Tetrazine Theoretical Study On The Thermal Decomposition Of Nitrogen Compound

Thermal decomposition mechanisms are very important for evaluating the safety and storage reliability of energetic materials. Tetrazines with high nitrogen content are a new class of energetic materials, which derive energy from their very high positive heats of formation. Therefore they have many good properties such as high energy, low sensitivity and oxygen balance easily to be achieved. The thermal decomposition mechanisms of tetrazine compounds were studied by combining ab initio molecular dynamics (AIMD) method with density functional theory (DFT). Tetrazine derivatives are constituted of ring, substituents and bridge connecting unit. Firstly, the thermal decomposition mechanisms of five kinds of simple hydronitrogen compounds used as substituents or bridge connecting units were studied, which were N₂H₂, N₂H₄, N₃H, N₄H₂ and N₄H₄. Then, s-tetrazine(T) as the mother molecule of tetrazine derivatives was studied. Lastly, five kinds of tetrazine derivatives with different subsistituents such as 3,6-diamino-1,2,4,5-tetrazine(DAT), 3,6-dihydrazino-1,2,4,5-tetrazine (DHT), 3,6-diazino-1,2,4,5-tetrazine (DiAT), 3,6-bis(1H-tetrazol-5-amino)-1,2,4,5-tetrazine (BTATz), and 3,3'-Azobis(6-amino-1,2,4,5-tetrazine) (DAAT) were studied.By VASP package, the thermal decomposition trajectories of above five kinds of simple hydronitrogen compounds and six kinds of tetrazine compounds were first simulated. Then the reaction channels were studied by Gaussian03 at B3LYP/6-311G(d,p) level to locate the local minimum points and the transition structures. To abtain more accurate and reliable reaction information, the high accuracy single point calculations were further performed at CCSD(T)=full/6-311+G(3df,2p) level for these simple hydronitrogen compounds. For s-tetrazine, the single point energy calculations at CCSD(T)/6-311G(d,p), B3LYP/6-311+G(2df,2p), G3MP2B3, G3B3 and CCSD(T)/6-311+G(2df,2p) levels were performed and compared, and G3MP2B3 was found both accurate and efficient. So, G3MP3B3 was selected to calculate the larger tetrazine derivatives, but the much larger BTATz and DAAT. For the reaction pathways with similar energy barriers, the rate constants were calculated with chemical reaction kinetics method to verify the main thermal decomposition pathway.Based on the study of the simple hydronitrogen compounds and s-tetrazine without substituents, the thermal decomposition mechanisms and rate-determining steps of five kinds of tetrazine derivatives were studied in detail to illuminate the influence of substituents and bridge connecting units on stability of tetrazine ring and understand their thermal decomposition mechanisms. The conclusions were drawn as below: (1) The tetrazine ring can be broken by concerted triple dissociation, concerted double dissociation or single dissociation. Generally speaking, the energy barrier of concerted triple dissociation is lowest. (2) The stability of the tetrazine ring can evidently be strengthened by delocalization of molecular orbitals between the ring and the substituent groups. (3) The reaction between the substituents and tetrazine ring (such as H transfer) is not main thermal decomposition pathway and the thermal decomposition mechanisms of the substituent (T)R are similar to that of its simple compound HR. (4) Tetrazines exhibit two principal modes of decomposition, which are the ring dissociation and the reaction of substituent groups. If the stability of the substituent is better than that of the tetrazine ring, decomposition occurs first through breaking of the ring. Otherwise, the substituent first reacts.The possible bimolecular reactions of DHT were calculated and the reaction energy barrier of the inter-hydrazino hydrogen transfer to loss NH₃ was found much lower than that of concerted triple dissociation as the main unimolecular decomposition pathway, which showing it was an important thermal decomposition pathway in crystal phase. This provides the theoretical base for further studying intermolecular interaction and the thermal decomposition mechanisms of tetrazine derivatives in crystal.