Intermediate Ion Structures in the Fragmentation of Metastable 3-Methylbutan-2-ol Radical Cations

Article abstract of DOI:10.1002/oms.1210251108

Ionized 3-methylbutan-2-ol displays four low-energy fragmentations, loss of CH3^. and C3H7^. and loss of CH4 and C3H8, the latter pair being produced by metastable ion decompositions.The electron-impact, metastable-ion and collision-induced dissociation mass spectra of 13C and 2H-labelled isotopomers have been recorded, together with appearance energy measurements.It was found that the fast (ion source) losses of CH3^. and C3H7^. involved only simple bond cleavages between C(1) and C(2) and between C(2) and C(3), respectively, and without any positional interchange of isotopes.The loss of C3H8 produces ionized vinyl alcohol containing only C(1) and C(2).The H atoms involved are only those attached to C(1) and C(3).In deuterium-labelled analogues, the deuterium is preferentially located in the propane, e.g. metastable CD3CH(OH)CH(CH3)2 yields predominantly C3H6D2.On the basis of all the observations, it is proposed that low-energy molecular ions can form a stable proton-bridged molecule-radical complex, +...^.CH(CH3)2>, and that this key intermediate is responsible for the isotope distribution in propane loss and also for the relatively low importance of the lowest energy dissociation, the simple C(1)-C(2) bond cleavage.