Photodissociation of nitromethane cluster anions

Three types of anionic fragments are observed in the photodissociation of nitromethane cluster anions, (CH(3)NO(2))(n)(-), n=1-6, at 355 nm: NO(2)(-)(CH(3)NO(2))(k), (CH(3)NO(2))(k)(-), and OH(-) (kn). The fragmentation trends are consistent with the parent clusters containing a monomer-anion core, CH(3)NO(2)(-), solvated by n-1 neutral nitromethane molecules. The NO(2)(-)(CH(3)NO(2))(k) and OH(-) fragments formed from these clusters are described as core-dissociation products, while the (CH(3)NO(2))(k)(-) fragments are attributed to energy transfer from excited CH(3)NO(2)(-) 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 A(2)A' and/or B(2)A' electronic states of CH(3)NO(2)(-) are believed responsible for photoabsorption leading to dissociation to NO(2)(-) based fragments, while the C(2)A" state is a candidate for the OH(-) pathway. Compared to neutral nitromethane, the photodissociation of CH(3)NO(2)(-) requires lower energy photons because the photochemically active electron occupies a high energy pi* orbital (which is vacant in the neutral). Although the electronic states in the photodissociation of CH(3)NO(2) and CH(3)NO(2)(-) are different, the major fragments, CH(3)+NO(2) and CH(3)+NO(2)(-), respectively, both form via C-N bond cleavage.