Photodissociation of nitromethane cluster anions

Three types of anionic fragments are observed in the photodissociation of nitromethane cluster anions, (CH₃ NO₂) n-, n=1-6, at 355 nm: NO₂^- (CH₃ NO₂) k, (CH ₃ NO₂)_k-, and OH^- (k<n). The fragmentation trends are consistent with the parent clusters containing a monomer-anion core, CH₃ NO₂^-, solvated by n-1 neutral nitromethane molecules. The NO₂^- (CH₃ NO₂) _k and OH- fragments formed from these clusters are described as core-dissociation products, while the (CH₃ NO ₂) k- fragments are attributed to energy transfer from excited CH₃ NO₂^- into the solvent network or a core-dissociation-recombination (caging) mechanism. As with other cluster families, the fraction of caged photofragments shows an overall increase with increasing cluster size. The low-lying A² A′ and/or B ² A′ electronic states of CH₃ NO₂ ^- are believed responsible for photoabsorption leading to dissociation to NO₂^- based fragments, while the C ² A″ state is a candidate for the OH- pathway. Compared to neutral nitromethane, the photodissociation of CH₃ NO ₂^- requires lower energy photons because the photochemically active electron occupies a high energy π orbital (which is vacant in the neutral). Although the electronic states in the photodissociation of CH₃ NO₂ and CH₃ NO₂^- are different, the major fragments, CH₃ + NO₂ and CH ₃ + NO₂^-, respectively, both form via C-N bond cleavage.