The crystal structures of N-{[4-(dimethylamino)phenyl] methylene}-2- benzoimidazoleamine(1), N-{[4-(dimethylamino) phenyl]methylene}-2- benzothiazoleamine (2) and N-(phenylmethylene)-2-naphthaleneamine (3) were determined. Twenty-three rotational eometries (θ=0°～-89°) of each of the molecules were optimized using AM1, RHF/STO-3G, 4-31G, 6- 311G, 6-311G and B3LYP/4-31G, 6-311G, 6-311G. The torsional angles of the preferential geometries of compound 1, 2 and 3 are close to the experimental values (1, θ=-10.69°; 2, θ=-11.99°; 3, θ=-52.88°) in the case of any optimized method. Although different optimized methods provide a specific molecule with different preferential geometries, the electron interaction always favors a twisted geometry and the nuclear repulsion is, practically, a resistance for distortion of th emolecule, which is not an artifact of a specific optimized method. The π system in the geometry with θ=0° is most destabilized no matter whether it is delocalized or not. It appears that the π system always prefers a twisted geometry.

Key words: BENZYLIDENE, NAPHTHALENE P, CONFORMATION, BENZIMIDAZOLE P, QUANTUM CHEMISTRY, CONJUGATE FUNCTIONS, CONJUGATE, CRYSTAL STRUCTURE