Threshold photoelectron photoion coincidence spectroscopy sheds light on the dissociation of pyrrole and thiophene molecular ions

Abstract Threshold photoelectron photoion coincidence spectroscopy was employed to examine the loss of ethyne from ionized pyrrole and thiophene near their dissociation thresholds as a function of internal energy and reaction time. The B3-LYP/6-311+G(3df,3pd) minimum energy reaction pathways for ethyne loss from both ions are presented. The experimental molecular ion breakdown curves were modeled with RRKM theory. For ionized pyrrole, the fitting results in confirmation of the transition state energies in the decomposition pathways. For ionized thiophene, the modeling suggests that ethyne loss is a concerted process rather than a true step-wise reaction. If so, the fitting yields a Δ f H 298 for CH 2 CS + of 1033 ± 2 kJ mol −1 .