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

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 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, Delta(double dagger)S). Fitted thresholds for NO(+) and CH(2)OHO(+) ions, originating from the isomeric methyl nitrite ion, are consistent with G3 level ab initio calculations.