On the Decomposition Mechanisms of New Imidazole-Based Energetic Materials

Abstract

New imidazole-based energetic molecules (1,4-dinitroimidazole, 2,4-dinitroimidazole, 1-methyl-2,4-dinitroimidazole, and 1-methyl-2,4,5-trinitroimidazole) are studied both experimentally and theoretically. The NO molecule is observed as a main decomposition product from the above nitroimidazole energetic molecules excited at three UV wavelengths (226, 236, and 248 nm). Resolved rotational spectra related to three vibronic bands (0-0), (0-1), and (0-2) of the NO (A (2)Σ(+) ← X (2)Π) electronic transition have been obtained. A unique excitation wavelength independent dissociation channel is characterized for these four nitroimidazole energetic molecules: this pathway generates the NO product with a rotationally cold (10-60 K) and vibrationally hot (1300-1600 K) internal energy distribution. The predicted reaction mechanism for the nitroimidazole energetic molecule decomposition subsequent to electronic excitation is the following: electronically excited nitroimidazole energetic molecules descend to their ground electronic states through a series of conical intersections, dissociate on their ground electronic states subsequent to a nitro-nitrite isomerization, and produce NO molecules. Different from PETN, HMX, and RDX, the thermal dissociation process (ground electronic state decomposition from the Franck-Condon equilibrium point) of multinitroimidazoles is predicted to be a competition between NO(2) elimination and nitro-nitrite isomerization followed by NO elimination for all multinitroimidazoles except 1,4-dinitroimidazole. In this latter instance, N-NO(2) homolysisis becomes the dominant decomposition channel on the ground electronic state, as found for HMX and RDX. Comparison of the stability of nitro-containing energetic materials with R-NO(2) (R = C, N, O) moieties is also discussed. Energetic materials with C-NO(2) are usually more thermally stable and impact/shock insensitive than are other energetic materials with N-NO(2) and O-NO(2) moieties. The imidazole aromatic ring also plays an important role in improving the stability of these energetic materials. Thus, multinitroimidazoles energetic materials can be of significant potential for both civilian and military applications.