Vibrational and Thermodynamic Properties of 2,2′,4,4′,6,6′-Hexanitroazobenzene and Its Derivatives: A Density Functional Theory Study

HNAB (2,2′,4,4′,6,6′-hexanitroazobenzene) and its derivatives have been optimized to obtain their molecular geometries and electronic structures by using density functional theory at the B3LYP/6-31G* level. Their IR spectra have been computed and assigned by vibrational analysis. The strongest peaks are attributed to the NO asymmetric stretching of nitro groups. Its central position moves towards higher frequency as the number of nitro groups increases. It is obvious that there is hydrogen-bonding between amino and nitro groups in amino derivatives. Based on the frequencies scaled by 0.96 and the principle of statistical thermodynamics, the thermodynamic properties have been evaluated, which are linearly related with the temperature, as well as the number of nitro and amino groups, respectively, obviously showing good group additivity. And the thermodynamic functions for the nitro derivatives increase much more than those for the amino derivatives with the increase of the number of substituents. The values of heat of formation (HOF) for the nitro derivatives increase gradually with n, while those of the amino derivatives decrease smoothly with n.