Unraveling Chain Branching in Cool Flames

In cool flames, autoxidation of organic compounds forms alkyl hydroperoxides and ketohydroperoxides, and this controls the critical rate of chain branching, but there have been large uncertainties in the decomposition rate constants. We synthesized a series of hydroperoxides and measured their decomposition rate constants in pyrolysis experiments by spray-vaporization jet-stirred-reactor synchrotron vacuum ultraviolet photoionization mass spectrometry. Structural variation of the hydroperoxides, including alkyl, cycloalkyl, aromatic, and heterocyclic functionalities, has only a slight effect on their decomposition rate constants. Calculated rate constants are in good agreement with the experiment. The rate constant of ketohydroperoxide decomposition was obtained by theoretical calculation of 3-hydroperoxy butanal and tested by the pyrolysis of synthesized 3-hydroperoxy-3-phenylpropionate. The rate constant of ketohydroperoxide decomposition is close to that of alkyl hydroperoxides. The new chain-branching rate constants improves the cool-flame kinetic model, which is essential for removing discrepancies in model predictions and for the design of high-efficiency and low-emission engines.