Internal rotation in energetic systems: TATB

We report second-order Moller-Plesset (MP2) and density functional (DFT) ca lculations of the rotational barrier and torsional potential for intramolec ular rotation of the nitro group in the energetic system 1,3,5-triamino-2,4 ,6-trinitrobenzene (TATB). The calculated rotational barrier is 11.0 and 17 .9 kcal/mol at the MP2/ 6-311G** and B3LYP/6-311G** levels, respectively, t wo to three times higher than that of nitrobenzene and nitroethylene. This is attributed to very strong intramolecular hydrogen bonding in this molecu le, where the oxygen-hydrogen bond is 1.7-1.8 Angstrom at the equilibrium s tructure and well below the van der Waals limit of 2.62 Angstrom. The two-d imensional torsional barrier is determined to be 10.5 kcal/mol at the B3LYP /6-311G** level, showing the effects of substantial nuclear relaxation. The se results should shed further insight on the unusual stability of TATB.