Isomeric cyclic [C₆H₁₀]^+• ions. The energy barrier to ring opening

Abstract

Appearance energies and metastable peak shapes for methyl loss from the molecular ions of cyclohexene, methylcyclopentenes, methylenecyclopentane, bicyclo[3.1.0]hexane, and 2-methyl-1,4-pentadiene indicate that they have a common reaction pathway which produces [cyclopentenium]+ as the daughter ion at threshold.From measurements of appearance energies and from relative peak abundances and kinetic energy releases for the metastable losses of CH3• and labelled methyl from the deuterium labelled cyclic [C6H10]+• molecular ions it was concluded that: (a) H/D mixing in [cyclohexene]+• occurs in the intact ring prior to a concerted methyl extrusion; (b) [methylcyclopentenes]+• loses methyl without any H/D mixing at or near to threshold by a straight C—C cleavage, possibly via the 3-isomer. Methyl losses involving H/D mixing are preceded by ring opening which has an activation energy of ca. 1 eV. However, [cyclopentenium]+• is again the daughter ion; (c) methylenecyclopentane and bicyclo[3.1.0]hexane molecular ions isomerize to methylcyclopentenes prior to methyl loss.H/D mixing (prior to methyl loss) in the molecular ion 1,1-2H2-2-methyl-1,4-pentadiene takes place before cyclisation to methylcyclopentene.

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

“…(13), this ion is a candidate. However, the energy release values for this group of compounds, the majority of which are conjugated dienes, all are significantly greater than those for the isolated dienes (and cycloalkenes (12)) which produce [cyclopentenium]+ at or near to threshold. Thus these AM, values may arise from the presence of an energy barrier for a common rearrangement (e.g., deCan.…”

“…Isomerization of a 1.3-diene to its 1,4-isomer requires a 1,3-H shift, where the itinerant hydrogen atom is allylic and primary (cf., 13-H allylic shifts (27)), an additional 1,2-H shift then affords the methylcyclopentene molecular ion (see Scheme 1). The significantly higher ANf[C,H7]+ values for the 1.3-dienes may arise from an activation energy for the deconjugation step, because the activation energy for a 1,3-allylic H-shift is negligible (12). AHf[@,H7]+ for 2-ethyl-l,3-butadiene can be similarly explained even though two 1,3-allylic H-shifts are required for deconjugation.…”

“…It has been shown recently (12) that isomeric cyclic C6HlO molecular ions decompose by methyl loss via a methylcyclopentene molecular ion, producing cyclopentenium ions at their thermochemical threshold; the latter is the most stable [C,H,]+ ion species, having a heat of formation (AH3 of 199 kcal m o l l (13).…”

“…(12). Ions having a greater internal energy and decomposing on the same time scale lose methyl containing randomly selected H atoms and the reaction is believed to proceed via a ring-opened species (12).…”

“…(12). Ions having a greater internal energy and decomposing on the same time scale lose methyl containing randomly selected H atoms and the reaction is believed to proceed via a ring-opened species (12). We have measured ionization and appearance energies (AE) for tniz 67 using energy selected electrons (15) (molecular ion lifetime -30 ps) and metastable peak shapes for [G,H,]+ ion production.…”