An ab initio study of stable conformation and thermal isomerization of p-aminoazobenzene

In this work, the stable conformation and thermal isomerization of p-aminoazobenzene (AAB) were investigated by using ab initial MP2/6-31+G∗ and DFT-B3LYP/6-31+G∗ calculations. For comparison reason, corresponding results of azobenzene (AB) were also obtained by the same theoretical methods. Compared with AB, the MP2 optimized geometries of AAB show little variation in the NN bond length and most bond angles, but show obvious variations in the CN bond length and CCNN dihedral angle that link with the NH2-substituted phenyl ring. Phenyl ring twist of both AB and AAB show two equivalent nonplanar minima in the potential energy surface (PES) of the NNCC dihedral angles, but the potential energy barriers at 0° (the planar structure) are low. For AAB, the energy barrier is even lower for CCNN dihedral angle of the NH2-substituted phenyl ring. The S0 PES of AB obtained by DFT method shows that the energy barrier for CNN inversion is lower than that for CNNC rotation. On the contrary, the energy barriers for AAB along both rotation and inversion pathways are almost equal. Calculation indicates that the molecular structures at the highest points on the PESs of CNNC rotation and CNN inversion of the unsubstituted phenyl ring are identical. On the basis of the calculation, it is believed that even the relative stability of trans- and cis-isomers is not obviously affected by the donor substitution, the thermal isomerization of AAB can go through both the inversion and rotation pathways.