1 H and 13 C NMR study of hindered rotation in N 1 , N 1 ‐dimethyl‐ N 2 ‐substituted phenylacetamidines

Barriers to rotation about the partial double CN 1 bond were determined from line shape analysis of 1 H and 13 C dynamic NMR spectra of N 1 , N 1 ‐dimethyl‐ N 2 ‐substituted phenylacetamidines with twelve different substituents on the phenyl ring. The values of Δ G   Tc ≠are 51.2–58.7 kJ mol −1 and the correlation with Hammett's σ values indicates an important contribution from the substituent effects to the barrier height. The decrease of the rotational barrier of approximately 10 kJ mol −1 in acetamidines in comparison with the respective formamidines is probably mainly due to a steric interaction of the CCH 3 group with aromatic protons; this leads to non‐planarity and a decrease of the conjugation of the aromatic ring with the acetamidine moiety.