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

Abstract

Energetic materials such as 1,3,5-trinitro-1,3,5triazinane (RDX) are known to photodissociate when exposed to UV light. However, the fundamental photochemical process(es) that initiate the decomposition of RDX is (are) still debatable. In this study we investigate the photodissociation of solid-phase RDX at four distinct UV wavelengths (254 nm (4.88 eV), 236 nm (5.25 eV), 222 nm (5.58 eV), 206 nm (6.02 eV)) exploiting a surface science machine at 5 K. We also conducted dose-dependent studies at the highest and lowest photon energy of 206 nm (6.02 eV) and 254 nm (4.88 eV). The products were monitored online and in situ via infrared spectroscopy. During the temperatureprogrammed desorption phase, the subliming products were detected with a reflectron time-of-flight mass spectrometer coupled with soft-photoionization at 10.49 eV (PI-ReTOF-MS). Infrared spectroscopy revealed the formation of small molecules including nitrogen monoxide (NO), nitrogen monoxide dimer ([NO] 2 ), dinitrogen trioxide (N 2 O 3 ), carbon dioxide (CO 2 ), carbon monoxide (CO), dinitrogen monoxide (N 2 O), water (H 2 O), and nitrite group (−ONO) while ReTOF-MS identified 32 cyclic and acyclic products. Among these, 11 products such as nitryl isocyanate (CN 2 O 3 ), 5-nitro-1,3,5-triazinan-2-one (C 3 H 6 N 4 O 3 ) and 1,5-dinitro-1,3,5-triazinan-2-one (C 3 H 5 N 5 O 5 ) were detected for the first time in photodecomposition of RDX. Dose-dependent in combination with wavelength-dependent photolysis experiments aid to identify key primary and secondary products as well as distinguished pathways that are more preferred at lower and higher photon energies. Our experiments reveled that N−NO 2 bond fission and nitro−nitrite isomerization are the initial steps in the UV photolysis of RDX. Reaction mechanisms are derived by comparing the experimental findings with previous electronic structure calculations to rationalize the origin of the observed products. The present study can assist in understanding the complex chemistry behind the photodissociation of electronically excited RDX molecule, thus bringing us closer to unraveling the decomposition mechanisms of nitramine-based explosives.

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

“…Although oxy-s-triazine (m/z = 97) is observed in high dose experiments at both 254 and 206 nm photolysis, in the case of very low dose (0.10 ± 0.02 eV molecule −1 at 254 nm and 0.20 ± 0.02 eV molecule −1 at 206 nm) experiments the signal at m/z = 97 is observed only at 206 nm, suggesting that the formation of oxy-s-triazine is preferential at a higher photon energy. 15 It is important to recall that 1,3,5-triazine represents also a wavelength-selective product, as was observed only during the photolysis at 254 nm (Figure 3g,i). These observations imply that the decomposition branching ratio is wavelength-dependent and that products such as oxy-s-triazine and 1,3,5-triazine could be tracers of wavelength-dependent decomposition pathways.…”

“…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.…”

“… , A silver substrate coated with a thin film of RDX (99.99%) at a thickness of 16 ± 1 μm was cooled to 5.0 ± 0.1 K. Ultrahigh purity RDX was prepared through recrystallization of production-grade RDX and characterized through NMR, UV, and FTIR spectroscopy (Figures S3–S5). The bond dissociation processes were triggered by exposing distinct films of RDX in separate experiments to 254 nm (4.88 eV) and 206 nm (6.02 eV) light accessing the n → π* and π R → π*transitions of RDX, respectively (Table S1). These photon energies are well below the adiabatic ionization energy of RDX in the condensed phase, which has been calculated to range from 9.9 to 10.2 eV. , The photolysis was performed at doses of 10.7 ± 1.0 and 22.3 ± 2.0 eV molecule –1 at 254 (4.88 eV) nm and 206 (6.02 eV) nm wavelengths, respectively (Table S2).…”

“…To identify these isomeric forms, we used SciFinder’s web-based structure search program. Considering the fact that the product at m / z = 97 is formed at the early stage of RDX decomposition, its structure must retain the −N–C–N–C–N– or −C–N–C–N–N– skeletal framework of RDX. Therefore, we selected only those cyclic and acyclic isomeric species that have the aforementioned structural backbone, are neutral, and possess functional groups similar to those of proposed structures of products at m / z = 97 and 81.…”

C2H5NO Isomers: From Acetamide to 1,2-Oxazetidine and Beyond

“…During UV photodissociation experiments 57 conducted at 5 K of the explosive RDX (1,3,5-trinitro-1,3,5-triazinane), mass to charge ratio peaks at 59 + were detected by a time-of-flight mass spectrometer during the temperature-programmed desorption phase; this was attributed to the presence of both methylamino methanol, HO–CH 2 –N=CH 2 , and O -methyloxime formaldehyde, H 3 C–O–N=CH 2 .…”