The conformation of N-phenylmethylene-2-thiazoleamine species and the driving forces for twisting molecule

The crystal structures of N-phenylmethylene-2-thiazoleamine (2a) and N-[(4- nitrophenyl) methylene]-2-thiazoleamine (2b) were determined. Twenty-two ro tational geometries(theta =0 degrees similar to 90 degrees) of each of two molecules were optimized using AM1, RHF/6-311G**, B3LYP/6-311G and B3LYP/6- 311G**. The twist angles of their preferential geometries obtained from DFT are both 0 degrees, which are most close to the experimental values (2a, t heta =8, 9 degrees; 2b, theta =3, 9 degrees) of all angles obtained from va rious optimized methods. Although there is a great difference in the twist angles of the five molecules, 2a, 2b, N-(4-nitrophrnyl)methylene-2-pyrimidi neamine (la), N-(4-nitrophenyl) methylene-2-pyridine-amine (Ib) and N-pheny l) methylene-3-pyridineamine (Ic), their lowest total electronic energies o ccur in the optimized geometries with the similar twist angles which are in the region of theta from 37 to 42 degrees and almost have nothing to do wi th the optimized methods (HF and DFT) and Gaussian basis sets. The pi syste m in the geometry with theta =0 degrees is destabilized no matter whether i t is delocalized or not. It appears that the pi system always prefers a twi sted geometry. The driving force for out-of-plane twist of the molecules ar ises from the electronic interaction, and the nuclear repulsion is, practic ally, a resistance for distortion of molecule.