Ultraviolet-Initiated Decomposition of Solid 1,1-Diamino-2,2-dinitroethylene (FOX-7)

Abstract

FOX-7 (1,1-diamino-2,2-dinitroethylene) was photolyzed with 202 nm photons to probe reaction energies, leading to the decomposition of this energetic material and to compare results from irradiations using lower-energy 532 and 355 nm photons as well as higher-energy electrons. The photolysis occurred at 5 K to suppress thermal reactions, and the solid samples were monitored using Fourier transform infrared spectroscopy (FTIR), which observed carbon dioxide (CO2), carbon monoxide (CO), cyanide (CN–), and cyanate (OCN–) after irradiation. During warming to 300 K, subliming products were detected using electron-impact quadrupole mass spectrometry (EI-QMS) and photoionization time-of-flight mass spectrometry (PI-ReTOF-MS). Five products were observed in QMS: water (H2O), carbon monoxide (CO), nitric oxide (NO), carbon dioxide (CO2), and cyanogen (NCCN). The ReTOF-MS results showed overlap with electron irradiation products but also included three intermediates for the oxidation of ammonia and nitric oxide: hydroxylamine (NH2OH), nitrosamine (NH2NO), and the largest product at 76 amu with the proposed assignment of hydroxyurea (NH2C(O)NHOH). These results highlight the role of reactive oxygen intermediates and nitro-to-nitrite isomerization as key early reactions that lead to a diverse array of decomposition products.

Initial decomposition pathways of 1,1-diamino-2,2-dinitroethylene (α-FOX-7) in the condensed phase

“…Since the product molecules in Fig. 3 have been shown to agree with the experiments well, 20–22,25 the focus of the condensed phase study is on how the encapsulated environments alternate the energetics. It should be noted that in the current setup, only the central FOX-7 molecule in the sheet is set to be reactive - in the initial structure for geometry optimization, only its structure deviates from the crystal structure.…”

“…Considering that almost all FOX-7 molecules remain intact after the radiation from 355 and 532 nm lasers, this setup mimics the initial decomposition. 22…”

“…18 Numerous experimental studies have been attempted to establish the interplay between the chemical structure and properties of FOX-7. 4,13,18–23 Jones and co-workers studied the thermal decomposition of FOX-7 in different phases with simultaneous thermogravimetry-differential thermal analysis-Fourier transform infrared spectroscopy-mass spectrometry (TG-DTA-FTIR-MS) measurements and differential scanning calorimetry. They observed a two-step decomposition process: the first step yields water (H 2 O), carbon dioxide (CO 2 ), dinitrogen oxide (N 2 O), hydrogen cyanide (HCN), nitrogen dioxide (NO 2 ), and isocyanic acid (HOCN), and the second step forms nitrous acid (HONO) and formic acid (HCOOH).…”

“…19 Recently, in a series of studies of photon-induced photolysis of FOX-7, Turner et al observed that the NO 2 and amino radical (NH 2 ) loss channels are not accessible with 355 and 532 nm photons, while nitrogen oxide (NO) was observed only with the 355 nm photon. 20,22,25…”