19F NMR chemical shifts. 1. Aliphatic fluorides.
The (19)F NMR shielding for the alkyl fluorides from methyl fluoride to tert-butyl fluoride has been calculated using IGAIM and has been separated into the contribution from each of the molecular orbitals. The relatively large change in fluorine shielding, in contrast to the adjacent carbon, was found to be due to the tensor components normal to the C-F bond axis. As the number of adjacent p-orbitals increases, the lone-pair p orbitals at fluorine become involved with MOs using these orbitals. The increase in the number of occupied orbitals associated with the fluorine leads to increased opportunities for mixing with virtual orbitals and to the increase in paramagnetic deshielding. The same pattern is seen on going from acetylene to 2-tert-butylacetylene and is also seen in the methyl (13)C shielding in the series ethane, propane, isobutane, and neopentane. The fluorine shielding in the series of fluoromethanes also decreases with increasing fluorine substitution. With carbon tetrafluoride, the decreased shielding arises from the highest occupied MO, which is a nonbonding linear combination of pure p functions at the fluorines.
Wiberg
K B
KB
Department of Chemistry, Yale University, New Haven, CT 06520-8107, USA. kenneth.wiberg@yale.edu
Zilm
K W
KW