RATE CONSTANTS FOR SOME RADICAL RADICAL CROSS-REACTIONS AND THE GEOMETRIC MEAN RULE

Rate constants for four radical-radical cross-reactions have been measured at 300 K. A laser pulse formed two different radicals simultaneously, and their decays were followed with a photoionization mass spectrometer. The major radical, formed at high concentration, decayed predominantly by reaction with itself. The minor radical decay was determined primarily by reaction with the major radical. Computer modeling was used to simultaneously fit the decays of both radicals. The measured rate constants, in units of 10-11 cm3 molecule-1 s-1, are 6.5 ± 2.0 for CH3 + C3H5, 9.3 ± 4.2 for CH3 + C2H5, 10.5 ± 4.0 for C3H5 + C2H5, and <0.6 for CH3 + CCl3. The slow rate observed for CH3 + CCl3 is not caused by a falloff effect. These results, and others in the literature, are compared to the predictions of the geometric mean rule. © 1990 American Chemical Society.