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 i n experimental and theoretical studies. To provide the theoretical basis fo r stable structures and vibrational properties of TAB and cis-azobenzene (C AB), we performed ab initio molecular orbital calculations based on the sec ond-order Moller-Plesset (MP2) method and density functional theory (DFT). Only the MP2 calculation accounting for the diffuse basis set (6-31+G*) lea ds to the distorted structure of TAB, which is consistent with the gas-phas e electron diffraction experiment. The Hartree-Fock and DFT (PW91PW91, MPW9 1, BP86, and B3LYP) calculations, on the other hand, result in the almost p lanar 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 B P86/6-31G* method. These results are expected to provide useful benchmark d ata on the accuracy of MP2 and DFT methods for describing the structural an d vibrational properties of azobenzene-like molecules, which consist of wea kly interacting phenyl rings.