Thermodynamics of intermediate ion–molecule complexes or kinetics of competing reactions? The reactions of low-energy isobutylamine and neopentylamine molecular ions

Abstract

Metastable isobutylamine molecular ions fragment predominantly by loss of either C₂H₅ or C₃H₇ radicals. In contrast, the principal reaction of the closely related metastable neopentylamine ions is loss of C₄H₇ radicals. Claims that competition between simple cleavage and rearrangement is governed in these systems by the heats of formation of transient ion–molecule complexes are shown to be unjustified. It is proposed that, in both systems, rearrangement predominates over ‘simple cleavage’ for low-energy ions, but that crossing of the k(E)vs. E curves occurs at slightly higher energies for neopentylamine than for isobutylamine. Reversible isomerization of the molecular ions to stable distonic isomers is suggested. These isomers are considered to be important intermediates in the rearrangement–fragmentation processes. The involvement of distonic isomers complicates a description of the competing reactions in terms of simple cleavage vs. rearrangment. α-Cleavage for the distonic ions is in fact a rearrangement reaction.