Lyman α photolysis of solid nitromethane (CH3NO2) and D3-nitromethane (CD3NO2) – untangling the reaction mechanisms involved in the decomposition of model energetic materials

Abstract

Solid nitromethane (CH₃NO₂) along with its isotopically labelled counterpart D3-nitromethane (CD₃NO₂) ices were exposed to Lyman α photons to investigate the mechanism involved in the decomposition of energetic materials in the condensed phase. The chemical processes in the ices were monitored online and in situ via infrared spectroscopy complimented by temperature programmed desorption studies utilizing highly sensitive reflectron time-of-flight mass spectrometry coupled with pulsed photoionization (ReTOF-PI) at 10.49 eV. The infrared data revealed the formation of cis-methylnitrite (CH₃ONO), formaldehyde (H₂CO), water (H₂O), carbon monoxide (CO), and carbon dioxide (CO₂). Upon sublimation of the irradiated samples, three classes of higher molecular weight products, which are uniquely formed in the condensed phase, were identified via ReTOF-PI: (i) nitroso compounds [nitrosomethane (CH₃NO), nitrosoethane (C₂H₅NO), nitrosopropane (C₃H₇NO)], (ii) nitrite compounds [methylnitrite (CH₃ONO), ethylnitrite (C₂H₅ONO), propylnitrite (C₃H₇ONO)], and (iii) higher molecular weight molecules [CH₃NONOCH₃, CH₃NONO₂CH₃, CH₃OCH₂NO₂, ONCH₂CH₂NO₂]. The mechanistical information obtained in the present study suggest that the decomposition of nitromethane in the condensed phase is more complex compared to the gas phase under collision-free conditions opening up not only hitherto unobserved decomposition pathways of nitromethane (hydrogen atom loss, oxygen atom loss, retro carbene insertion), but also the blocking of several initial decomposition steps due to the ‘matrix cage effect’.