Periodic DFT study of structural transformations of cocrystal NTO/TZTN under high pressure.

Density functional theory (DFT) periodic calculations were performed to study the geometrical and electronic structures of energetic cocrystal NTO/TZTN under pressures ranging from 0 to 80 GPa. The optimized crystal structure by the GGA/PW91 (Perdew-Wang-91) and dispersion corrections corresponds well with the experimental values under ambient pressure. With the pressure increasing, the lattice constants, unit cell volumes, interatomic distances, H-bond energies, atomic charges, and bond populations of cocrystal NTO/TZTN change gradually. At pressures of 4, 8, and 23 GPa, three structural transformations occurred, shown by the results. The cyclization plays an important role in stabilizing the systems. The increasing pressure contributes to the increase of interaction force gradually. At 4 GPa, a new hydrogen bond O3⋯H5 is formed. At 8 GPa, the formation of eight membered rings is because of the existence of a covalent bond O1-H3 between two NTO molecules. In addition, a covalent interaction is formed between N2 and H4 atoms with the biggest H-bond energy compared to the others. As the pressure reaches 23 GPa, another new hydrogen bond forms between N8 and H5 atoms, which contributes to the formation of a five membered ring between NTO and TZTN. The electrons can move freely according to the results of the density of states between the valence and conduction bands when the pressure is high. This work will provide useful information in understanding the high-pressure effect on the structural transformation.