Double-Imaging Photoelectron Photoion Coincidence Spectroscopy Reveals the Unimolecular Thermal Decomposition Mechanism of Dimethyl Carbonate

We studied the thermal decomposition of dimethyl carbonate (DMC, C3H6O3) in a flash vacuum pyrolysis reactor 1100–1700 K range. The reaction products and intermediates were probed by ultraviolet synchrotron radiation photoelectron photoion coincidence (PEPICO) spectrometer to record isomer-specific mass-selected threshold (ms-TPE) spectra. Reaction pathways explored using quantum chemical calculations, which confirmed experimental observation that intramolecular migration methyl group, yielding ether (DME, C2H6O) carbon dioxide, dominates initial unimolecular chemistry. role second potentially important channel, namely, C–O bond fission yield radicals, could not be determined experimentally due short lifetime ·C2H3O3 radical overlapping sequential products. However, potential energy surface microcanonical rate constant calculations predict 2 3 orders magnitude lower rates for this channel than decarboxylation DME. Consequently, DMC shows bewilderingly similar product abundances as DME pyrolysis. This coincides with combustion modeling studies, found is key intermediate mechanism. Furthermore, we have detected traces formate formaldehyde, produced after hydrogen shift central atom DMC. Ethylene acetylene established bimolecular because their abundance depended strongly on concentration. It intriguing compare structurally methylal (dimethoxymethane, DMM). While methanol formaldehyde are quantities DMM, thanks low-energy hydrogen-transfer reactions, almost fully suppressed absence hydrogens at thermodynamically favored decarboxylation. These new mechanistic insights may help development predictive models fuel additives biofuels.