Conformational properties of the free and methylated 2-amino group in benzimidazole, benzoxazole, and benzothiazole. X-Ray crystallographic analysis and nuclear magnetic resonance study of the internal rotation

The conformational properties of the free and methylated 2-amino group in benzimidazole, 1-methyl-benzimidazole, benzo-oxazole, and benzothiazole were studied, as well as the modifications induced by N-protonation and N-methylation at the heterocyclic ring. Both 1H n.m.r. spectra and X-ray analysis show that in the ground state of the molecules examined the amino group is coplanar with respect to the plane of the rings. The NN-dimethylamino group tends to be distorted from coplanarity when a methyl group is present at the heterocyclic nitrogen: the degree of distortion determined experimentally compares satisfactorily with that corresponding to the energy minimum of the molecules calculated by semi-empirical methods. For the benzoxazole and benzothiazole derivatives it was also possible through 1H dynamic n.m.r. measurements to determine the thermodynamic parameters for the internal rotation of the NN-dimethylamino group; and in the N-protonated forms the free energy of activation turns out to be higher than that in the free bases.