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

Abstract

AbstractThe 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.