On the dissociation dynamics of energy-selected nitrobenzene ion

Fragmentation mechanisms of nitrobenzene ions have been investigated using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown curves were obtained at internal energies of the molecular ion from 0 to 8 eV, and fragmentation pathways were elucidated. The precursor of C4H+3 has been identified clearly as the C6H+5 ion from the breakdown graph. Decay rates of metastable decompositions were measured from the asymmetric peak shapes of their coincidence time-of-flight (TOF) peaks for the C6H5O+, C6H+5, C4H+3, and NO+ ions. The activation energies for NO+ and C6H5O+ formation were estimated as 1.02 eV on the basis of RRKM/QET calculations. Average kinetic energy release for the formation of phenyl cation by direct C–N bond cleavage showed that about 4.2% of the internal energy is converted to translation; this is in good quantitative agreement with the calculations based on the statistical phase space theory. On the other hand, the dependence of the fragmentation rate constant on internal energy is in good agreement with the RRKM/QET calculation assuming a slightly tight transition state. However, the decay of C6H+5 to C4H+3 and C2H2 was one to two orders of magnitude faster than predicted by RRKM/QET assuming direct fragmentation. This result is ascribed to isomerization of phenyl cation to a linear structure before dissociation.