Unraveling\nChain Branching in Cool Flames

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