CHARGE-REMOTE AND CHARGE-PROXIMATE FRAGMENTATION PROCESSES IN ALKALI-CATIONIZED FATTY-ACID ESTERS UPON HIGH-ENERGY COLLISIONAL ACTIVATION -A NEW MECHANISTIC PROPOSAL

The effect of the metal ion on the high-energy collision-induced disso ciation (CID) of alkali metal-cationized n-butyl and methyl ester deri vatives of palmitic and oleic acid was examined. The results show that the alkali metal ion has a pronounced effect and does not act as a me re 'spectator' ion with respect to the fragmentation process. While C- H cleavage is a dominant process for [M + Li](+) as well as [M + Na]() precursor ions, C-C cleavage is also significant for the [M + Na](+) ions, Homolytic mechanisms involving the formation of a transient bir adical cation are proposed which enable us to rationalize in a straigh tforward manner all product ions formed by both charge-remote and char ge-proximate fragmentations. The mechanistic proposal is discussed in view of available knowledge on electron impact, CID and related proces ses. In order to predict how the alkali metal ion could affect the rea ctivity of the postulated biradical state termed following electronic excitation of the alkali metal-cationized molecules, quantum chemical calculations were performed on methyl and n-butyl acetate as model sub stances. The decreased spin density at the carbonyl oxygen atom in the biradical state may provide an explanation for the greater tendency t owards C-C cleavage reactions of the sodium-cationized fatty acid este rs relative to the corresponding lithium complexes. (C) 1988 John Wile y & Sons, Ltd.