Infrared multiphoton dissociation (IRMPD) of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in a molecular beam has been performed in order to investigate the mechanism of RDX thermal decomposition. A beam of molecules was crossed by a pulsed TEA CO2 laser and velocity distributions of the various products were measured by the time-of-flight (TOF) technique as a function of the laboratory angle using a mass spectrometric detector. The dissociation channels, their branching ratios, and the translational energy distributions of the products were determined. In contrast to the conventional view of simple bond rupture through loss of NO2 as the dominant primary channel in RDX decomposition, it was found that the dominant primary channel is concerted symmetric triple fission to produce three CH2N2O2 fragments which subsequently undergo secondary concerted dissociation to produce HCN, H2CO, HONO (or HNO2), and N2O. A total of two primary and four secondary dissociation channels were observed. Concerted reactions predominate over simple bond rupture not only in the number of channels (four vs two) but also in the amount of products. A fair amount of translational energy release through concerted reaction channels was observed, which is significant for an explanation of the energies of RDX decomposition.
2-Bromoethanol and 2-chloroethanol were photodissociated in a molecular beam at 193 nm. Only one primary reaction channel was observed, elimination of the halogen atom, with an average translational energy release of 33 kcal/mol. In the case of 2-bromoethanol, some of the C2 H4 OH partner fragment survived and some underwent secondary dissociation to produce C2 H4 and OH. The surviving C2 H4 OH contained up to 43 kcal/mol of internal energy, far more than the expected C2 H4 -OH bond energy of ∼28 kcal/mol. The initial C–Br recoil occurs with a large exit impact parameter and leaves most of the internal energy in C2 H4 OH rotation, creating rotationally metastable fragments. The angular distributions of the secondary C2 H4 and OH products were strongly forward–backward peaked with respect to the primary (C2 H4 OH) velocity vector, consistent with the decay of a long-lived complex in which the total angular momentum is perpendicular to the velocity vector and mainly carried away as orbital angular momentum. This effect is analogous to that observed in the decay of similar long-lived complexes in crossed molecular beam experiments.
The photodissociation processes of benzene following excitation at 193 and 248 nm have been studied by molecular beam photofragmentation translational spectroscopy. When benzene was excited to the 1 B1u state by absorption at 193 nm, dissociation occurred through three primary channels, C6H5+H (80%), C6H4+ H2 (16%), and C5H3+CH3 (4%), following internal conversion to the vibrationally excited ground state. When benzene was excited to the 1 B2u state at 248 nm, two primary dissociation channels, C6H4+H2 (96%), and C5H3+CH3 (4%), were observed. Photodissociation to produce two C3H3 was induced by two photon absorption of benzene at both 193 and 248 nm. Numerous secondary photodissociation processes of the primary photoproducts were also observed at both 193 and 248 nm.
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