A high-pressure far- and mid-infrared study of 1,1-diamino-2,2-dinitroethylene

Abstract

High pressure infrared (IR) measurements of the insensitive explosive, 1,1-diamino-2,2-dinitroethylene (DADNE or FOX-7), have been performed in the far- and mid-IR spectral regions up to ∼28 GPa using a synchrotron source. Petroleum jelly and KBr were employed as the pressurizing media for the far- and mid-IR experiments, respectively. In both experiments, IR spectra were collected at various pressures both in compression and decompression to determine reversibility under pressure cycling. There is evidence for at least two and possibly three phase transitions in the samples at pressures near 2, 5, and10 GPa, respectively. In the high frequency region (∼3000–3300 cm−1), the NH2 symmetric and antisymmetric vibrational modes soften with increasing pressure, suggesting strengthening intermolecular hydrogen bonding, and then stiffen with further pressure increase. At higher pressures (above 5 GPa), we suspect progressive flattening of the zig zag structure characteristic of the α phase occurs, which was observed in the low temperature, quenched γ phase. FOX-7 was also observed to survive pressure cycling up to 28 GPa contrary to earlier reports of pressure-induced molecular decomposition.

Density Functional Theory Calculations of Pressure Effects on the Structure and Vibrations of 1,1-Diamino-2,2-dinitroethene (FOX-7)

“…This behavior was attributed to the strengthening of hydrogen bonding in FOX-7 under pressure. 18,22 As we have already mentioned, the experimental Raman spectra showed several changes around 2 GPa, which were attributed to changes in the molecular conformation. 18 The spectral changes at this pressure included occurrence of new peaks at 151 and 607 cm −1 and disappearance of a peak at 499 cm −1 .…”

Initial decomposition mechanism for the energy release from electronically excited energetic materials: FOX-7 (1,1-diamino-2,2-dinitroethene, C2H4N4O4)

“…The FOX-7 crystal appears golden yellow and has two-dimensional wave-shaped layers with extensive intermolecular hydrogen bonding within the layers and with weaker van der Waals interactions between the layers. 14,15 Stresses in the crystal are reduced by the relative movement of layers, and in this manner, the crystal is not sensitive to mechanical strain. 5 Compared to the presently most widely used and powerful explosives RDX and HMX, FOX-7 yields the same number of moles (0.0405) of gaseous product per gram of compound for the complete decomposia) Author to whom correspondence should be addressed.…”

Structural, vibrational, and quasiparticle band structure of 1,1-diamino-2,2-dinitroethelene from ab initio calculations

“…However, they found that no lattice transformations could be observed up to 8 GPa under hydrostatic conditions. Recent experiments at different temperatures and pressures found three possible phase transitions (at 2 GPa, 5 GPa and above 10 GPa) in mid and far IR regions 11,12 . Pravica et al 11 reported the strengthening of the hydrogen bond and softening of NH 2 stretching frequencies at high pressure by using IR experiments.…”

“…Starting from the synthesis of this compound in 1998 3 , several studies were carried out to investigate its structural properties from ambient 4 to extreme conditions [5][6][7][8] , its thermal behavior with decomposition mechanism 9,10 and vibrational spectroscopy (IR and Raman) at different pressures and temperatures 5,11,12 . Beeman and Ostrak 4 determined the crystal structure of FOX-7 to be monoclinic with the space group P2 1 /n, which contains four molecules (56 atoms) per unit cell.…”