The role of pi-type nonbonding orbitals for spin-orbit induced NMR chemical shifts: DFT study of C-13 and F-19 shifts in the series CF3IFn (n=0, 2, 4, 6)

pi-type nonbonding orbitals on heavy halogen or related substituents are la rgely responsible for significantly shielding spin-orbit-induced heavy-atom effects on nuclear magnetic resonance (NMR) chemical shifts of the neighbo ring atoms. This suggestion has been examined and confirmed by density func tional theory (DFT) calculations on C-13 shifts Of trifluoromethyl compound s CF3IFn (n = 0, 2, 4, 6), including both one- and two-electron spin-orbit corrections. Indeed, the "removal of iodine pi-type lone pairs" upon oxidat ion leads to a dramatic reduction in the absolute values of the negative C- 13 spin-orbit shifts along the first three members of the series (-57, -29, and 0 ppm for n = 0, 2, and 4, respectively). The inclusion of the spin-or bit effects is mandatory to reach even qualitative agreement between theore tical and experimental trends. Analyses of the shift tensors provide furthe r insight into the spin-orbit effects, in particular, the orientation of th e C-13 shift tensors for CF3I and CF3IF2 is altered dramatically by spin-or bit coupling. Structural and chemical shift predictions are made for the as yet unknown CF3IF6. Spin-orbit effects on F-19 shifts for fluorine atoms b ound to iodine are much less pronounced than those for carbon shifts. This is related to the low fluorine s-character in the bonds, and thus to a less effective Fermi-contact mechanism. (C) 1999 John Wiley & Sons, Inc.