Thermal Dissociation and Roaming Isomerization of Nitromethane: Experiment and Theory

Abstract

The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH3NO2 decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH3 and NO2 radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH3NO2, and for the reaction of CH3 with NO2.

Insights into nitromethane combustion from detailed kinetic modeling – Pyrolysis experiments in jet-stirred and flow reactors

“…These were identified by Zhu et al [45] suggesting that the roaming channel is active at pressures lower than 2 Torr over a wide temperature range (400-3000 K). This finding was confirmed by the succeeding study of Annesley et al [23] who also performed shock tube experiments. Their theoretical analysis suggested that at pressures above 2 Torr the product distribution undergoes a sharp transition from the roaming dominated (CH3ONO → CH3O + NO) to the bond-fission dominated (CH3 + NO2) channel.…”

“…However, in [31], several recent studies on the thermal CH3NO2 decomposition were not taken into account. In these studies [23,25,[43][44][45], it was suggested that CH3NO2 not only thermally dissociates to CH3 + NO2 but also isomerizes to methyl nitrate (CH3ONO) via a roaming type transition state finally dissociating to CH3O + NO. In 2009, Zhu and Lin [43] studied the CH3NO2 decomposition theoretically and suggested that the channel to CH3O + NO could be detected in experiments.…”

“…Though studies on nitromethane are not new, there are a number of studies on nitromethane detonation [19][20][21] and nitromethane decomposition [22][23][24][25]. None of these studies took into account the detailed kinetic study of CH3NO2 combustion.…”

A modular, multi-diagnostic, automated shock tube for gas-phase chemistry

“…In the BST, instead of the helium-neon laser originally prescribed by Kiefer, 13 a Fabrey-Perot red diode laser of similar wavelength (637 vs 633 nm) has been employed, following the work of Annesley et al 18 The detector is a quadsegmented photodiode (UDT SPOT-9DMI) with electronics developed and constructed by Argonne National Laboratory.…”

The impact of roaming radicals on the combustion properties of transportation fuels

“…Since the work of Townsend et al, [1] there has been an explosion of interest in roaming reactions . Most of the initial attention to roaming focused on formaldehyde [1-4, 6-8, 40] and acetaldehyde [5,10,11,15,34,38,41], but more recent studies considered larger carbonyls [12,13,27,32,39], alkanes [14,20,24], ethers [19,26], and NO 2 -containing compounds [22,29,30,33,35,37].…”