Electron-Induced Decomposition of Solid 1,1-Diamino-2,2-dinitroethylene (FOX-7) at Cryogenic Temperatures

Abstract

Solid FOX-7 (1,1-diamino-2,2-dinitroethylene), an energetic material of interest due to its high stability and low shock/thermal sensitivity, was exposed to energetic electrons at 5 K to explore the fundamental mechanisms leading to decomposition products and provide a better understanding of the reaction pathways involved. As a result of the radiation exposure, infrared spectroscopy revealed carbon dioxide (CO2) and carbon monoxide (CO) trapped in the FOX-7 matrix, while these compounds along with water (H2O), nitrogen monoxide (NO), and cyanogen (C2N2) were detected exploiting quadrupole mass spectrometry both during irradiation and during the warming phase from 5 to 300 K. Photoionization reflectron time-of-flight mass spectrometry detected small molecules such as ammonia (NH3), nitrogen monoxide (NO), and nitrogen dioxide (NO2) as well as more complex molecules up to 96 amu. Potential reaction pathways are presented and assignments are discussed. Among the reaction mechanisms, the importance of an initial nitro-to-nitrite isomerization is highlighted by the observed decomposition products.

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

“…Reaction schemes from FOX-7 (top) showing decomposition pathways toward observed products (boxes with the mass-to-charge ratio). Products in red boxes were also observed during the irradiation with electrons, while those in green boxes are newly observed in this study. The pathway for reaction 1 is labeled R1 and so forth.…”

“…While computational studies often consider the energetics and decomposition mechanisms for individual reactions, purely experimental investigations of FOX-7 usually focus on bulk properties, and thus molecular-scale computational findings rarely reconcile with experiments. Here, we present experimental results of the 202 nm photon-initiated decomposition of solid FOX-7 at 5 K that extends a systematic investigation of previously reported irradiations using 532 and 355 nm photons along with energetic electrons in order to elucidate how molecular decomposition processes relate to the impinging energy and reaction energetics . The 532 nm photolysis only generated a small quantity of molecular oxygen (O 2 ), while 355 nm photolysis produced four times greater molecular oxygen (O 2 ) along with nitric oxide (NO).…”

“…In contrast, the rate constants from electron irradiation ( k CC = 6.8 × 10 –4 s –1 , k NH = 2.1 × 10 –3 s –1 , and k NO 2 = 1.3 × 10 –3 s –1 ) display greater variance between their valuesa 1:3:2 ratioand are 30 to 90 times larger in magnitude than the rate constants from 202 nm photolysis. Unlike the infrared signals from electron irradiation, which showed an asymptotic behavior as the uppermost 320 nm FOX-7 layer decomposed, the spectra from 202 nm irradiation show no such behavior, suggesting that the FOX-7 sample transmits a large portion of the irradiation and the sample is processed at all depths. Based on the number of reacted molecules and the photon flux, 300 photons impinged the sample on average for every reacted molecule.…”

“…In contrast, a negligible quantity of products degassed from the sample during 202 nm irradiation, and instead, the products were observed during TPD. By depositing known quantities of CO 2 or NO ices and utilizing ionization cross sections, previous experiments performed QMS calibrations that reported conversion factors ( K ) between the number of detected molecules and the number of molecules that had sublimed. The factors for these five products are K H 2 O = 6.5 × 10 5 , K CO = 5.8 × 10 5 , K NO = 4.2 × 10 5 , K CO 2 = 5.0 × 10 5 , and K NCCN = 3.5 × 10 5 , meaning one in every 3.5 × 10 5 molecules of NCCN that sublime from the sample is detected by QMS.…”

“…During photolysis, Fourier transform infrared (FTIR) spectra were collected using an external Thermo Nicolet 6700 along a dry nitrogen-purged beam path that passes into and out of the UHV chamber using differentially pumped potassium bromide (KBr) windows. The infrared beam interacts with the sample through absorption–reflection–absorption at a 44° angle of incidence and is recorded using an external liquid nitrogen-cooled mercury–cadmium–telluride type-B (MCT-B) detector. ,,, After 2.5 h of irradiation, the sample was warmed to 320 K at 1 K min –1 as a temperature-programmed desorption (TPD) protocol. During TPD, subliming molecules were detected using photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS, Jordan TOF Products) utilizing 10.49 eV photons generated by tripling the third harmonic (355 nm) of the Nd:YAG laser using nonresonant four-wave mixing (ω 2 = 3ω 1 ) with pulsed xenon gas as the nonlinear medium .…”