On the excited electronic state dissociation of nitramine energetic materials and model systems

Abstract

In order to elucidate the difference between nitramine energetic materials, such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), and their nonenergetic model systems, including 1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine, and dimethylnitramine, both nanosecond mass resolved excitation spectroscopy and femtosecond pump-probe spectroscopy in the UV spectral region have been employed to investigate the mechanisms and dynamics of the excited electronic state photodissociation of these materials. The NO molecule is an initial decomposition product of all systems. The NO molecule from the decomposition of energetic materials displays cold rotational and hot vibrational spectral structures. Conversely, the NO molecule from the decomposition of model systems shows relatively hot rotational and cold vibrational spectra. In addition, the intensity of the NO ion signal from energetic materials is proportional to the number of nitramine functional groups in the molecule. Based upon experimental observations and theoretical calculations of the potential energy surface for these systems, we suggest that energetic materials dissociate from ground electronic states after internal conversion from their first excited states, and model systems dissociate from their first excited states. In both cases a nitro-nitrite isomerization is suggested to be part of the decomposition mechanism. Parent ions of dimethylnitramine and nitropyrrolidine are observed in femtosecond experiments. All the other molecules generate NO as a decomposition product even in the femtosecond time regime. The dynamics of the formation of the NO product is faster than 180 fs, which is equivalent to the time duration of our laser pulse.

Electronic spectra of nitroethylene

“…Electronic excitation phenomena are important in the detonation initiation of energetic materials, as recently observed through mass resolved excitation spectroscopy and femtosecond pump-probe spectroscopy in the ultraviolet (UV) spectral region for a set of component molecules of these materials [5].…”

The electronically excited states of RDX (hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine): Vertical excitations

“…This molecule belongs to the class of nitramines with general formula RRЈN(NO 2 ). Like most of the compounds containing NO 2 groups, RDX has a diffuse electronic spectrum, a tendency to dissociation and shows a variety of associated photochemical phenomena [4]. In particular, it is known that UV excitation markedly reduces the power requirements for detonation of secondary explosives [5].…”

“…Because of a wide range of practical applications, RDX has been studied extensively both theoretically and experimentally [2,7]. Solid and gas-phase properties have been experimentally investigated by means of mass spectroscopy, femtosecond pump-probe spectroscopy [2], and measurements of the UV spectrum [8].…”

Further Studies into the Photodissociation Pathways of 2-Bromo-2-nitropropane and the Dissociation Channels of the 2-Nitro-2-propyl Radical Intermediate

“…Dissociation of Vibrationally Excited HONO and CH 3 C(Br)CH 2 Resulting from Primary HONO Photoelimination. A. Overview.…”

“…The HONO photoelimination channel is confirmed by signal at m/e = 122 (CH 3 C(Br)CH 2 + ) (TOF spectrum shown in Figure 4). The detected 2-bromopropene fragments could arise from two alternative dissociation channels: secondary H-loss from CH 3 C(Br)CH 3 or dissociative ionization of CH 3 C(Br)CH 3 to CH 3 C(Br)CH 2 + via a hydrogen + , using two different methods of detection. Upper frame: acquired at a source angle of 10°using electron bombardment ionization of 200 eV.…”