results in exce llent agreement when comparing them with values taken from the literature, which were obtained with state-of-the-art calculations. To obtain an insigh t into the relative importance of the spin-dipolar contribution in unsatura ted compounds, calculations of J( F, C), J( F, F) and J( F, H) couplings in 1, 2-, 1,3-, and 1,4-difluorobenzenes were performed. An important spin-di polar contribution to (3)J( F, F) and (5)J( F, F) was found, suggesting tha t this term might be important in some cases. When performing DFT calculati ons the non-singlet instabilities usually found in unsaturated compounds ar e overcome.