A theoretical study of the origin of rotational barriers in push-pull ethylenes
Using the B3LYP/6-31G∗ ab initio method, we have studied the rotation about the CC bonds in 15 push–pull ethylenes of the general formula (X,Y)CC(CHO)2 [X, Y=NH2, NHCH3, N(CH3)2, OCH3, SCH3] in the gas phase. Two stationary points (minimum and transition state) were located for all compounds. The geometry, dipole moments, natural bond orbital atomic charges, as well as the rotational barriers were examined. The torsion angle θ depends essentially on the presence or absence of intramolecular hydrogen bonds, and the barrier is a function of the torsion angle.