Application of NMR spectroscopy of chiral association complexes 11 —Rotation about the C(sp 2 )—C(aryl) Bond in 2,6‐Difluorobenzamides Studied by 19 F NMR

The energy barrier to rotation about the C(sp 2 )C(aryl) single bond in non‐planar N , N ‐dimethyl‐, N , N ‐tetramethylene‐, N , N ‐diisopropyl‐2,6‐difluorobenzamides and in N , N ‐dimethyl‐2‐chloro‐6‐fluorobenzamide was investigated by 19 F{ 1 H} NMR in the presence of an optically active shift reagent. The free energy of activation was calculated from the coalescence temperature of the fluorine signals; the calculation was accomplished using the simple, approximative equation and, also, by the graphical procedure including the (eigen) line width, i.e. without the broadening caused by two‐site exchange. A distinct effect of the size of the N ‐substituents on the Δ G c ‡values was observed, in accordance with expectation.