Competition between isomerization and fragmentation of gaseous ions. II. Nitromethane and methylnitrite ions

Major fragmentation pathways of the nitromethane and methylnitrite ions have been examined. Ionization and fragmentation onsets for these processes have been determined and breakdown graphs have been constructed for both compounds using a threshold photoelectron–photoion coincident (TPEPICO) mass spectrometer. At onset, fragmentation of the nitromethane ion to NO+ is preceded by isomerization to the energetically favored methylnitrite ion; the barrier to isomerization is 0.64 eV. Symmetry considerations indicate that the nitromethane ion in its 2A2 (π) state correlates with the 2A″(π) state of the methylnitrite ion through which dissociation to the NO+ fragment occurs.