Proton and carbon-13 nuclear magnetic resonance studies of substituted pyrimidines. Part 3. Hindered internal rotation in some 4-(NN-dimethylamino)pyrimidines
The free energy of activation for internal rotation about the C–N exocyclic bond of some substituted 4-(NN-di-methylamino)pyrimidines has been determined using 1H and 13C n.m.r. line-shape analysis. Substituent effects on the rotational barrier of the dimethylamino group are evaluated. The influence of the position of the dimethyl-amino group with respect to the nitrogen atoms is discussed, the rotational barrier being higher in the 4- than in the 2-position. An interesting linear correlation between free energies of activation and 1J(C,H) coupling constants for the 4-dimethylamino group has been found.