The role of ?-type nonbonding orbitals for spin-orbit induced NMR chemical shifts: DFT study of13C and19F shifts in the series CF3IFn (n=0, 2, 4, 6)
✍ Scribed by Kaupp, Martin; Malkina, Olga L.; Malkin, Vladimir G.
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 246 KB
- Volume
- 20
- Category
- Article
- ISSN
- 0192-8651
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✦ Synopsis
type nonbonding orbitals on heavy halogen or related substituents are largely responsible for significantly shielding spin᎐orbit-induced Ž . heavy-atom effects on nuclear magnetic resonance NMR chemical shifts of the neighboring atoms. This suggestion has been examined and confirmed by Ž .
13 density functional theory DFT calculations on C shifts of trifluoromethyl Ž . compounds CF IF n s 0, 2, 4, 6 , including both one-and two-electron 3 n spin᎐orbit corrections. Indeed, the ''removal of iodine -type lone pairs'' upon oxidation leads to a dramatic reduction in the absolute values of the negative 13 Ž C spin᎐orbit shifts along the first three members of the series y57, y29, and . 0 ppm for n s 0, 2, and 4, respectively . The inclusion of the spin᎐orbit effects is mandatory to reach even qualitative agreement between theoretical and experimental trends. Analyses of the shift tensors provide further insight into the spin᎐orbit effects. In particular, the orientation of the 13 C shift tensors for CF I and CF IF is altered dramatically by spin᎐orbit coupling. Structural and 3 3 2 chemical shift predictions are made for the as yet unknown CF IF .