The Elusive Ketene (H₂ CCO) Channel in the Infrared Multiphoton Dissociation of Solid 1,3,5-Trinitro-1,3,5-Triazinane...

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 (H₂ CCO), but not to diazomethane (H₂ CNN; 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 (H₂ CCO) 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.