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 mol^–1. In addition, evidence was found for a transition state switch in the bond cleavage reaction in 1 leading to CH₃^• + NO₂⁺. As internal energy increases, the effective transition state for this reaction becomes tighter (i.e. is characterized by a lower entropy of activation, Δ^‡S). Fitted thresholds for NO⁺ and CH₂OHO⁺ ions, originating from the isomeric methyl nitrite ion, are consistent with G3 level ab initio calculations.

Keywords: nitromethane ion, methylnitrite ion, isomerization, variational transition state theory, computational chemistry, ion dissociation, threshold photoelectron–photoion coincidence spectroscopy