Kinetics and thermochemistry of the reaction of 3-methylpropargyl radical with molecular oxygen

Abstract

We have measured the kinetics and thermochemistry of the reaction of 3-methylpropargyl radical (but-2-yn-1-yl) with molecular oxygen over temperature (223–681 K) and bath gas density (1.2−15.0×1016cm−3) ranges employing photoionization mass-spectrometry. At low temperatures (223–304 K), the reaction proceeds overwhelmingly by a simple addition reaction to the −CH2 end of the radical, and the measured CH3CCCH2•+O2 reaction rate coefficient shows negative temperature dependence and depends on bath gas density. At intermediate temperatures (340–395 K), the addition reaction equilibrates and the equilibrium constant was determined at different temperatures. At high temperatures (465–681 K), the kinetics is governed by O2 addition to the third carbon atom of the radical, and rate coefficient measurements were again possible. The high temperature CH3CCCH2•+O2 rate coefficient is much smaller than at low T, shows positive temperature dependence, and is independent of bath gas density. In the intermediate and high temperature ranges, we observe a formation signal for ketene (ethenone). The reaction was further investigated by combining the experimental results with quantum chemical calculations and master equation modeling. By making small adjustments (2−3kJmol−1) to the energies of two key transition states, the model reproduces the experimental results within uncertainties. The experimentally constrained master equation model was used to simulate the CH3CCCH2•+O2 reaction system at temperatures and pressures relevant to combustion.