Structure and vibrational spectra of conjugated acids of trans- and cis-azobenzene

The equilibrium geometry and vibrational infrared (IR) and Raman spectra of conjugated acid of trans-azobenzene with the sigma (NH) bond, tABH+, were calculated by the DFT model B3LYP/6-31+G(d,p). In selecting the functional the comparison of not only the vibrational frequencies but also IR and Raman intensities with the experiment was taken into account. The agreement between the experimental Raman spectra of tAB in acidic solutions and the simulated B3LYP/6-31+G(d,p) spectrum of tABH+ can be taken as reliable evidence for formation of the classical azonium monocation and enables us to assign all observed bands. The calculations were extended to the cis isomer, cAB. In both isomers protonation leads to the NN bond elongation and considerable red-shift of the NN stretching mode. It disturbs highly not only normal modes including nitrogen atoms, but also the pure ring modes. According to the calculations the protonated trans and cis species should be easily distinguished by vibrational spectroscopy. It is noteworthy that the B3LYP/6-31+G(d,p) model does not succeed in reproducing Raman intensities of characteristic phenyl ring vibrations. The changes upon the proton binding have been rationalized by considering electron density redistribution using the natural population analysis (NPA) and the natural bond orbital (NBO) energetic analysis.