THE CRYSTAL-STRUCTURE OF 1,3,5-TRIAMINO-2,4,6-TRINITROBENZENE - CENTROSYMMETRIC OR NONCENTROSYMMETRIC

The crystal structure of the title compound (TATB) has been determined as centrosymmetric (space group P1OBAR, Z=2) by X-ray diffraction, and yet the solid substance displays non-negligible second-harmonic generation. The contradiction is analyzed by means of potential energy calculations for the crystal packing of TATB, using a set of newly calibrated empirical potentials for the N=O...H-N hydrogen bond. It is shown that two non-centrosymmetric (P1) crystal structures are energetically similar to the one with P1OBAR, Z=2, and that libration or reorientation of the whole molecule in its plane within the crystal may be possible, especially at high temperatures. An explanation of the unusual optical behaviour of TATB is attempted in the light of these results.