Potential energy profiles for unimolecular reactions of organic ions: [C₃H₈N]⁺ and [C₃H₇O]⁺

Abstract

Abstract The unimolecular decompositions of two isomers of [C 3 H 8 N] + , \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} {\rm CH}_{\rm 2} {\rm CH} = \mathop {\rm N}\limits^ + {\rm H}_2 $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} {\rm CH}_{\rm 2} \mathop {\rm N}\limits^ + {\rm H = CH}_{\rm 2} $\end{document} , are discussed in terms of the potential energy profile over which reaction may be considered to occur. The energy needed to promote slow (metastable) dissociations of either ion is found to be less than that required to cause isomerization to the other structure. This finding is supported by the observation of different decomposition pathways, different metastable peak shapes for C 2 H 4 loss, the results of 2 H labelling studies, and energy measurements on the two ions. The corresponding potential energy profile for decomposition of the oxygen analogues, \documentclass{article}\pagestyle{empty}\begin{document}${\rm CH}_{\rm 3} {\rm CH}_{\rm 2} {\rm CH =\!= }\mathop {\rm O}\limits^ + {\rm H} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_{\rm 3} {\rm CH}_{\rm 2} \mathop {\rm O}\limits^ + {\rm = CH}_{\rm 2} $\end{document} , is compared and contrasted with that proposed for the [C 3 H 8 N] + isomers. This analysis indicates that for the oxygen analogues, the energy needed to decompose either ion is very similar to that required to cause isomerization to the other structure. Consequently, dissociation of either ion is finely balanced with rearrangement to the other and similar reactions are observed. Detailed mechanisms are proposed for loss of H 2 O and C 2 H 4 from each ion and it is shown that these mechanisms are consistent with 2 H and 13 C labelling studies, the kinetic energy release associated with each decomposition channel, the relative competition between H 2 O and C 2 H 4 loss and energy measurements.