Nitro-nitrite isomerization and transition state switching in the dissociation of ionized nitromethane: a threshold photoelectron-photoion coincidence spectroscopy study

摘要

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been employed to investigate the competition between bond cleavage and rearrangement reactions in the dissociation of ionized nitromethane, 1. Modeling TPEPICO breakdown diagrams with a combination of RRKM theory and ab initio calculations at the G3 level of theory allowed the derivation of the activation energy for the isomerisation of 1 to ionized methyl nitrite, 2, 82 kJ mor~(-l). In addition, evidence was found for a transition state switch in the bond cleavage reaction in 1 leading to CH_3 + NO_2.~+. As internal energy increases, the effective transition state for this reaction becomes tighter (i.e. is characterized by a lower entropy of activation, ASS). Fitted thresholds for NO~+ and CH20H 0+ ions, originating from the isomeric methyl nitrite ion, are consistent with 03 level ab initio calculations.