RESUMO
Understanding the thermal decomposition behavior of TATB (1,3,5-triamino-2,4,6-trinitrobenzene) is a major focus in energetic materials research because of safety issues. Previous research and modelling efforts have suggested benzo-monofurazan condensation producing H2O is the initiating decomposition step. However, early evolving CO2 (m/z 44) along with H2O (m/z 18) evolution have been observed by mass spectrometric monitoring of head-space gases in both constant heating rate and isothermal decomposition studies. The source of the CO2 has not been explained, until now. With the recent successful synthesis of 13C6-TATB (13C incorporated into the benzene ring), the same experiments have been used to show the source of the CO2 is the early breakdown of the TATB ring, not adventitious C from impurities and/or adsorbed CO2. A shift in mass m/z 44 (CO2) to m/z 45 is observed throughout the decomposition process indicating the isotopically labeled 13C ring breakdown occurs at the onset of thermal decomposition along with furazan formation. Partially labeled (N18O2)3-TATB confirms at least some of the oxygen comes from the nitro-groups. This finding has a significant bearing on decomposition computational models for prediction of energy release and deflagration to detonation transitions, with respect to conditions which currently do not recognize this oxidation step.
RESUMO
The electronic structure and geometries of (Z)- and (E)-H-CON- N+(CH3)3 have been examined at two levels of theory: B3LYP (basis sets 6-311+G(d,p), 6-311++G(d,p), and 6-311G(3df,3pd)) and MP2(full)/6-311++G(d,p). The (Z) conformation about the C(O)-N(-) bond is thermodynamically preferred over the (E) configuration. Natural bond orbital calculation locates one lone pair of the N- in the HOMO, which is the p(z) natural hybrid orbital (perpendicular to the O=CN- N+ plane). The second lone pair (of lower energy) of N- occupies the HOMO-3, which is the natural hybrid orbital sp(1.12) (sp(1.01) for the (E) conformation, sp(1.74) in the rotational transition state). The carbonyl pi bond is the HOMO-2. The charge-transfer ability of the negative nitrogen in H-CON- N+ (CH3)3 is more powerful than that of the neutral amidic nitrogen in dimethylformamide. The following facts convincingly sustain this view: (1) the higher rotational barrier (stronger C-N(-) bond) in the case of H-CON- N+ (CH3)3, (2) natural resonance theory analysis predicts almost equal weights for the (Z)-H-C(=O)N- N+ (CH3)3 and the (Z)-H-C(O-)=NN+ (CH3)3 canonical resonance structures whereas the weight of the HCON(CH3)2 structure is almost twice as large as that of HC(O-)=N+ (CH3)2, and (3) the second-order perturbation stabilization, as a result of the donor (N-)/acceptor (carbonyl) interaction, is 101.3 kcal/mol for H-CON- N+ (CH3)3 and only 64.4 kcal/mol for dimethylformamide.
