Isomerization and Fragmentation of Methylfuran Ions and Pyran Ions in the Gas Phase

Article abstract of DOI:10.1002/oms.1210210803

The mutual interconversion of the molecular ions ^+. of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to ⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energies and by a MNDO calculation of the minimum energy reaction path.The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1-3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions ⁺ arise by the loss of a H atom.This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions.Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an isotope effect k_H/k_D=1.67-2.16 for metastable ions.In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions.However, the loss of a H atom from all metastable ^+. ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26+/-1.5 kJ mol^-1.The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1^+. and 2^+. favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions.This agrees with the experimental results.