The enolic [C5H10O]+· and [C3H6O]+· ions formed from aliphatic ketones.

The structures of reactive and nonreactive ions formed by McLafferty rearrangement in a variety of ketones are studied using metastable ion abundances and the kinetic energy released in metastable ion fragmentations. The loss of methyl radicals is studied in [C₅H₁₀O]^+. ions and the mechanism delineated for ions formed in an initial configuration corresponding both to ketonic and enolic forms; the major mechanism in both cases appears to involve a common intermediate. Partial scrambling of hydrogen atoms on the C-1 and C-5 carbons and on the oxygen precedes methyl elimination. Loss of neutral ethylene from [C₅H₁₀O]^+. ions is also studied and again the major mechanism occurs via a common species, intermediate in structure between a ketone and an enol; scrambling of hydrogen is much faster for ions that lose ethylene than for those which lose a methyl radical. Keto ions of formula [C₃H₆O]^+. show no unimolecular loss of methyl. The stable [C₃H₆O]^+. ions generated in the keto and enol forms are shown to preserve their initial structures, confirming the results of ion cyclotron resonance experiments. Reactive [C₃H₆O]^+. ions formed from 2-pentanone molecular ions by McLafferty rearrangement are shown to have the same structures as reactive ions originally generated as enolic structures from 1-methylcyclobutanol. The paper highlights the relationship between hydrogen scrambling and keto-enol isomerization reactions in aliphatic ketones. [ABSTRACT FROM AUTHOR]