Carbon‐13 spin–lattice relaxation in some polyfluoroaromatic compounds

Carbon‐13 chemical shifts, spin‐lattice relaxation times and nuclear Overhauser enhancement factors are reported for five polyfluoroaromatic compounds at 28°C. In all cases the relaxation of the fluorine bearing carbon is predominantly dipolar. Effective correlation times are smaller than those of the analogous benzene derivatives by a factor of 3–4, in qualitative agreement with predictions from the Stokes–Einstein diffusion theory. The T 1 values for the para ‐carbon of monosubstituted fluorobenzenes is clearly shorter than the T 1 values for the ortho ‐ and meta ‐carbons. This phenomenon was traced to anisotropic tumbling, and D ∥ and D⊥ diffusion coefficients were computed using Woessner's equations for molecules assumed to behave like symmetric rotors about their C 2 in‐plane principal symmetry axis. Equal tumbling ratios, D ∥/ D ⊥, were found in this way for toluene and perfluorotoluene.