First-Principle Studies of the Initiation Mechanism of Energetic Materials

It is important to understand the initiation mechanism of energetic materials to improve and engineer them. In this thesis first-principle calculation is used to study the initiation of several explosives and propellants. The second chapter is focused on a new energetic material, silicon pentaerythritol tetranitrate (Si-PETN), DFT calculations have identified the novel rearrangement that explains the very dramatic increase in sensitivity observed experimentally. The critical difference is that Si-PETN allows a favorable five-coordinate transition state in which the new Si−O and C−O bonds form simultaneously, leading to a transition state barrier of 33 kcal/mol (it is 80 kcal/mol for PETN) and much lower than the normal O−NO2 bond fission observed in other energetic materials (40 kcal/mol). In addition this new mechanism is very exothermic (45 kcal/mol) leading to a large net energy release at the very early stages of Si-PETN decomposition. The third chapter is about nitrogen-rich compounds, which has high heat of formation and releases the energy by decomposing into stable N2 molecules. Two families of compounds, azobistetrazoles and azobistriazoles, were studied. Based on the calculated mechanisms, for azobistetrazoles with four N atoms in the five-member ring, a clearly-defined N=N fragment can always be found in the ring, and its decomposition starts with ring-opening to free one end of N=N followed by N2 dissociation and heat generation. This barrier is around 28-35 kcal/mol, which is low enough to dominate the sensitivity of material. For azobistriazoles, only 1,1’-azobis-1,2,3-triazole has a N=N fragment in the original 5-member ring and similar ring-opening - N2 dissociation pathway is favored. For the remaining compounds, an additional isomerization is necessary to release N2, which gives the barrier around 55~60 kcal/mol, making these compound less sensitive. The fourth chapter shifts focus to hypergolic propellants. DFT calculations with B3LYP functional was applied to study the hypergolic reaction between N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-Tetramethylmethylenediamine (TMMDA) and HNO3. Bond energies in TMEDA and TMMDA were calculated and compared with their alkane analogues to demonstrate that the lone-pair electrons on N atoms plays the role of activating adjacent chemical bonds. Two key factors relating to the ignition delay were calculated at atomistic level. The first factor is the exothermicity of the formation of the dinitrate salt of TMEDA and TMMDA. Because of the shorter distance between basic amines in TMMDA, it is more difficult to protonate both amines for the stronger electrostatic repulsion, resulting in the smaller heat of dinitrate salt formation by 6.3kcal/mol. The second factor is the reaction rate of TMEDA and TMMDA reacting with NO2 to the step that releases enough heat and more reactive species to propagate reaction. In TMEDA, the formation of the intermediate with C-C double bond and the low bond energy of C-C single bond provide a route with low barrier to oxidize C. Both factors can contribute to the shorter ignition delay of TMEDA. The fifth chapter is about the other pair of hypergolic propellant, monomethylhydrazine (MMH) with oxidizers NO2/N2O4. Experimentally several IR-active species were identified in the early reactions, including HONO, monomethylhydrazinium nitrite (MMH•HONO), methyl diazene (CH3N=NH), methyl nitrate (CH3ONO2), methyl nitrite (CH3ONO), nitromethane (CH3NO2), methyl azide (CH3N3), H2O, N2O and NO. In order to elucidate the mechanisms by which these observed products are formed, we carried out quantum mechanics calculations (CCSD(T)/6-31G**//M06-2X/6-311G**++) for the possible reaction pathways. Based on these studies, we proposed that the oxidation of MMH in an atmosphere of NO2 occurs via two mechanisms: (1) sequential H-abstraction and HONO formation, and (2) reaction of MMH with asymmetric ONONO2, leading to formation of methyl nitrate. These mechanisms successfully explain all intermediates observed experimentally. We concluded that the formation of asymmetric ONONO2 is assisted by an aerosol formed by HONO and MMH that provides a large surface area for ONONO2 to condense, leading to the generation of methyl nitrate. Thus we proposed that the overall pre-ignition process involves both gas-phase and aerosol-phase reactions. The sixth chapter is about another pair of hypergolic propellant, unsymmetrical dimethylhydrazine (UDMH) with oxidizers NO2/N2O4. We carried out the same level of quantum mechanics calculations as MMH to study this pair. We proposed that the oxidation of UDMH in an atmosphere of NO2 occurs via two mechanisms, similar with MMH: (1) sequential H-abstraction and HONO formation in gas phase, which has no more than 20 kcal/mol barrier and leads to the production of (CH3)2NNO and HONO. (2)UDMH reacts with asymmetric ONONO2 in aerosol phase, leading to formation of CH3N3 and then CH3ONO2, with a 26.8 kcal/mol enthalpic barrier, which is 10 kcal/mol higher than the corresponding reaction barrier for MMH. Thus we predicted the low production rate of CH3ONO2 for UDMH/NO2 pair. Experimental evidences support our mechanisms for both MMH and UDMH reacting with NO2.</p

Provable Sparse Tensor Decomposition

We propose a novel sparse tensor decomposition method, namely Tensor Truncated Power (TTP) method, that incorporates variable selection into the estimation of decomposition components. The sparsity is achieved via an efficient truncation step embedded in the tensor power iteration. Our method applies to a broad family of high dimensional latent variable models, including high dimensional Gaussian mixture and mixtures of sparse regressions. A thorough theoretical investigation is further conducted. In particular, we show that the final decomposition estimator is guaranteed to achieve a local statistical rate, and further strengthen it to the global statistical rate by introducing a proper initialization procedure. In high dimensional regimes, the obtained statistical rate significantly improves those shown in the existing non-sparse decomposition methods. The empirical advantages of TTP are confirmed in extensive simulated results and two real applications of click-through rate prediction and high-dimensional gene clustering.Comment: To Appear in JRSS-

First Principles Study of the Ignition Mechanism for Hypergolic Bipropellants: N,N,N′,N′-Tetramethylethylenediamine (TMEDA) and N,N,N′,N′-Tetramethylmethylenediamine (TMMDA) with Nitric Acid

We report quantum mechanics calculations (B3LYP flavor of density functional theory) to determine the chemical reaction mechanism underlying the hypergolic reaction of pure HNO_3 with N,N,N′,N′-tetramethylethylenediamine (TMEDA) and N,N,N′,N′-tetramethylmethylenediamine (TMMDA). TMEDA and TMMDA are dimethyl amines linked by two CH_2 groups or one CH_2 group, respectively, but ignite very differently with HNO_3. We explain this dramatic difference in terms of the role that N lone-pair electrons play in activating adjacent chemical bonds. We identify two key atomistic level factors that affect the ignition delay: (1) The exothermicity for formation of the dinitrate salt from TMEDA or TMMDA. With only a single CH_2 group between basic amines, the diprotonation of TMMDA results in much stronger electrostatic repulsion, reducing the heat of dinitrate salt formation by 6.3 kcal/mol. (2) The reaction of NO_2 with TMEDA or TMMDA, which is the step that releases the heat and reactive species required to propagate the reaction. Two factors of TMEDA promote the kinetics by providing routes with low barriers to oxidize the C: (a) formation of a stable intermediate with a C–C double bond and (b) the lower bond energy for breaking the C–C single bond (by 18 kcal/mol comparing to alkane) between two amines. Both factors would decrease the ignition delay for TMEDA versus TMMDA. The same factors also explain the shorter ignition delay of 1,4-dimethylpiperazine (DMPipZ) versus 1,3,5-trimethylhexahydro-1,3,5-triazine (TMTZ). These results indicate that TMEDA and DMPipZ are excellent green replacements for hydrazines as the fuel in bipropellants

First-Principles Study of the Role of Interconversion Between NO_2, N_(2)O_4, cis-ONO-NO_2, and trans-ONO-NO_2 in Chemical Processes

Experimental results, such as NO_2 hydrolysis and the hypergolicity of hydrazine/nitrogen tetroxide pair, have been interpreted in terms of NO_2 dimers. Such interpretations are complicated by the possibility of several forms for the dimer: symmetric N_(2)O_4, cis-ONO-NO_2, and trans-ONO-NO_2. Quantum mechanical (QM) studies of these systems are complicated by the large resonance energy in NO_2 which changes differently for each dimer and changes dramatically as bonds are formed and broken. As a result, none of the standard methods for QM are uniformly reliable. We report here studies of these systems using density functional theory (B3LYP) and several ab initio methods (MP2, CCSD(T), and GVB-RCI). At RCCSD(T)/CBS level, the enthalpic barrier to form cis-ONO-NO_2 is 1.9 kcal/mol, whereas the enthalpic barrier to form trans-ONO-NO_2 is 13.2 kcal/mol, in agreement with the GVB-RCI result. However, to form symmetric N_(2)O_4, RCCSD(T) gives an unphysical barrier due to the wrong asymptotic behavior of its reference function at the dissociation limit, whereas GVB-RCI shows no barrier for such a recombination. The difference of barrier heights in these three recombination reactions can be rationalized in terms of the amount of B_2 excitation involved in the bond formation process. We find that the enthalpic barrier for N_(2)O_4 isomerizing to trans-ONO-NO_2 is 43.9 kcal/mol, ruling out the possibility of such an isomerization playing a significant role in gas-phase hydrolysis of NO_2. A much more favored path is to form cis-ONO-NO_2 first then convert to trans-ONO-NO_2 with a 2.4 kcal/mol enthalpic barrier. We also propose that the isotopic oxygen exchange in NO_2 gas is possibly via the formation of trans-ONO-NO2 followed by ON^+ migration

Explanation of the colossal detonation sensitivity of silicon pentaerythritol tetranitrate (Si-PETN) explosive

DFT calculations have identified the novel rearrangement shown here for decomposition of the Si derivative of the PETN explosive [pentaerythritol tetranitrate (PETN), C(CH_2ONO_2)_4] that explains the very dramatic increase in sensitivity observed experimentally. The critical difference is that Si-PETN allows a favorable five-coordinate transition state in which the new Si−O and C−O bonds form simultaneously, leading to a transition state barrier of 33 kcal/mol (it is 80 kcal/mol for PETN) and much lower than the normal O−NO_2 bond fission observed in other energetic materials (40 kcal/mol). In addition this new mechanism is very exothermic (45 kcal/mol) leading to a large net energy release at the very early stages of Si-PETN decomposition that facilitates a rapid temperature increase and expansion of the reaction zone