Unimolecular kinetics of pyridine ion fragmentation

Abstract

The fragmentation of the pyridine ion to form C 4H 4 + and HCN has been studied by means of photoelectron—photoion coincidence mass spectrometry with variable ion-source residence time. A detailed analysis of the time-dependent breakdown curves leads to a T = 0 K fragmentation threshold of 12.15 ± 0.02 eV, Δ H f 0 0(C 4H 4 +) = 1195 ± 12 kJ mol −1, somewhat higher than the value reported by Eland et al. The fragmentation process requires a tight transition state, in agreement with earlier conclusions of Eland et al. There is some disagreement with earlier work on the energy-dependence of the fragmentation rate. It is suggested that this may be due to distortion of the thermal vibrational population distribution of the ion after vertical ionization.