MOLECULAR DECOMPOSITION MECHANISMS OF ENERGETIC MATERIALS

La détonation des matériaux énergétiques implique la libération de l'énergie chimique résultant du réarrangement des liaisons chimiques pour former des molécules plus stables. Une compréhension de ces processus chimiques au niveau moléculaire demande une connaissance de la stabilité thermochimique des différents intermédiaires moléculaires qui peuvent être formés. Elle nécessite aussi une connaissance des chemins de réaction possibles pour un réarrangement moléculaire. En particulier, on a besoin de connaître les hauteurs de barrières d'énergie (énergies d'activation) ainsi que les variations d'entropie (facteurs préexponentiels) au goulot de la réaction. Pour déterminer la thermochimie et les chemins de réaction au cours de la décomposition, nous avons développé la méthode de chimie quantique BAC-MP4. Utilisant les calculs de corrections d'additivité de liaison à la théorie des perturbations au 4ème ordre Moller Plesset (Bond - Additivity - Corrections to Moller - Plesset 4th Order), on peut déterminer les énergies des différentes liaisons, les chaleurs de formation, les entropies, et les énergies libres le long des chemins possibles de réaction. Les réactions chimiques dominantes sont fortement dépendantes de la vitesse avec laquelle l'énergie est portée sur le front de choc. La thermochimie et les mécanismes de décomposition sont discutés en fonction de la vitesse de chauffage et de la température. Nous distinguons dans l e processus de décomposition les étapes chimiques qui sont exothermiques, de celles qui sont, par nature, endothermiques. Les résultats sont présentés pour une variété de composés nitrés comprenant les nitro-aliphatiques et les nitramines HMX et RDX.The detonation of energetic materials involves the release of chemical energy resulting from the rearrangement of the chemical bonds to form more stable molecules. An understanding of these chemical processes at the molecular level requires a knowledge of the thermochemical stability of the various molecular intermediates which can be formed. It also necessitates a knowledge of possible reaction pathways for molecular rearrangement. In particular, one needs to know the heights of the energy barriers (activation energies) as well as the changes in entropy (preexponential factors) at the bottleneck to reaction. To determine the thermochemistry and reaction pathways occurring during decomposition, we have developed the BAC-MP4 quantum chemical method. Using Bond- Additivity-Corrections to Møller-Plesset 4th-order pertubation theory calculations, the various bond energies, heats of formation, entropies, and free energies along possible reaction pathways can be calculated. The dominating chemical reactions are strongly dependent on the rate of energy deposited at the shock front. The thermochemistry and decomposition mechanisms are discussed as a function of heating rate and temperature. We distinguish those chemical steps in the decomposition process which are exothermic from those which are inherently endothermic. Results are presented for various nitro compounds including the nitro-aliphatics and the nitramines HMX and RDX

Thermochemistry and Reaction Mechanisms of Nitromethane Ignition

The thermochemistry and reaction mechanisms of nitromethane initiation are modeled using detailed chemical kinetics. Initial conditions correspond to gaseous nitromethane at atmospheric and liquid-like densities and initial temperatures between 1100 and 2000 K. Global reactions as well as elementary reactions are identified for each of the two stages of ignition. The chemical steps to convert the nitro group to N2 involve a complex set of elementary reactions. The time-dependence of the ignition steps (ignition delay times) as a function of temperature and pressure is used to determine effective activation energies and pressure dependencies to ignition. The ignition delay times range from several nanoseconds to tens of microseconds. At atmospheric conditions, the delay times for both ignition stages are in excellent agreement with observed experimental data. At the high densities, the ignition times at these elevated temperatures appear to be dominated by the same reaction mechanism that occurs for atmospheric gaseous nitromethane initiation. This is to be contrasted with lower temperature, condensedphase ignition studies where it appears that solvent-assisted reactions dominate

Heats of formation and bond energies in group III compounds

We present heats of formation and bond energies for Group-III compounds obtained from calculations of molecular ground-state electronic energies. Data for compounds of the form MX0 are presented, where M = B, Al, Ga, and In, X = H, Cl, and CH3, and n = 1-3. Energies for the B, Al, and Ga compounds are obtained from G2 predictions, while those for the In compounds are obtained from CCSD(T)/CBS calculations ; these are the most accurate calculations for indium-containing compounds published to date. In most cases, the calculated thermochemistry is in good agreement with published values derived from experiments for those species that have well-established heats of formation. Bond energies obtained from the heats of formation follow the expected trend (Cl >> CH3 - H). However, the CH3M-(CH3)2 bond energies obtained for trimethylgallium and trimethylindium are considerably stronger (> 15 kcal mol-1) than currently accepted values

Contribution to the modeling of CVD silicon carbide growth

The modeling of the growth of silicon carbide from the vapor phase in the Si-C-H system requires a good understanding of the gas-phase chemistry. The object of this paper is to complement the previous studies on the kinetic modeling of the gas-phase in the system SiH4 / C3H8. To date, kinetic approaches to modeling the gas-phase chemistry have not been fully developed Previous kinetic models have only dealt with the pyrolysis of individual precursors (silane and propane) without allowing for the formation of organosilicon species. This study provides a progress report on our efforts to develop a full gas-phase mechanism that includes organosilicon compounds. Rate constants for this mechanism are determined where possible from experimental data available in the literature. However, for several important reactions, experimental data are not available. Consequently, we are performing ab initio calculations to determine activation energies and are using RRKM calculations to estimate pressure fall-off effects for unimolecular reactions. In this contribution, we focus on the formation of methylsilane H3SiCH3 and discuss the importance of such species in the gas-phase chemistry of SiC deposition

Variational transition state theory calculations of tunneling effects on concerted hydrogen motion in water clusters and formaldehyde/water clusters

The direct participation of water molecules in aqueous phase reaction processes has been postulated to occur via both single-step mechanisms as well as concerted hydrogen atom or proton shifts. In the present work, simple prototypes of concerted hydrogen atom transfer processes are examined for small hydrogen-bonded water clusters -- cyclic trimers and tetramers -- and hydrogen-bonded clusters of formaldehyde with one and two water molecules. Rate constants for the rearrangement processes are computed using variational transition state theory, accounting for quantum mechanical tunneling effects by semiclassical ground-state adiabatic transmission coefficients. The variational transition state theory calculations directly utilize selected information about the potential energy surface along the minimum energy path as parameters of the reaction path Hamiltonian. The potential energy information is obtained from ab ignite electronic structure calculations with an empirical bond additivity correction (the BAC-MP4 method). Tunneling is found to be very important for these concerted rearrangement processes -- the semiclassical ground-state adiabatic transmission coefficients are estimated to be as high as four order of magnitude at room temperature. Effects of the size of the cluster (number of water molecules in the cyclic complex) are also dramatic -- addition of a water molecule is seen to change the calculated rates by orders of magnitude. 36 refs., 10 figs