Initial Thermal Decomposition Mechanism of (NH2)2C=C(NO2)(ONO) Revealed by Double-Hybrid Density Functional Calculations

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

The initial decomposition pathways of α-FOX-7 in the condensed phase (crystal) were investigated via density functional theory. Calculations were carried out using three FOX-7 systems with increasing complexity from 1-layer (sheet) via 2-layer (surface) to 3-layer (bulk). The encapsulated environment of the central α-FOX-7 molecule, where decomposition takes place, is reconstructed by neighbouring molecules following a crystal structure. A minimal number of neighbouring molecules that have an impact on the energetics of decomposition are identified among all surrounding molecules. The results show that the presence of intermolecular hydrogen bonds due to the encapsulated environment in the condensed phase decreases the sensitivity of α-FOX-7, i.e. it increases the barrier of decomposition, but it does not alter the initial decomposition pathways of the reaction compared to the gas phase. Moreover, increasing the complexity of the system from a single gas phase molecule via sheet and surface to bulk increases the decomposition barriers. The calculations reveal a remarkable agreement with experimental data [A. M. Turner, Y. Luo, J. H. Marks, R. Sun, J. T. Lechner, T. M. Klapötke and R. I. Kaiser, Exploring the Photochemistry of Solid 1, 1-Diamino-2, 2-Dinitroethylene (FOX-7) Spanning Simple Bon Ruptures, Nitro-to-Nitrite Isomerization, and Nonadiabatic Dynamics, J. Phys. Chem. A, 2022, 126, 29, 4747-4761] and suggest that the initial decomposition of α-FOX-7 likely takes place at the surface of the crystal.

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

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.

The potential energy profile of the decomposition of 1,1-diamino-2,2-dinitroethylene (FOX-7) in the gas phase

The decomposition products of 1,1-diamino-2,2-dinitroethylene (FOX-7) in the gas phase.