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Browsing by Author "Bernstein, Elliot R., advisor"

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    Excited electronic state decomposition mechanisms and dynamics of nitramine energetic materials and model systems
    (Colorado State University. Libraries, 2007) Greenfield, Margo, author; Guo, Yuanqing, advisor; Bernstein, Elliot R., advisor
    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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    ItemOpen Access
    Part 1, executed electronic state decomposition of energetic molecules. Part 2, conformation specific reactivity of radical cation intermediates of bioactive molecules
    (Colorado State University. Libraries, 2010) Bhattacharya, Atanu, author; Bernstein, Elliot R., advisor; Levinger, Nancy E., committee member; Van Orden, Alan K., committee member; Szamel, Grzegorz, committee member; Bartels, Randy A., committee member
    Energetic materials have a wide variety of industrial, civil, and military applications. They include a number of organic compounds such as RDX (1,3,5- trinitroheahydro-s-triazine), HMX (octahydro-1,3,5,7-tetranitro-l ,3,5,7-tetrazocine), DAAF (3,3'-diamino-4,4'-azoxyfurazan), DAATO35 (3,3'-azobis(6-amino-l,2,4,5- tetrazine)-mixed N-oxides), etc. These materials release huge chemical energy during their decomposition. The decomposition of energetic materials is initiated with a shock or compression wave or a spark. Such events in solids generate molecules in the excited electronic states. Hence, in order to maximize release of the stored chemical energy in the most efficient and useful manner and to design new energetic materials, the unimolecular decomposition mechanisms and dynamics from excited electronic states should be understood for these systems. This thesis describes understanding about unimolecular decomposition of energetic materials from their excited electronic states. A few fundamental questions at molecular level dealing with electronic excitation of energetic materials are addressed here: (a) what happens immediately after electronic excitation of energetic molecules?; (b) how is excess energy partitioned among product molecules following electronic excitation?; (b) what are the mechanism and dynamics of molecular decomposition?; (d) does nonadiabatic chemistry (a process that span multiple electronic potential energy surfaces) through conical intersection (crossing of different potential energy surfaces) dominate system behavior? Both energy and time resolved spectroscopic techniques are used in this effort. The product internal state (rotational and vibrational) distributions are probed using mass and energy resolved spectroscopic techniques using time-of-flight mass spectrometry (TOFMS) and laser induced fluorescence (LIF) spectroscopy. Analyzing the product internal state distributions, the mechanisms of unimolecular decomposition of energetic molecules from excited electronic states are determined. The femtosecond pump-probe spectroscopic technique is utilized to determine ultrafast decomposition dynamics of these molecules. From a theoretical point of view, multiconfigurational methodologies such as, CASSCF and CASMP2 are used to model the processes involving excited electronic states of energetic molecules. Influence of nonadiabatic chemistry in the overall decomposition of energetic molecules is also theoretically judged. The primary energetic systems whose nonadiabatic chemistry discussed here are the nitramine (e.g., RDX, HMX), furazan (e.g., DAAF), and tetrazine-N-oxide (e.g., DAATO3.5) based energetic species. A number of model systems, which are simple analogue molecules of the large and more complex energetic materials, are studied in detail to understand nonadiabatic energetic behavior of a single energetic moiety of particular class. Subsequently, the decomposition mechanism for more complex energetic systems are studied and compared with that of their model systems. Nitramine energetic materials and model systems undergo nitro-nitrite isomerization followed by IV NO elimination. Nitramine energetic materials dissociates in the ground state generating rotationally cold (20 K) distribution of the NO product. Nitramine model systems dissociates in the excited state surface producing rotationally hot (-120 K) distribution of the NO product. The nitro-nitrite isomerization happens through conical intersection. Furazan based model molecules (e.g., furazan) possess two different pathways of decomposition: ring contraction and ring opening. These two pathways are electronically nonadiabatic. The ring contraction mechanism generates rotationally cold (20 K) product NO and the ring opening mechanism generates rotationally hot (100 K) product NO. Furazan based energetic material (DAAF), however, dissociates only through a ring contraction mechanism. Thus nonadiabatic pathways control the decomposition of furazan based molecules. Decomposition of tetrazine-2,4-dioxide based molecules involves a ring contraction mechanism through (Si/So)ci, producing only rotationally cold (20 K) but vibrationally hot (1200 K) distributions of the NO product. Tetrazine-l,4-dioxde undergoes similar decomposition pathway through (Si/So)ci; however, it produces rotationally hotter (50 K) but vibrationally colder distribution of the NO product. Thus the relative position of the oxygen atoms attached to the tetrazine ring is important parameter along with their nonadiabatic chemistry controlling their final energetic reactivity. Decomposition dynamics of all energetic materials is faster than 180 fs. Considering the influence of conical intersections in the excited electronic state decomposition of energetic materials, rotationally cold N2 product is predicted to be the major decomposition product of high nitrogen content energetic species. The present work infers that generation of internally cold product is an important characteristics of a true energetic molecule. Presence of low lying chemically relevant conical intersections provides a direct pathway of ultrafast decomposition chemistry of energetic molecules. The energy barrier to the low lying chemically relevant conical intersection, in principle, would be a point of interest to make a system more or less energetic.