The 19F NMR shieldings for several remotely substituted rigid polycyclic alkyl fluorides with common sets of substituents covering a wide range of electronic effects were calculated using the DFT-GIAO theoretical model. The level of theory, B3LYP/6-311+G(2d,p), was chosen based on trial calculations which gave good agreement with experimental values where known. The optimized geometries were used to obtain various molecular parameters (fluorine natural charges, electron occupancies on fluorine of lone pairs and of the C-F bond, and hybridization states) by means of natural bond orbital (NBO) analysis which could help in understanding electronic transmission mechanisms underlying 19F substituent chemical shifts (SCS) in these systems. Linear regression analysis was employed to explore the relationship between the calculated 19F SCS and polar substituent constants and also the NBO derived molecular parameters. The 19F SCS are best described by an electronegativity parameter. The most pertinent molecular parameters appear to be the occupation number of the NBO p-type fluorine lone pair and the occupation number of the C-F antibonding orbital. This trend suggests that in these types of rigid saturated systems hyperconjugative interactions play a key role in determining the 19F SCS. Electrostatic field effects appear to be relatively unimportant.
- Bridgehead-substituted bicyclic fluorides
- F chemical shift
- Hyperconjugative interactions
- Substituent effects