The free energy of activation for hindered rotation about the C—N exocyclic bond in someN,N-dimethylaminopyrimidine hydrochlorides has been determined by1H and13C nmr line-shape analysis.Monoprotonation ofN,N-dimethylaminopyrimidines induces a large increase of the free energy of activation (from 14 to 24 kJ mol−1). This increase is larger for the 4-dimethylamino group than for the 2-dimethylamino group due to the predominance of the monoprotonated (N-1 H) form. Consequently, the difference of conjugation of the 4- and 2-dimethylamino groups with the pyrimidine ring is more pronounced in the monoprotonated species.