Laser-induced UV photodissociation of 2-bromo-2-nitropropane: Dynamics of OH and Br formation

Abstract

Photoexcitation of 2-bromo-2-nitropropane (BNP) at 248 and 193 nm generates OH, Br, and NO(2) among other products. The OH fragment is detected by laser-induced fluorescence spectroscopy, and its translational and internal state distributions (vibration, rotation, spin-orbit, and Λ-doubling components) are probed. At both 248 and 193 nm, the OH fragment is produced translationally hot with the energy of 10.8 and 17.2 kcal∕mol, respectively. It is produced vibrationally cold (v" = 0) at 248 nm, and excited (v" = 1) at 193 nm with a vibrational temperature of 1870 ± 150 K. It is also generated with rotational excitation, rotational populations of OH(v" = 0) being characterized by a temperature of 550 ± 50 and 925 ± 100 K at 248 and 193 nm excitation of BNP, respectively. The spin-orbit components of OH(X(2)Π) are not in equilibrium on excitation at 193 nm, but the Λ-doublets are almost in equilibrium, implying no preference for its π lobe with respect to the plane of rotation. The NO(2) product is produced electronically excited, as detected by measuring UV-visible fluorescence, at 193 nm and mostly in the ground electronic state at 248 nm. The Br product is detected employing resonance-enhanced multiphoton ionization with time-of-flight mass spectrometer for better understanding of the dynamics of dissociation. The forward convolution analysis of the experimental data has provided translational energy distributions and anisotropy parameters for both Br((2)P(3∕2)) and Br∗((2)P(1∕2)). The average translational energies for the Br and Br∗ channels are 5.0 ± 1.0 and 6.0 ± 1.5 kcal∕mol. No recoil anisotropies were observed for these products. Most plausible mechanisms of OH and Br formation are discussed based on both the experimental and the theoretical results. Results suggest that the electronically excited BNP molecules at 248 and 234 nm relax to the ground state, and subsequently dissociate to produce OH and Br through different channels. The mechanism of OH formation from BNP on excitation at 193 nm is also discussed.

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

“…This implies that the amount of energy partitioned into translation does not change on the basis of the total amount of available energy in the system. Although this is quite strange for C−Br photofission pathways, Saha et al 21 performed the photodissociation of 2-bromo-2-nitropropane at Figure S17. The NO formed from the unimolecular dissociation of NO 2 products formed from primary C−NO 2 photofission is show as the solid blue line fit and it uses the P(E T ,2°) shown in the Supporting Information as Figure S18.…”

“…This plethora of fragments (many with the same mass) causes difficulty in unambiguously determining the primary photodissociation pathway and the subsequent unimolecular decomposition of the photoproduct. This is illustrated in the study of Saha et al 21 where they performed both state selective photodetection of OH radicals and fluorescence experiments on 2-bromo-2-nitropropane after photodissociation at 193 nm. They speculated that the OH radicals could arise from several different pathways but were unable to determine which one(s).…”

“…One class of materials that have the potential to model the decomposition of such intermediates is halonitroalkanes. 21,22 Studies have shown that upon irradiation with 193 nm photons, these materials undergo both Br and HBr loss yielding a nitroalkyl radical and a nitroalkene, respectively. 21,22 Because FOX-7 is being investigated as a potential replacement material for the commonly used RDX and geminal dinitro compounds are commonly used as energetic materials, it is imperative to determine the exact mechanistic steps that these materials undergo upon decomposition.…”

Thermal decomposition pathways for 1,1-diamino-2,2-dinitroethene (FOX-7)

“…Despite the large body of work on this subject, the initial decomposition steps in energetic materials are still debated and the decomposition steps responsible for the large exothermicity are often unresolved. [1][2][3][4][5][6][7][8][9] For energetic materials containing nitro groups, the three most likely initial steps are: HONO elimination, NO 2 loss, and NO loss via a nitro-nitrite isomerization. 1 For example, gas-phase 1 studies of the thermal decomposition of dimethylnitramine (DMNA) and 1,3,3trinitroazetidine (TNAZ) conclude that C-NO 2 fission is the first step in the thermal decomposition mechanism, in agreement with prior infrared multiphoton dissociation experiments in molecular beams.…”

A Novel Mechanism for Nitric Oxide Production in Nitroalkyl Radicals that Circumvents Nitro–Nitrite Isomerization

“…The accepted mechanism for NO loss in nitroalkanes (and other nitroalkyl species) is isomerization to a nitrite intermediate followed by O–N bond cleavage; the energetic barrier to this isomerization also tends to be comparable to but higher than C–NO 2 fission. Interestingly, the local geometry (at the C–NO 2 group) of the nitro–nitrite isomerization transition state (TS) is very similar for a series of nitroalkanes, − 2-nitropropene, and nitroaromatics . Early calculations on the nitro–nitrite isomerization TS in nitromethane were done by Dewar et al in 1985.…”