Ab initio Kinetics and Thermal Decomposition Mechanism of 5 a . CONTRACT NUMBER FA 9300 - 06 - C - 0023

Mononitrobiuret (MNB) and 1,5-Dinitrobiuret (DNB) are, tetrazole-free, nitrogen-rich, energetic compounds. For the first time, the thermal decomposition mechanisms of MNB and DNB have been investigated by advanced ab initio kinetics theories. In particular, the intramolecular interactions of amine H-atom with electronegative nitro O-atom and carbonyl O-atom have been analyzed for MNB, DNB, and biuret at the M06-2X/aug-cc-pVTZ level of theory. The results show that the MNB and DNB molecules are stabilized through six-member-ring moieties via intramolecular H-bonding with interatomic distances between 1.8 2.0 Å, due to electrostatic as well as polarization and dispersion interactions. Furthermore, it was found that the stable molecules in the solid state have the smallest dipole moment amongst all the conformers in the nitrobiuret series of compounds, thus revealing a simple way for evaluating reactivity of fuel conformers. The potential energy surfaces for thermal decomposition of MNB and DNB were characterized at the RCCSD(T)/cc-pV∞Z//M06-2X/aug-cc-pVTZ level of theory. It was found that the thermal decomposition of MNB is initiated by the elimination of HNCO and HNN(O)OH intermediate. Intramolecular transfer of an H-atom, respectively, from the terminal NH2 group to the adjacent carbonyl O-atom via a six-member-ring transition state eliminates HNCO with an energy barrier of 35 kcal/mol, and from the central NH group to the adjacent nitro O-atom eliminates HNN(O)OH with an energy barrier of 34 kcal/mol. Elimination of HNN(O)OH is also the primary channel