Study of the spectra and recombination kinetics of alkyl radicals by molecular modulation spectrometry. Part 2.—The recombination of ethyl, isopropyl and t-butyl radicals at room temperature and t-butyl radicals between 250 and 450 K

Abstract

Using the technique of molecular modulation spectrometry, we have found transient ultraviolet absorptions which we assign to ethyl, isopropyl and t-butyl radicals. The spectra are broad continua with maxima near 250, 233 and 230 nm, respectively. From the rate constants of their mutual reactions, measured by monitoring the radicals in the ultraviolet, and the disproportionation/recombination ratios found from separate experiments, the rates of recombination are measured at room temperature to be: ethyl, (1.3 ± 0.3)× 10^–11 cm³ molecule^–1 s^–1; isopropyl, (8.3 ± 2.0)× 10^–12 cm³ molecule^–1 s^–1; t-butyl, (4.0 ± 1.0)× 10^–12 cm³ molecule^–1 s^–1. The t-butyl rate falls by a factor of 2 as the temperature is increased to 423 K. These results are compared with other recent gas- and liquid-phase determinations, and we conclude that the currently accepted thermochemistry of larger alkyl radicals and our results are incompatible.