THE UNIMOLECULAR CHEMISTRY OF THE ENOL OF IONIZED METHYL GLYCOLATE - FORMATION OF THE HYDROGEN-BRIDGED RADICAL-CATION R-DOT-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-O=CH](.+)

Dissociative ionization of methyl 2-hydroxy-isovalerate and dimethyl t artrate cleanly generate, via McLafferty rearrangements, the 1-methoxy -ethene-1,2-diol ion HOCH=C(OH)OCH3+., 2. The unimolecular chemistry o f 2, the enol form of ionized methyl glycolate, HOCH2C(=O)OCH3+., 1, w as investigated by a variety of tandem-mass spectrometry-based techniq ues using D- and O-18-labelled precursor molecules. The enol ion under goes four major dissociations viz. loss of CH3., CO, CH3OH and C2HO2.. Loss of CH3. involves isomerization of 2, via a 1,4 H shift, into the distonic ion HC(O-.)=C(OHO+(H)CH3, 4, followed by direct bond cleavag e yielding the product ion HC(=O)C(OH)(2)(+). A second 1,4 H shift yie lds the hydroxyketene/methanol ion-dipole complex which serves as the precursor for the losses of CH3OH and C2HO2., yielding HO(H)C=C=O-+. a nd CH3OH2+ respectively. A further isomerization step leads to the los s of CO, yielding the -O ... H ... O- bridged ion [CH3O(H) ... H ... O =CH](+.), one of the most stable isomers on the C2H6O2+. potential ene rgy surface. Ionized methyl acetate, CH3C(=O)OCH3+. and related alipha tic esters, readily interconvert with their enol isomers prior to diss ociation, but no such tautomerization occurs in 1. This is because the HO-CH functionality opens up facile rearrangement/dissociation pathwa ys in 1 and 2 whose energy requirements lie below the tautomerization barrier 1-->2.