ClH⋯O and O⋯H⋯O bonded intermediates in the dissociation of low energy methyl glycolate radical cations

Low energy methyl glycolate radical cations HOCH2C(O)OCH3+, 1, abundantly lose HCO, yielding protonated methyl formate HC(OH)OCH3+. Tandem mass spectrometry based experiments on 2H, 13C and 18O labelled isotopologues show that this loss is largely (about 75%) atom specific. Analysis of the atom connectivity in the product ions indicates that the reaction proceeds analogously to the loss of HCO and CH3CO from ionized acetol HOCH2C(O)CH3+ and acetoin HOCH(CH3)C(O)CH3+, respectively. The mechanism, it is proposed, involves isomerization of 1 to the key intermediate CH2O⋯ HC(O)OCH3+, an H-bridged ion-dipole complex of neutral formaldehyde and ionized methyl formate. Next, charge transfer takes place to produce CH3OC(H)O⋯HC(H)O+, an H-bridged ion-dipole complex of ionized formaldehyde and neutral methyl formate, followed by proton transfer to generate the products.Preliminary ab initio calculations executed at the UMP3/6-31G∗//6-31G∗+ZPVE level of theory are presented in support of this proposal. The non-specific loss of HCO from 1 (about 25%) is rationalized to occur via the same mechanism, but after communication with isomeric dimethyl carbonate ions CH3OC(O)OCH3+, 2, via the O⋯H⋯O bonded intermediate [CH2O⋯H⋯OCOCH3]+. The latter pathway is even more important in the formation of CH2OH+ ions from 1 which, it is shown, is not a simple bond cleavage reaction, but may involve consecutive or concerted losses of CH3 and CO2 from the above O⋯H⋯O bonded species.Ionized methyl lactate HOCH(CH3)C(O)OCH3+, the higher homologue of 1, shows a unimolecular chemistry which is akin to that of 1.