The unimolecular chemistry of the enol of ionized methyl glycolate: Formation of the hydrogen‐bridged radical cation [CH 3 O(H)…︁H…︁OCH] .+

Dissociative ionization of methyl 2‐hydroxy‐isovalerate and dimethyl tartrate cleanly generate, via McLafferty rearrangements, the 1‐methoxy‐ethene‐1,2‐diol ion HOCHC(OH)OCH   3 +· , 2. The unimolecular chemistry of 2, the enol form of ionized methyl glycolate, HOCH 2 C(O)OCH   3 +· , 1, was investigated by a variety of tandem‐mass spectrometry‐based techniques using D‐ and 18 O‐labelled precursor molecules. The enol ion undergoes four major dissociations viz. loss of CH   3 . , CO, CH 3 OH and C 2 HO   2 . . Loss of CH   3 ·involves isomerization of 2, via a 1,4 H shift, into the distonic ion HC(O . )C(OH)O + (H)CH 3 , 4, followed by direct bond cleavage yielding the product ion HC(O)C(OH)   2 · . A second 1,4 H shift yields the hydroxyketene/methanol ion–dipole complex which serves as the precursor for the losses of CH 3 OH and C 2 HO   2 · , yielding HO(H)CCO +· and CH 3 OH   2 +respectively. A further isomerization step leads to the loss of CO, yielding the O…H ⃛O bridged ion [CH 3 O(H) ⃛H… OCH] +· , one of the most stable isomers on the C 2 H 6 O   2 +·potential energy surface. Ionized methyl acetate, CH 3 C(O)OCH   3 +·and related aliphatic esters, readily interconvert with their enol isomers prior to dissociation, but no such tautomerization occurs in 1. This is because the HOCH functionality opens up facile rearrangement/dissociation pathways in 1 and 2 whose energy requirements lie below the tautomerization barrier 1→2.