Rearrangement and fragmentation pathways of [C₃H₇Z]⁺ ions (Z = NH and S): Are ion-neutral complexes important?

High level ab initio calculations at the G2(ZPE = MP2) level have been used to characterize the potential energy surfaces for rearrangement/fragmentation of various [C₃H₈N]⁺ and [C₃H₇S]⁺ isomers. In contrast to the behavior in the corresponding [C₃H₇O]⁺ system, it is found that ion-neutral complexes are only of minor importance in determining the fragmentation characteristics. Either dissociation of such complexes occurs too fast due to a large barrier to their formation ([C₃H₈N]⁺ system), or alternative lower-energy rearrangement routes that do not involve ion-neutral complexes are available ([C₃H₇S]⁺ system). Calculated thermochemical quantities such as heats of formation and reaction barriers are found to be in reasonable agreement with experimental results. Metastable ion product abundances and results of both deuterium- and ¹³C-labeling experiments are rationalized in terms of the calculated potential energy surfaces and rate constants obtained using Rice-Ramsperger-Kassel-Marcus theory. (C) 2000 Elsevier Science B.V.