The Elusive Ketene (H₂CCO) Channel in the Infrared Multiphoton Dissociation of Solid 1,3,5‐Trinitro‐1,3,5‐Triazinane (RDX)

Abstract

Understanding of the fundamental mechanisms involved in the decomposition of 1,3,5‐trinitro‐1,3,5‐triazinane (RDX) still represents a major challenge for the energetic materials and physical (organic) chemistry communities mainly because multiple competing dissociation channels are likely involved and previous detection methods of the products are not isomer selective. In this study we exploited a microsecond pulsed infrared laser to decompose thin RDX films at 5 K under mild conditions to limit the fragmentation channels. The subliming decomposition products during the temperature programed desorption phase are detected using isomer selective single photoionization time‐of‐flight mass spectrometry (PI‐ReTOF‐MS). This technique enables us to assign a product signal at m/z=42 to ketene (H2CCO), but not to diazomethane (H2CNN; 42 amu) as speculated previously. Electronic structure calculations support our experimental observations and unravel the decomposition mechanisms of RDX leading eventually to the elusive ketene (H2CCO) via an exotic, four‐membered ring intermediate. This study highlights the necessity to exploit isomer‐selective detection schemes to probe the true decomposition products of nitramine‐based energetic materials.

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

“…H 2 CO (formaldehyde) either dissociates to CO (carbon monoxide; 26 ; 28 amu) and hydrogen and/or it reacts with another formaldehyde molecule to form ketene, 27 (H 2 CCO; 42 amu), and water (H 2 O; 18 amu). The later pathway, i.e., formation of ketene was predicted computationally and verified experimentally in our previous study . The detection of products holding masses of 105, 101, 74, 58, 59, 42, and 31 amu in our experiment (Figure ) further validates the likelihood of the C–N bond rupture channel in the UV decomposition of RDX.…”

“…Product signal at m / z = 42 has been observed in several studies investigating the decomposition of RDX. Although, m / z = 42 is assigned to diazomethane (H 2 CNN; IE = 8.99 eV) in most of the studies, ,, our recent investigation on RDX decomposition exploiting isomer selective soft-photoionization technique enabled us to assign it to ketene (H 2 CCO; IE = 9.62 eV) . The ion signal at m / z = 44 can be ascribed to dinitrogen monoxide (N 2 O; IE = 12.89 eV) and/or methane diazene holding the molecular formula CH 4 N 2 (IE = 8.81 eV).…”

“…Finally, the decomposition mechanisms reported through gas-phase and condensed-phase calculations ,,,, were exploited to rationalize the formation of the observed species. Further experiments are planned, exploiting isomer-selective photoionization ,, to explicitly verify the structures of the decomposition products and hence the nature of the isomer(s) formed. Experimental results will be combined with theoretical simulations to unravel the complex decomposition pathways of RDX.…”

“…The decomposition of condensed phase RDX via thermal energy, shock waves, and infrared laser exposure suggest that the N–NO 2 bond fission represents the primary pathway. ,, On the contrary, infrared multiphoton dissociation of RDX in molecular beams in the gas phase reveals a triple C–N bond rupture leading to three methylene nitramine molecules (H 2 CN–NO 2 ) as the dominating channel . 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 .…”

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

“…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. An identification of these transient species is fundamental in our understanding of the dominating decomposition chemistry of complex organic molecules, including molecular energetic materials like RDX. − Transients such as 2a and 2b could be a key source of hydroxy (OH) radical; ,, thus, the identification of enols would critically constrain and impact our knowledge on the decomposition mechanisms of nitramine-based energetic materials.…”