Initial mechanisms for the unimolecular decomposition of electronically excited nitrogen-rich energetic salts with tetrazole rings: (NH4)2BT and TAGzT

Abstract

Unimolecular decomposition of nitrogen-rich energetic salt molecules bis(ammonium)5,5 ′bistetrazolate (NH 4) 2 BT and bis(triaminoguanidinium) 5,5 ′-azotetrazolate TAGzT, has been explored via 283 nm laser excitation. The N 2 molecule, with a cold rotational temperature (<30 K), is observed as an initial decomposition product, subsequent to UV excitation. Initial decomposition mechanisms for the two electronically excited salt molecules are explored at the complete active space self-consistent field (CASSCF) level. Potential energy surface calculations at the CASSCF(12,8)/6-31G(d) ((NH 4) 2 BT) and ONIOM (CASSCF/6-31G(d):UFF) (TAGzT) levels illustrate that conical intersections play an essential role in the decomposition mechanism as they provide non-adiabatic, ultrafast radiationless internal conversion between upper and lower electronic states. The tetrazole ring opens on the S 1 excited state surface and, through conical intersections (S 1 /S 0) CI , N 2 product is formed on the ground state potential energy surface without rotational excitation. The tetrazole rings open at the N2-N3 ring bond with the lowest energy barrier: the C-N ring bond opening has a higher energy barrier than that for any of the N-N ring bonds: this is consistent with findings for other nitrogen-rich neutral organic energetic materials. TAGzT can produce N 2 either by the opening of tetrazole ring or from the N==N group linking its two tetrazole rings. Nonetheless, opening of a tetrazole ring has a much lower energy barrier. Vibrational temperatures of N 2 products are hot based on theoretical predictions. Energy barriers for opening of the tetrazole ring for all the nitrogen-rich energetic materials studied thus far, including both neutral organic molecules and salts, are in the range from 0.31 to 2.71 eV. Energy of the final molecular structure of these systems with dissociated N 2 product is in the range from −1.86 to 3.11 eV. The main difference between energetic salts and neutral nitrogen-rich energetic material is that energetic salts usually have lower excitation energy.

Proposed Proton-Transfer Mechanism for the Initial Decomposition Steps of BTATz

“…The loss of a nitrogen molecule has previously been reported as a primary step in the photochemical decomposition of different tetrazole derivatives . Previous reports on nonsubstituted 1H-tetrazole discuss the elimination of a nitrogen molecule originating from nitrogen atoms at positions N2 and N3 of the ring .…”

A comparison of methods for theoretical photochemistry: Applications, successes and challenges

“…Yuan and Bernstein et al have also performed several number of multireference studies in the past few years, applying CASSCF calculations to the excited state decomposition reactions of 3,3 0 -diamino-4,4 0 -bisfuroxan and 4,4 0 -diamino-3,3 0 -bisfuroxan, 60 bis(ammonium)5,5 0 -bistetrazolate and bis(triaminoguanidinium) 5,5 0 -azotetrazolate, 61 dihydroxylammonium 5,5 0bistetrazole-1,1 0 -diolate and dihydroxylammonium 3, 62 nitrophenyl derivatives, 63 and 1,1-diamino-2,2dinitroethylene, 64 as well as the fragmentation of excited state ammonia-based clusters. 65 In the five decomposition studies, the products are a ring-opened tetra/triazoles, NO and/or N 2 .…”

A Photophysical Deactivation Channel of Laser-Excited TATB Based on Semiclassical Dynamics Simulation and TD-DFT Calculation

“…The presence of nitro groups significantly changes the photochemical and photophysical behavior of molecules and affect the stability and sensitivity of energetic molecules [ 4 , 5 , 6 ]. Recent literature shows [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ] excited energetic molecules deactivated from the S 1 excited state to the S 0 ground state through a conical intersection (CI). Experimental and theoretical studies by Bernstein and coworkers [ 6 ] found that the conical intersection plays an important role in the non-adiabatic decay of the energetic molecule dimethylnitramine (DMNA).…”

“…This isomerization process was predicted to take place through a “loose transition state-like geometry”. Bernstein’s group studied the non-adiabatic transitions of many energetic molecules [ 8 , 9 , 10 , 11 , 12 , 13 , 14 ] and found that their (S 1 /S 0 ) CI are similar in structure. Soto, Arenas and collaborators [ 15 , 16 , 17 ] used the complete active space with second-order perturbation theory (CASPT2) to predict NO 2 –ONO isomerization in nitramide molecules through a (S 1 /S 0 ) CI ; a similar CI structure in the nitromethane molecule has led to the nitro elimination reaction.…”