F-19 NMR chemical shifts. 1. Aliphatic fluorides

The F-19 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 foun d to be due to the tenser components normal to the C-F bond axis. As the nu mber 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 opport unities for mixing with virtual orbitals and to the increase in paramagneti c deshielding. The same pattern is seen on going from acetylene to 2-tert-b utylacetylene and is also seen in the methyl C-13 Shielding in the series e thane, propane, isobutane, and neopentane. The fluorine shielding in the se ries of fluoromethanes also decreases with increasing fluorine substitution , With carbon tetrafluoride, the decreased shielding arises from the highes t occupied MO, which is a nonbonding linear combination of pure p functions at the fluorines.