DFT-GIAO and DFT-NBO studies of the origin of ¹⁹F NMR shielding effects in alkyl fluorides

Electron delocalization into the local environment of the fluorine nucleus in the anti (1) and syn (2) epimers of 7-norbornenyl fluoride, 7-fluoronorbornane (3), 7-fluoronobornadiene (4) and the equatorial and axial epimers of cyclohexyl fluoride (5 and 6, respectively) were studied using the natural bond orbital (NBO) approach. The level of theory, B3LYP/6-311G**, was chosen in such a way that the GIAO-calculated ¹⁹F magnetic shielding constants in all these compounds were in very good agreement with experimental values. The large ¹⁹F deshielding effects known experimentally in 1 and 4 with respect to 2 and 3 are rationalized in terms of differences in such electron delocalizations. The main contribution is that corresponding to the π → (C-F)* interaction, which is only efficient for an anti orientation of the C-F bond and the vinyl moiety. ¹⁹F and ¹³C NMR spectra of 5 and 6 were also measured because some ambiguity was detected in the literature. Despite the large calculated charge donation to the fluorine environment in 6, its ¹⁹F resonance appears upfield from that in 5. This trend is rationalized as originating in γ effects of the C-H_ax bonds of the kind which have been invoked to explain ¹⁷O chemical shift trends in different cyclohexanols and their respective ethers. Electron delocalization effects on ¹J(C,F) and ²J(C,F) couplings are also discussed.