Probing the Reaction Mechanisms Involved in the Decomposition of Solid 1,3,5-Trinitro-1,3,5-triazinane by Energetic Electrons

Abstract

The decomposition mechanisms of 1,3,5-trinitro-1,3,5-triazinane (RDX) have been explored over the past decades, but as of now, a complete picture on these pathways has not yet emerged, as evident from the discrepancies in proposed reaction mechanisms and the critical lack of products and intermediates observed experimentally. This study exploited a surface science machine to investigate the decomposition of solid-phase RDX by energetic electrons at a temperature of 5 K. The products formed during irradiation were monitored online and in situ via infrared and UV–vis spectroscopy, and products subliming in the temperature programmed desorption phase were probed with a reflectron time-of-flight mass spectrometer coupled with soft photoionization at 10.49 eV (ReTOF-MS-PI). Infrared spectroscopy revealed the formation of water (H2O), carbon dioxide (CO2), dinitrogen oxide (N2O), nitrogen monoxide (NO), formaldehyde (H2CO), nitrous acid (HONO), and nitrogen dioxide (NO2). ReTOF-MS-PI identified 38 cyclic and acyclic products arranged into, for example, dinitro, mononitro, mononitroso, nitro–nitroso, and amines species. Among these molecules, 21 products such as N-methylnitrous amide (CH4N2O), 1,3,5-triazinane (C3H9N3), and N-(aminomethyl)­methanediamine (C2H9N3) were detected for the first time in laboratory experiments; mechanisms based on the gas phase and condensed phase calculations were exploited to rationalize the formation of the observed products. The present studies reveal a rich, unprecedented chemistry in the condensed phase decomposition of RDX, which is significantly more complex than the unimolecular gas phase decomposition of RDX, thus leading us closer to an understanding of the decomposition chemistry of nitramine-based explosives.

Investigating the Photochemical Decomposition of Solid 1,3,5-Trinitro-1,3,5-triazinane (RDX)

“…m / z = 81 has been often associated with triazine (C 3 H 3 N 3 ; IE = 9.80 eV) , while m / z = 87 is likely associated as a fully hydrogenated counterpart of 1,3,5-triazine, i.e., 1,3,5-triazinane (C 3 H 9 N 3 ). Although m / z = 81 has been observed in several experiments, , there is only a single report of product at m / z = 87 . Signal at m / z = 88 can be ascribed to mononitro compound with the molecular formulas CN 2 O 3 or C 2 H 4 N 2 O 2 following the assignments reported by Schroeder .…”

“…The signal at m / z = 98 is allocated to a mononitroso compound holding the molecular formula C 2 H 2 N 4 O. Mononitroso species have been proposed to form during RDX decomposition in previous experimental studies. , The TPD profile recorded at m / z = 99 can be associated with the hydrogenated form of oxy- s y m -triazine (C 3 H 3 N 3 O; m / z = 97), hence assigned to the molecular formula C 3 H 5 N 3 O. Although, m / z = 97 has been observed in previous condensed phase studies investigating the decomposition of RDX, the signature of m / z = 99 has not been observed before.…”

“…This channel has also been observed in the decomposition of solid phase RDX via infrared laser leading eventually to the formation of ketene (H 2 CCO) and water (H 2 O) . Thermal decomposition studies disclose yet another primary mechanism, which involves a concerted elimination of nitrous acid (HONO) along with the formation of triazine (C 3 H 3 N 3 ). ,, Very recently, the dissociation of RDX conducted via energetic electrons revealed that the products involving the N–NO 2 homolysis and HONO elimination are more prevailing than the triple dissociation . Although several discrepancies revolve around the initial step of decomposition, the proposed reaction schemes in the aforementioned studies could elucidate the decomposition of RDX from its ground electronic state (S 0 ).…”

“…Over the past decades, copious studies have been devoted to investigate the fundamental chemical processes involved in the fragmentation of RDX (Scheme ). − Diverse methods ranging from shock waves, infrared (IR) lasers, electron beams to ultraviolet (UV) lasers have been employed to initiate the complex dissociation mechanism(s), but the very first reaction step(s) that initialize the decomposition is (are) still debated. ,− …”

Identification of Elusive Keto and Enol Intermediates in the Photolysis of 1,3,5-Trinitro-1,3,5-Triazinane

“…Behrens and co-workers predicted the existence of both the keto (C 3 H 3 N 3 O; 2a ) and enol (C 3 H 2 N 3 OH; 2b ) tautomers of oxy- s -triazine (OST) in the thermal decomposition of RDX ( 1 ) (Scheme ). , Successive studies exploiting photolysis, electron exposure, and pyrolysis − of RDX detected signal at m / z = 97, i.e., the molecular parent ion of C 3 H 3 N 3 O, upon nonisomer selective ionization of the decomposition products in the gas phase but failed to unambiguously assign the structural isomer(s). Therefore, despite the potentially critical role of the tautomer pair 2a – 2b in the elucidation of the underlying decomposition mechanisms of RDX, the identification of 2a and 2b is still elusive.…”