Abstract: The crystal structures of N-phenylmethylene-2-thiazoleamine(2a) and N-[(4-nitrophenyl)methylene]-2-thiazoleamine (2b) were determined. Twenty-two rotational geometries(θ=0°⁹0°) of each of two molecules were optimized using AM1, RHF/6-311G^** B3LYP/6-311Gand B3LYP/6-311G^** The twist angles of their preferential geometries obtained from DFTare both 0°, which are most close to the experimental values (2a, θ=8.9°; 2b, θ=3.9°) of all angles obtained from various optimized methods. Although there is a great difference in the twist angles of the five molecules, 2a, 2b, N-(4-nitrophrnyl)methylene-2-pyrimidineamine (1a), N-(4-nitrophenyl)methylene-2-pyridine-amine (1b) and N-phenyl)methylene-3-pyridineamine (1c ), their lowest total electronic energies occur in the optimized geometries with the similar twist angles which are in the region of θ from 37 to 42° and almost have nothing to do with the optimized methods (HFand DFT) and Gaussian basis sets. The π system in the geometry with θ=0° is destabilized no matter whether it is delocalized or not. It appears that the π system always prefers a twisted geometry. The driving force for out-of-plane twist of the molecules arises from the electronic interaction, and the nuclear repulsion is, practically, a resistance for distortion of molecule.

Key words: Resonance effect, Driving force for distorting stilbene-like species, Conformation, N-phenylmethylene-2-thiazoleameine, N-(4-Nitrophemyl)methylene-2-thiazoleameine