Molecular-mechanics and 13C NMR spectroscopic study of the electronic and steric structure of 9-alkenylcarbazoles, 10-alkenylphenoxazines, and 10-alkenylphenothiazines showed that the phenoxazine and phenothiazine derivatives are chatacterized by nonplanar heterocyclic systems with quasiaxial orientation of the alkenyl group. The phenothiazine system exerts stronger shielding of the terminal carbon atom in the N-vinyl group, as compared to phenoxazine and carbazole. The carbon chemical shifts are directly related to the torsion angles with respect to the C-N bond, indicating a competition between the olefinic double bond and aromatic rings for the unshared electron pair on the nitrogen