Carbon‐13 spin–lattice relaxation in some polyfluoroaromatic compounds

Abstract

AbstractCarbon‐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 T1 values for the para‐carbon of monosubstituted fluorobenzenes is clearly shorter than the T1 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 C2 in‐plane principal symmetry axis. Equal tumbling ratios, D∥/D⊥, were found in this way for toluene and perfluorotoluene.