Internal Rotation of Amino and Nitro Groups in TATB: MP2 Versus DFT (B3LYP)

We report second-order Møller−Plesset (MP2) and density functional (B3LYP) fully optimized amino and nitro group torsional potential barriers in 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a molecule with unusually strong intramolecular hydrogen bonding. The calculated barriers show optimum values of 17.0 (25.5) kcal/mol for the amino group and 5.6 (10.5) kcal/mol for the nitro group rotation at the MP2 (B3LYP) level. The large barrier associated with the amino rotation is due to the double-bond character of the respective C−N bond. Because the B3LYP results are 50−87% higher in energy than those provided by MP2, a more detailed comparison with other functionals and higher-level ab initio methods is needed for an accurate description of these intramolecular motions.