Ab Initio Molecular Orbital and Density Functional Studies on the Stable Structures and Vibrational Properties of trans- and cis-Azobenzenes

The structure of trans -azobenzene (TAB) has been a subject of controversy in experimental and theoretical studies. To provide the theoretical basis for stable structures and vibrational properties of TAB and cis -azobenzene (CAB), we performed ab initio molecular orbital calculations based on the second-order Møller−Plesset (MP2) method and density functional theory (DFT). Only the MP2 calculation accounting for the diffuse basis set (6-31+G*) leads to the distorted structure of TAB, which is consistent with the gas-phase electron diffraction experiment. The Hartree−Fock and DFT (PW91PW91, MPW91, BP86, and B3LYP) calculations, on the other hand, result in the almost planar structure of TAB. The frequencies of normal modes of TAB and CAB are calculated most accurately by the PW91PW91/6-31+G* method followed by the BP86/6-31G* method. These results are expected to provide useful benchmark data on the accuracy of MP2 and DFT methods for describing the structural and vibrational properties of azobenzene-like molecules, which consist of weakly interacting phenyl rings.