Dissociation chemistry of the hydrogen-bridged radical cation [CH 2O ⋯ H ⋯ OC–OCH 3] ·+: proton transport catalysis and charge transfer

Abstract

Tandem mass spectrometry based experiments on the decarbonylation products of ionized methyl-β-hydroxypyruvate (MHP) and dimethyloxalate (DMO) show that the hydrogen-bridged radical cation (HBRC) CH 2O ⋯ H ⋯ OC–OCH 3 ·+ is a stable species in the gas phase. Its low energy dissociation products are protonated methylformate, HOC(H)OCH 3 +, and the formyl radical, HCO ·. The HBRC isomers HOCH 2C(O)OCH 3 ·+ (ionized methylglycolate) and (CH 3O) 2CO ·+ (ionized dimethylcarbonate) show the same dissociation characteristics. Deuterium labeling experiments dictate that loss of HCO · from the title HBRC cannot be formulated as a simple H shift from the formaldehyde moiety to the C atom of the OC ·–OCH 3 group. Ab initio molecular orbital (MO) calculations support the proposal that this dissociation proceeds via sequential transfers of a proton, electron, and another proton within ion–dipole complexes. The first step in this rearrangement process is a 1,2-proton shift catalyzed by a formaldehyde dipole. This yields an ion/dipole complex, CH 2O ⋯ H–C(O)OCH 3 ·+, that is in the correct configuration for electron transfer to occur at the energetic threshold dictated by experiment. The resulting intermediate triggers the transfer of yet another proton from the formaldehyde unit, thereby generating another stable H-bridged radical cation viz. HCO ⋯ H ⋯ OC(H)OCH 3 ·+. This final intermediate dissociates with little or no activation energy into HOC(H)OCH 3 + 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 methylglycolate and dimethylcarbonate via the title ion quite unlikely; instead an alternative route for this communication is proposed.