DFT calculation of NMR JFF spin-spin coupling constants in fluorinated pyridines

All four isotropic contributions to the NMR fluorine-fluorine coupling constants (Fermi contact, FC, spin-dipolar, SD, paramagnetic spin-orbit, PSO, and diamagnetic spin-orbit, DSO) have been calculated for 2,6-difluoropyridine, 2,4,6-trifluoropyridine, perfluoropyridine, and 2-Br-3,4,5,6,7,8-hexafluoroquinoline by means of density functional theory in combination with the rather modest 6-311G** basis set. Experimental values ranging from -20.3 to +45.8 Hz are semiquantitatively reproduced for three- to seven-bond couplings, suggesting that the different electronic effects responsible for the spin-spin interactions are adequately taken into account. In all cases, the relative importance of noncontact terms was examined. With few exceptions, the sum of the SD and PSO noncontact terms is larger than the FC contact contribution, even though in most cases the two noncontact values have opposite signs. The widespread assumption that the Fermi contact term dominates scalar spin-spin couplings in the case of light atoms would appear to be an oversimplification for J(FF) in polyfluorinated organic molecules. In addition, the CPU performance of the Fermi contact contribution calculated separately by the coupled-perturbed and the finite-perturbation methods was investigated showing the latter to be-slightly more efficient.