Shock-Induced Chemical Changes in Neat Nitromethane:  Use of Time-Resolved Raman Spectroscopy

Abstract

Time-resolved Raman spectroscopy was used to examine chemical changes in neat liquid nitromethane subjected to stepwise loading to peak pressures of 14−17 GPa. After a peak pressure at 14 GPa was reached, no changes in the CN (917 cm-1), NO2 (1400 cm-1), and CH3 (2968 cm-1) symmetric stretching modes were observed. After a peak pressure at 16 GPa was reached, time-dependent changes were observed during the induction period reported in previous absorption experiments. At this peak pressure, the extent of reaction was small, and the observed changes in the CH3 and CN modes indicated prereaction changes in the sample bulk. After a peak pressure at 17 GPa was reached (980 K peak temperature), all three peaks disappeared, indicating that the extent of reaction was substantial under these conditions. The broadening of the CH3 peak and the softening of the CN mode observed in this work suggest strong intermolecular interactions. These interactions lead to a reaction precursor involving proposed head-to-tail intermolecular associations with decomposition proposed to follow through a bimolecular reaction, put forward by Bardo, which forms nitrosomethane and nitromethanol. Confirmation of these ideas will require different spectroscopic methods, since Raman measurements are primarily useful for probing initial changes in the sample.

Vibrational Properties and Structure of Pentaerythritol Tetranitrate

“…In the past, vibrational spectroscopy has been applied successfully to study various materials subjected to shock wave compression. [13][14][15][16][17][18][19][20][21] There have also been attempts to obtain vibrational spectra of PETN under shock wave and static high-pressure loading. 22, 23 However, these attempts were hampered by experimental difficulties and by the lack of understanding of the complex structure of the PETN vibrational spectrum.…”

Effect of Shock Precompression on the Critical Diameter of Liquid Explosives

“…It has previously been shown that increased presence of the aci ion increases the reaction sensitivity of nitromethane [11] and could possibly explain the results obtained in this study. However, the role of pressure in the reaction mechanism of nitromethane has been widely discussed [4]. It was suggested that the reaction mechanism of nitromethane at high pressures could possibly involve a reaction precursor via an intermediate molecule [4,12].…”

“…However, the role of pressure in the reaction mechanism of nitromethane has been widely discussed [4]. It was suggested that the reaction mechanism of nitromethane at high pressures could possibly involve a reaction precursor via an intermediate molecule [4,12].…”

“…Past studies have focused on the effects of shock compression on the initiation of reaction in NM [4]. This study looks at the propagation of a detonation through a previously shocked explosive.…”

Modeling Defect-Induced Phenomena

“…Based on observed CH 3 , NO 2 , and CH 3 O products, by using a molecular beam in conjunction with infrared multiphoton dissociation technique and pyrolysis experiments, it was suggested that below 700 • C, the initial stage of the reaction involves cleavage of the C-N bond, which must compete with the rearrangement of CH 3 NO 2 to give methyl nitrite and by its subsequent decomposition [15], which was also predicted theoretically [16]. Recent experiments enriched possible scenarios by adding bimolecular [17,18] and ionic [19] decompositions. Theoretical studies propose that nitro-to-nitrite rearrangement and HONO elimination reactions are the most favorable and are about 15 kcal mol −1 lower in energy than C-NO 2 break [20], in some qualitative agreement with recent calculations [21,22].…”