Excited electronic state decomposition mechanisms and dynamics of nitramine energetic materials and model systems
Date
2007
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Abstract
Energetic materials play an important role in aeronautics, the weapon industry, and the propellant industry due to their broad applications as explosives and fuels. RDX (1,3,5-trinitrohexahydro-s-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), and CL- 20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane)-j are compounds which contain high energy density (J/cm3) or (J/g). Although RDX and HMX have been studied extensively over the past several decades, a complete understanding of their decomposition mechanisms and dynamics is unknown. This work describes the novel approach taken to assist in the overall understanding of the decomposition of these energetic materials, namely their gas phase single molecule excited state decomposition. Excited electronic states can be generated by shock and compression and therefore play an important role in the initiation/decomposition of RDX, HMX, and CL-20. Energy (ns lasers) and time resolved (fs lasers) UV-photodissociation experiments have been performed to elucidate the mechanisms and dynamics of gas phase energetic material decomposition from excited electronic states. Time of flight mass spectroscopy (TOFMS), laser induced fluorescence (LIF), and pump-probe experiments performed on three energetic materials, as well as five model systems, illustrate the unique behavior of energetic materials. TOFMS UV photodissociation (ns) experiments of gas phase RDX, HMX, and CL-20 generate the NO molecule as the initial decomposition product. Four different vibronic transitions of the initial decomposition product, the NO molecule, are observed: A2Σ(υ'=0)<—X2Π(υ"=0,l,2,3). Simulations of the rovibronic intensities for the A<— Xtransitions demonstrate that NO dissociated from RDX, HMX, and CL-20 is rotationally cold (~ 20 K) and vibrationally hot (~ 1800 K). Conversely, experiments on the five model systems (nitromethane, dimethylnitramine, nitropyrrolidine, nitropiperidine and dinitropiperazine) produce rotationally hot and vibrationally cold NO spectra. LIF experiments are performed to rule out the possible decomposition product OH, generated along with NO, perhaps from the suggested HONO elimination mechanism. The OH radical is not observed in the fluorescence experiments, indicatingthe HONO decomposition intermediate is not an important pathway for the excited electronic state decomposition of cyclic nitramines. The NO molecule is also employed to measure the dynamics of the excited statedecomposition. A 226 nm, 180 fs light pulse is utilized to photodissociate the gas phase systems. Stable ion states of DMNA and nitropyrrolidine are observed while the energetic materials and remaining model systems present the NO molecule as the only observed product. Pump-probe transients of the resonant A<—X (0-0) transition of the NO molecule show a constant signal indicating these materials decompose faster than the time duration of the 226 nm laser light. Comparison of NO from the three energetic materials to NO from NO2 gas generated by a 180 fs light pulse at 226 nm indicates that NO2 is not an intermediate product of the excited electronic state photodissociation of RDX, HMX, or CL-20. Two possible excited state decomposition mechanisms are suggested for the three energetic materials. The first mechanism involves a dissociative excited electronic statein which the nitramine moieties (CNNO2) in the electronically excited energetic material isomerize (CNONO) and further dissociate. In the second possible decomposition mechanism the electronically excited molecules undergo internal conversion to very highly excited (~5 eV of vibrational energy) vibrational states of their ground electronic state. Once in the ground state, isomerization of the nitramine moieties occurs and thematerial further decomposes. Calculational results together with the experimental results indicate the energetic materials decompose according to the second mechanism, relaxation to the ground state, while the model systems follow the excited electronic state decomposition pathway. An additional path in which the -NO2 moiety loses an O atom, becomes linear with the CN attachment, and then NO is released, is also consistent with experimental observations but is, as yet, not supported by calculations. The keys to generating better cyclic nitramine energetic materials would then beto enhance the propensity to form Si - So conical intersections, improve Si - So Franck-Condon factors for internal conversion near the Si zero point level, and to enhance the So density of vibronic states at high So vibrational energy. Additionally, one would like to generate NO with less internal vibrational excitation, so altering the NONO vibrational excitation in the dissociation process could be important. These ideas would suggest that more flexible cyclic nitramines, with increased internal degrees of freedom, might be useful to explore for new energetic systems. Perhaps larger ring structures along the lines of CL-20 might be useful compounds to explore.
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Subject
combustion
Explosives -- Biodegradation
nitramines
nitroamines