Dissociation chemistry of the hydrogen-bridged radical cation [CH2=O center dot center dot center dot H center dot center dot center dot O=C-OCH3](center dot+): proton transport catalysis and charge transfer

Tandem mass spectrometry based experiments on the decarbonylation products of ionized methyl-P-hydroxypyruvate (MHP) and dimethyloxalate (DMO) show th at the hydrogen-bridged radical cation (HBRC) CH2=O...H...O=C-OCH3.+; is a stable species in the gas phase. Its low energy dissociation products are p rotonated methylformate, HOC(H)OCH:, and the formyl radical, HC=O-.. The HB RC isomers HOCH2C(=O)OCH3.+ (ionized methylglycolate) and (CH3O)(2)C=O.+ (i onized dimethylcarbonate) show the same dissociation characteristics. Deute rium labeling experiments dictate that loss of HC=O-. from the title HBRC c annot be formulated as a simple H shift from the formaldehyde moiety to the C atom of the O=C-.-OCH3, group. Ab initio molecular orbital (MO) calculat ions support the proposal that this dissociation proceeds via sequential tr ansfers of a proton, electron, and another proton within ion-dipole complex es. The first step in this rearrangement process is a 1,2-proton shift cata lyzed by a formaldehyde dipole. This yields an ion/dipole complex, CH2=O H- C(=O)OCH3+, that is in the correct configuration for electron transfer to o ccur at the energetic threshold dictated by experiment. The resulting inter mediate triggers the transfer of yet another proton from the formaldehyde u nit, thereby generating another stable H-bridged radical cation viz. HC=O.. .H...OC(H)OCH3.+. This final intermediate dissociates with little or no act ivation energy into HOC(H)OCH3+ and HC=O-.. It is further predicted by the calculations that ionized methylglycolate isomerizes into the title HBRC by a fairly high barrier that makes the communication between ionized methylg lycolate and dimelhylcarbonate via the title ion quite unlikely; instead an alternative route for this communication is proposed. (Int J h iass Spectr om 195/196 (2000) 85-99) (C) 2000 Elsevier Science B.V.