Initial mechanisms for the decomposition of electronically excited energetic salts: TKX-50 and MAD-X1

Decomposition of energetic salts TKX-50 and MAD-X1 (dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate and dihydroxylammonium 3,3'-dinitro-5,5'-bis-1,2,4-triazole-1,1'-diol, respectively), following electronic state excitation, is investigated both experimentally and theoretically. The NO and N2 molecules are observed as initial decomposition products from the two materials subsequent to UV excitation. Observed NO products are rotationally cold (<25 K) and vibrationally hot (>1500 K). The vibrational temperature of the NO product from TKX-50 is (2600 ± 250) K, (1100 ± 250) K hotter than that produced from MAD-X1. Observed N2 products of these two species are both rotationally cold (<30 K). Initial decomposition mechanisms for these two electronically excited salts are explored at the complete active space self-consistent field (CASSCF) level. Potential energy surface calculations at the CASSCF(8,8)/6-31G(d) level illustrate that conical intersections play an essential role in the decomposition mechanisms. Electronically excited S1 molecules can nonadiabatically relax to the lower electronic state through (S1/S0)CI conical intersections. Both TKX-50 and MAD-X1 have two (S1/S0)CI conical intersections between S1 and S0 states, related to and leading to two different reaction paths, forming N2 and NO products. N2 products are released by the opening of the tetrazole or triazole rings of TKX-50 and MAD-X1. NO products are released from the amine N-oxide moiety of TKX-50, and for MAD-X1, they are produced through nitro-nitrite isomerizations. The observed rotational energy distributions for NO and N2 products are consistent with the final structures of the respective transition states for each molecule on its S0 potential energy surface.