Hydrogen Bonding and Molecular Rearrangement in 1,3,5-Triamino-2,4,6-trinitrobenzene under Compression

We studied the structural behavior and properties of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under hydrostatic compression using atomistic and electronic (ab initio) level computations. We observed a marked change in the intermolecular hydrogen-bonding network upon compression of the crystal without change in crystal symmetry. The changes in molecular arrangement are found to have a profound impact on various observable properties: energetic, vibrational spectra, structural, and elastic properties. From the analysis of vibrational modes, we observed that the changes are mainly due to the nitro and amino groups. An increase in the number of hydrogen bonding interactions along the c-axis of the crystals results in providing the extra stabilization energy. In addition to analyze the isolated molecule and dimer, this molecular rearrangement is systematically studied and characterized in the condensed phase. From higher-level ab initio calculations, the potential energy surface of the dimer indicates the presence of a region with two local minima within 3.42 kcal/mol difference in energy. Since this behavior is not associated with a change of symmetry of the crystal unit cell, the possible coexistence of two molecular arrangements might lead to the loss of a definitive inversion center for bulk. The calculated elastic constants of the crystal dramatically reveal the changes via large increases in certain components. Implications on the observed pressure-induced rearrangement behavior on the mechanical, optical, and thermodynamic properties of TATB are further discussed, and correlations with experimental spectroscopic data are provided.