Ab initio reaction pathways for photodissociation and isomerization of nitromethane on four singlet potential energy surfaces with three roaming paths

Photodissociation pathways of nitromethane following π → π electronic excitation are reported. The potential energy surfaces for four lowest singlet states are explored, and structures of many intermediates, dissociation limits, transition states, and minimum energy conical intersections were determined using the automated searching algorism called the global reaction route mapping strategy. Geometries are finally optimized at CASSCF(14e,11o) level and energies are computed at CAS(14o,11e)PT2 level. The calculated preferable pathways and important products qualitatively explain experimental observations. The major photodissociation product CH₃ and NO₂ (²B₂) is formed by direct dissociation from the S₁ state. Important pathways involving S₁ and S₀ states for production of various dissociation products CH ₃NO + O (¹D), CH₃O(X²E) + NO (X ²Π ), CH₂NO + OH, and CH₂O + HNO, as well as various isomerization pathways have been identified. Three roaming processes also have been identified: the O atom roaming in O dissociation from CH ₃NO₂, the OH radical roaming in OH dissociation from CH₂N(O)(OH), and the NO roaming in NO dissociation from CH ₃ONO.