| Abstract: | A McLafferty-type rearrangement of a trimethylsilyl group is demonstrated. Abundant rearrangement ions are formed for trimethylsilyl derivatives of α,β-dihydroxy carbonyl compounds. Data are given for a large number of hydroxy acids and hydroxy ketones including aldonic acids, aldaric acids, acyclic ketoses and hydroxy keto acids. A few branched lactones are also shown to rearrange. The positive charge and the unpaired electron are extensively delocalized in the odd-electron rearrangement ions. Those substituents at the α-carbon atom which offer a favourable delocalization promote the rearrangement. Spectra of specifically methylated species reveal that migration over larger than six-membered rings occurs to some extent. An observed dependence of abundance on configuration is associated with steric strains in the transition state. The rearrangement ions are of great diagnostic value in structural analysis. They are relatively more abundant at low electron energies and give rise to the base peak at 20 eV for many compounds. Ions produced by the competing conventional McLafferty rearrangement are less abundant. |
