Photodissociation dynamics of nitromethane and methyl nitrite by infrared multiphoton dissociation imaging with quasiclassical trajectory calculations: Signatures of the roaming pathway

Abstract

We combine the techniques of infrared multiphoton dissociation (IRMPD) with state selective ion imaging to probe roaming dynamics in the unimolecular dissociation of nitromethane and methyl nitrite. Recent theoretical calculations suggest a "roaming-mediated isomerization" pathway of nitromethane to methyl nitrite prior to decomposition. State-resolved imaging of the NO product coupled with infrared multiphoton dissociation was carried out to examine this unimolecular decomposition near threshold. The IRMPD images for the NO product from nitromethane are consistent with the earlier IRMPD studies that first suggested the importance of an isomerization pathway. A significant Λ-doublet propensity is seen in nitromethane IRMPD but not methyl nitrite. The experimental observations are augmented by quasiclassical trajectory calculations for nitromethane and methyl nitrite near threshold for each dissociation pathway. The observation of distinct methoxy vibrational excitation for trajectories from nitromethane and methyl nitrite dissociation at the same total energy show that the nitromethane dissociation bears a nonstatistical signature of the roaming isomerization pathway, and this is possibly responsible for the nitromethane Λ-doublet propensity as well.

Characterization of molecular channel in photodissociation of SOCl2 at 248 nm: Cl2 probing by cavity ring-down absorption spectroscopy

“…40 Such a roaming-mediated isomerization pathway was also found in nitrobenzene 42 and nitromethane 43,44 to account for a significant yield of the NO elimination. For instance, Suits, Bowmen and coworkers 45 carried out the infrared multiphoton dissociation of nitromethane to examine this unimolecular decomposition near threshold. They found that the roaming-mediated isomerization pathway dominates the NO loss channel, yielding a small translational energy release and a feature of the L-doublet propensity of NO, favoring an unpaired electron lying in the pp orbital perpendicular to the plane of rotation.…”

Lyman α photolysis of solid nitromethane (CH3NO2) and D3-nitromethane (CD3NO2) – untangling the reaction mechanisms involved in the decomposition of model energetic materials

“…These studies revealed three channels: (1) the unimolecular decomposition of nitromethane (CH 3 NO 2 ) to the methyl radical (CH 3 ) and to nitrogen dioxide (NO 2 ) 30,[35][36][37][38][39] via reaction (1) as well as (2) a roamingmediated nitromethane (CH 3 NO 2 )-methylnitrite (CH 3 ONO) isomerization in IRMPD studies followed by unimolecular decomposition via (2a) a radical pathway to the methoxy radical (CH 3 O) plus nitrogen monoxide (NO) (reaction (2a) and (2b)) through a molecular elimination pathway yielding formaldehyde (H 2 CO) plus nitrosylhydride (HNO) (reaction (2b)). [37][38][39][40][41] CH 3 NO 2 -CH 3 + NO 2 (1)…”

Perspective: chemical dynamics simulations of non-statistical reaction dynamics

“…Roaming has now been reported for numerous reactions, as described in [83]. Extensive analyses of roaming have been reported for CH 3 CHO [85][86][87] and NO 3 [88][89][90] dissociation and isomerization in CH 3 NO 2 [91][92][93]. Roaming is expected to be important for hydrocarbon dissociation [94] and for C 3 H 8 → CH 3 + C 2 H 5 may lead to the possible disproportionation products CH 4 + C 2 H 4 and CH 2 + C 2 H 6 .…”