CH₃NO₂ decomposition/isomerization mechanism and product branching ratios: An ab initio chemical kinetic study

The low-lying energy pathways for the decomposition/isomerization of nitromethane (NM) have been investigated using different molecular orbital methods. Our results show that in addition to the commonly known CH₃ + NO₂ products formed by direct C-N bond breaking and the trans-CH₃ONO formed by nitro-nitrite isomerization, NM can also isomerize to cis-CH₃ONO via a very loose transition state (TS) lying 59.2 kcal/mol above CH₃NO₂ or 0.6 kcal/mol below the CH₃ + NO₂ asymptote predicted at the UCCSD(T)/CBS level of theory. Kinetic results indicate that in the energy range of 59 ± 1 kcal/mol, production of CH₃O + NO is dominant, whereas above the C-N bond breaking threshold, the formation of CH₃ + NO₂ sharply increases and becomes dominant. The k(E) values predicted at different energies clearly indicate that CH₃O + NO could be detected in an infrared multi-photon dissociation study, whereas in UV dissociation experiments with energies high above the C-N bond breaking threshold the CH₃ + NO₂ products are generated predominantly.