Further Studies into the Photodissociation Pathways of 2‑Bromo-2-nitropropane and the Dissociation Channels of the 2‑Nitro-2-propyl Radical Intermediate

These experiments investigate the decomposition mechanisms of geminal dinitro energetic materials by photolytically generating two key intermediates: 2-nitropropene and 2-nitro-2-propyl radicals. To characterize the unimolecular dissociation of each intermediate, we form them under collision-free conditions using the photodissociation of 2-bromo-2-nitropropane; the intermediates are formed at high internal energies and undergo a multitude of subsequent unimolecular dissociation events investigated herein. Complementing our prior work on this system, the new data obtained with a crossed-laser molecular beam scattering apparatus with VUV photoionization detection at Taiwan’s National Synchrotron Radiation Research Center (NSRRC) and new velocity map imaging data better characterize two of the four primary 193 nm photodissociation channels. The C–Br photofission channel forming the 2-nitro-2-propyl radicals has a trimodal recoil kinetic energy distribution, P(E T), suggesting that the 2-nitro-2-propyl radicals are formed both in the ground electronic state and in two low-lying excited electronic states. The new data also revise the HBr photoelimination P(E T) forming the 2-nitropropene intermediate. We then resolved the multiple competing unimolecular dissociation channels of each photoproduct, confirming many of the channels detected in the prior study, but not all. The new data detected HONO product at m/e = 47 using 11.3 eV photoionization from both intermediates; analysis of the momentum-matched products allows us to establish that both 2-nitro-2-propyl → HONO + CH3CCH2 and 2-nitropropene → HONO + C3H4 occur. Photoionization at 9.5 eV allowed us to detect the mass 71 coproduct formed in OH loss from 2-nitro-2-propyl; a channel missed in our prior study. The dynamics of the highly exothermic 2-nitro-2-propyl → NO + acetone dissociation is also better characterized; it evidences a sideways scattered angular distribution. The detection of some stable 2-nitropropene photoproducts allows us to fit signal previously assigned to H loss from 2-nitro-2-propyl radicals. Overall, the data provide a comprehensive study of the unimolecular dissociation channels of these important nitro-containing intermediates.