Rates of reaction of butyl radicals with molecular oxygen

Rate constants for the reaction of four different butyl radicals with molecular oxygen have been measured at room temperature. The radicals were generated by flash photolysis and their time decay was followed with a photoionization mass spectrometer. The radical concentrations were kept low to avoid complications from radical–radical reactions. Radical lifetimes were long, up to 50 msec, thus assuring that thermalized radicals were being studied. The rate constants, in units of 10−11 cm3 molecule−1 sec−1, are: n-butyl (0.75±0.14); s-butyl (1.66±0.22); t-butyl (2.34±0.39); 3-hydroxy s-butyl (2.8±1.8). No pressure dependence of the rate constants was observed over the range 1 to 4 Torr. In the absence of O2, the butyl radicals decay mainly by loss on the quartz surface of the reaction cell, with sticking coefficients in the range of 10−2 to 10−3. The Adiabatic Channel Model can predict the approximate absolute values of these rate constants using reasonable molecular parameters, but it fails to reproduce the observed trend of rate constants with radical ionization potential.