DFT-GIAO and DFT-NBO studies of the origin of F-19 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-fluoronorb ornane (3), 7-fluoronobornadiene (4) and the equatorial and axial epimers o f cyclohexyl fluoride (5 and 6, respectively) were studied using the natura l bond orbital (NBO) approach. The level of theory, B3LYP/6-311G**, was cho sen in such a way that the GIAO-calculated F-19 magnetic shielding constant s in all these compounds were in very good agreement with experimental valu es. The large F-19 deshielding effects known experimentally in 1 and 4 with respect to 2 and 3 are rationalized in terms of differences in such electr on delocalizations. The main contribution is that corresponding to the pi - -> (C-F)* interaction, which is only efficient for an anti orientation of t he C-F bond and the vinyl moiety. F-19 and C-13 NMR spectra of 5 and 6 were also measured because some ambiguity was detected in the literature. Despi te the large calculated charge donation to the fluorine environment in 6 it s F-19 resonance appears upheld from that in 5. This trend is rationalized as originating in gamma effects of the C-H-ax bonds of the kind which have been invoked to explain O-17 chemical shift trends in different cyclohexano ls and their respective ethers. Electron delocalization effects on (1)J(C,F ) and (2)J(C,F) couplings are also discussed. Copyright (C) 1999 John Wiley & Sons, Ltd.