Initial Mechanisms for the Decomposition of Electronically Excited Energetic Salts: TKX-50 and MAD-X1

Abstract

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.

DFT Study of the Decomposition Pathways of 5,5'-bitetrazole-1,1'-diide as a Parent Anion in the Family of Highly Energetic Green Explosives

“…Nevertheless, the activation barriers for real crystalline salts (like TKX-50 or MAD-X1) is expected to be high enough for a simple thermal occupation. This suggests the fairness of the previous results concerning conical intersections between S1 and S0 states [11].…”

“…To date, a two-step process initiated by a proton transfer from hydroxylammonium cations to 5,5'-bistetrazole-1,10-diolate is generally accepted [8][9][10][11][12][13][14][15]. Early molecular dynamics studies of thermal decomposition of TKX-50 revealed that the latter decays with release of molecular nitrogen or N2O with the activation energies of 188.7 and 302.1 kJ mol -1 , respectively [8].…”

“…It is w o r t h w h i l e t o n o t e t h a t t h e s o l i d s u b s t a n c e b e a r i n g t h e a m m o n i u m c a t i o n s i n s t e a d o f t h e hydroxylammonium cations was detected experimentally among the intermediate products of d e c o m p o s i t i o n [ 1 0 ] . A n u m b e r o f i n t e r m e d i a t e s t r u c t u r e s w e r e c a l c u l a t e d a s a r e s u l t o f t h e decomposition of electronically excited TKX-50 [11]. It was found that TKX-50 has two conical intersections between S1 and S0 states, related to and leading to two different reaction paths, forming N2 and NO products [11].…”

“…A n u m b e r o f i n t e r m e d i a t e s t r u c t u r e s w e r e c a l c u l a t e d a s a r e s u l t o f t h e decomposition of electronically excited TKX-50 [11]. It was found that TKX-50 has two conical intersections between S1 and S0 states, related to and leading to two different reaction paths, forming N2 and NO products [11].…”

High Pressure Behavior of Crystalline Dihydroxylammonium 5,5'-Bistetrazole- 1,1'-Diolate: First-Principles Study

“…Goddard et al, 18 developed a flexible classical force field for TKX-50, which reproduces the cell parameters, densities, lattice energy and mechanical properties derived from quantum mechanics (QM) and experiments. Bernstein et al, 19 explored the decomposition of the energetic salts TKX-50 and MAD-X1 (dihydroxylammonium 3,3'-dinitro-5,5'-bis-1,2,4-triazole-1,1'-diol) following electronic excitation via nanosecond energy resolved spectroscopy.…”