On the formation of cyclopentenium cations from all C6H10+• molecular ion structures

Abstract

The structures of [C5H7]+ ions produced by methyl loss from thirty isomeric C6H10 molecular ions have been assessed from measurements of ionic heats of formation and the kinetic energy release associated with the fragmentation. It is proposed that the thermodynamically most stable [C5H7]+ ions [cyclopentenium]+ are formed at the dissociation threshold irrespective of the initial molecular ion structure. Mechanistic aspects of the fragmentations were studied by mean of deuterium labelling.Observations of metastable peak shapes for labelled compounds permitted the identification of reaction pathways having different energy requirements. The lowest energy routes involved little or no positional mixing of H and D prior to fragmentation. It is proposed that the molecular ions with sufficient energy to fragment on the μs time scale undergo successive (and reversible) isomerization by allylic 1,3-hydrogen shifts to their isomeric isolated diene molecular ions which, by an allylic 1,2-hydrogen shift, may isomerize to a methylcyclopentene molecular ion prior to loss of methyl. The sequence of isomerization of the molecular ions is alkyne → allene → conjugated diene → isolated diene. The de-conjugation step has an activation energy of ca. 10 kcal mol−1. We suggest, however, that allene molecular ions rearrange directly to isolated diene molecular ions.