Initial mechanisms for the decomposition of electronically excited energetic materials: 1,5'-BT, 5,5'-BT, and AzTT

Decomposition of nitrogen-rich energetic materials 1,5'-BT, 5,5'-BT, and AzTT (1,5'-Bistetrazole, 5,5'-Bistetrazole, and 5-(5-azido-(1 or 4)H-1,2,4-triazol-3-yl)tetrazole, respectively), following electronic state excitation, is investigated both experimentally and theoretically. The N2 molecule is observed as an initial decomposition product from the three materials, subsequent to UV excitation, with a cold rotational temperature (<30 K). Initial decomposition mechanisms for these three electronically excited materials are explored at the complete active space self-consistent field (CASSCF) level. Potential energy surface calculations at the CASSCF(12,8)/6-31G(d) level illustrate that conical intersections play an essential role in the decomposition mechanism. Electronically excited S1 molecules can non-adiabatically relax to their ground electronic states through (S1/S0)CI conical intersections. 1,5'-BT and 5,5'-BT materials have several (S1/S0)CI conical intersections between S1 and S0 states, related to different tetrazole ring opening positions, all of which lead to N2 product formation. The N2 product for AzTT is formed primarily by N-N bond rupture of the -N3 group. The observed rotational energy distributions for the N2 products are consistent with the final structures of the respective transition states for each molecule on its S0 potential energy surface. The theoretically derived vibrational temperature of the N2 product is high, which is similar to that found for energetic salts and molecules studied previously.