Vibrational analysis of trans-azobenzene

Molecular orbital calculations were performed to determine the normal modes and vibrational energies of azobenzene. A semiempirical calculation using the PM3 Hamiltonian and an ab initio calculation carried out at the SCF level using the 6-31G basis set gave unsatisfactory predictions especially for vibrations dominated by azo atom displacements. High-level electron correlation ab initio calculations carried out at the MP2 level improved the fit with experiment but the choice of basis set was found to be critical. When the basis set for the nitrogens of the azo group was changed to the 6-31+G(d) basis set, the calculation gave a satisfactory fit. Normal-mode diagrams and energies are presented, and assignments to experimentally observed vibrational energies of azobenzene are made. The main azo stretch, v(10), observed at 1440 cm(-1), is theoretically predicted at 1450 cm(-1). The calculation correctly predicts an increase in frequency in the azo stretch mode upon deuteration of the phenyl rings. Coupling of several phenyl modes with azo vibrations are revealed by the calculation, in agreement with previous assignments of the vibrational spectra of azobenzene and azobenzene derivatives. The calculation indicates why certain in-plane stretching frequencies give rise to relatively intense Raman and resonance Raman scattering. In Raman scattering, the modes giving rise to the strongest scattering involve displacements along the N=N and C-N bonds. The same modes give intense resonance Raman scattering with the stretches along the azo bond providing the greatest intensity.