Experimental and theoretical studies of the decomposition of new imidazole based energetic materials: Model systems

Abstract

Decomposition of three imidazole based model energetic systems (2-nitroimidazole, 4-nitroimidazole, and 1-methyl-5-nitroimidazole) is investigated both experimentally and theoretically. The initial decomposition mechanism for these three nitroimidazoles is explored with nanosecond energy resolved spectroscopy, and quantum chemical theory at the complete active space self-consistent field (CASSCF) level. The NO molecule is observed as an initial decomposition product from these three nitroimidazoles subsequent to UV excitation. A unique, excitation wavelength independent dissociation channel is observed for these three nitroimidazoles that generates the NO product with a rotationally cold (∼50 K) and a vibrationally mildly hot (∼800 K) distribution. Potential energy surface calculations at the CASSCF∕6-31G(d) level of theory illustrate that conical intersections play an important and essential role in the decomposition mechanism. Electronically excited S(2) nitroimidazole molecules relax to the S(1) state through the (S(2)∕S(1))(CI) conical intersection, and undergo a nitro-nitrite isomerization to generate the NO product from the S(1) potential energy surface. Nevertheless, NO(2) elimination and nitro-nitrite isomerization are expected to be competitive reaction mechanisms for the decomposition of these molecules on the ground state potential energy surface from the Franck-Condon equilibrium geometry through thermal dissociation.

Reactions in Nitroimidazole and Methylnitroimidazole Triggered by Low-Energy (0–8 eV) Electrons

“…26 It was found that in both molecules NO • is formed as an initial product, irrespective of the excitation wavelength. Thereby NO • is generated with a rotationally cold (50 K) and vibrationally warm (800 K) distribution, independent of the excitation wavelength.…”

Isoconversional Kinetic Analysis of Decomposition of Nitroimidazoles: Friedman method vs Flynn–Wall–Ozawa Method

“…9 The reaction would follow: (i) NO 2 elimination, (ii) nitro-nitrite isomerization, and (iii) a step-wise mechanism involving initial ring opening followed by bond dissociation with a concerted hydrogen shift.…”

Dispersion Corrected Structural Properties and Quasiparticle Band Gaps of Several Organic Energetic Solids

“…Currently, experiments on these materials are increasing in order to explore important properties such as structural, optical and chemical decomposition mechanisms. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Indeed complex crystal structure and high sensitivity to a small external stimuli of these compounds present difficulties in experiments to examine the desired properties such as analysis of the bonding, stability, and studying the polymorphic phases of these materials. On the other hand, theoretical assessment on these materials is an alternative method to provide a description of the structural and electronic properties at various conditions.…”