DENSITY-FUNCTIONAL CALCULATIONS OF STRUCTURES, VIBRATIONAL FREQUENCIES, AND NORMAL-MODES OF TRANS-AZOBENZENE AND CIS-AZOBENZENE

The vibrational structures of trans- (TAB) and cis-azobenzene (CAB) ar e of interest due to their importance in optoelectronic applications a s well as due to the unique isomerization mechanism involving the inve rsion process (at the nitrogen site). In this paper, we report the equ ilibrium structures, harmonic frequencies, and mode assignments for TA B and CAB and their isotopic analogues, using restricted Hartree-Fock (RHF), hybrid Hartree-Fock/density functional (HF/DF), and pure densit y functional theoretical (DFT) methods utilizing the 6-31G basis set. The results of the optimized molecular structure obtained on the basi s of RHF and all the DFT calculations are presented and then criticall y compared with the experimental electron diffraction results. It is o bserved that best structural parameters are predicted by the hybrid HF /DF method, viz. B3LYP and B3P86 followed by the pure DFT method BP86. In the case of harmonic vibrational frequencies (unsealed) and the no rmal modes, it is found that the BP86/6-31G is the most accurate. The data obtained here has been used to reassign (in contrast to the prev iously reported MP2 results) some of the vibrational frequencies, part icularly, for the main N=N and C-N vibrations of TAB. On the basis of BP86/6-31G force field, the infrared intensities for both TAB and CAB and their isotopomers have also been calculated. Moreover, the main d ifferences in the vibrational spectra of the two isomers of azobenzene have been discussed from normal mode analysis.